The Pictet-Spengler Reaction Updates Its Habits

The Pictet-Spengler reaction (P-S) is one of the most direct, efficient, and variable synthetic method for the construction of privileged pharmacophores such as tetrahydro-isoquinolines (THIQs), tetrahydro-β-carbolines (THBCs), and polyheterocyclic frameworks. In the lustro (five-year period) following its centenary birthday, the P-S reaction did not exit the stage but it came up again on limelight with new features. This review focuses on the interesting results achieved in this period (2011–2015), analyzing the versatility of this reaction. Classic P-S was reported in the total synthesis of complex alkaloids, in combination with chiral catalysts as well as for the generation of libraries of compounds in medicinal chemistry. The P-S has been used also in tandem reactions, with the sequences including ring closing metathesis, isomerization, Michael addition, and Gold- or Brønsted acid-catalyzed N-acyliminium cyclization. Moreover, the combination of P-S reaction with Ugi multicomponent reaction has been exploited for the construction of highly complex polycyclic architectures in few steps and high yields. The P-S reaction has also been successfully employed in solid-phase synthesis, affording products with different structures, including peptidomimetics, synthetic heterocycles, and natural compounds. Finally, the enzymatic version of P-S has been reported for biosynthesis, biotransformations, and bioconjugations.

It should be noted Gholamzadeh recently published a relevant chapter of a book, analyzing the versatility of P-S reaction in the construction of heterocyclic scaffolds [12]. Moreover, Kumar and coworkers published in 2018 a review depicting an overview on synthetic versus enzymatic P-S reaction [13]. Seidel brought to light a procedure that can be considered as a redox variant of classic P-S reaction [23]. Seidel and coworkers discovered that indole aldehyde 15 engage cyclic amines 16, such as THIQ, pyrrolidine or proline esters, at a high temperature under microwave conditions to form the corresponding ring fused products 17 in moderate to good yields and in short times (Scheme 5). The addition of appropriate additives (e.g., carboxylic acids) and particular substrate combinations allowed for the reaction to occur at lower temperatures [24].
In spite of a fierce competition, the P-S cyclization maintains the role of a protagonist, enriching the chemical literature with examples of its versatility. If we take into consideration for an update the five years (2011)(2012)(2013)(2014)(2015) following the one-century birthday, we can highlight that the reaction is still active in the syntheses of biologically relevant benzoannulated nitrogen heterocycles. Finally, the novel Lewis-acid catalyzed [3 + 3]-annulation process for the synthesis of THBCs 5a and THIQs 14c from sulfonyl aziridines 18 and readily available benzylic alcohols 19a and 19b (Scheme 6), can be a valuable complement to the more widely used Pictet-Spengler condensation [25,26]. Scheme 6. Syntheses of 4-aryl-tetrahydro-β-carbolines and 4-aryl-tetrahydroisoquinolines from aziridines and benzyl alcohols.
In spite of a fierce competition, the P-S cyclization maintains the role of a protagonist, enriching the chemical literature with examples of its versatility. If we take into consideration for an update the five years (2011)(2012)(2013)(2014)(2015) following the one-century birthday, we can highlight that the reaction is still active in the syntheses of biologically relevant benzoannulated nitrogen heterocycles. Finally, the novel Lewis-acid catalyzed [3 + 3]-annulation process for the synthesis of THBCs 5a and THIQs 14c from sulfonyl aziridines 18 and readily available benzylic alcohols 19a and 19b (Scheme 6), can be a valuable complement to the more widely used Pictet-Spengler condensation [25,26]. Scheme 6. Syntheses of 4-aryl-tetrahydro-β-carbolines and 4-aryl-tetrahydroisoquinolines from aziridines and benzyl alcohols.

I think it's time
In spite of a fierce competition, the P-S cyclization maintains the role of a protagonist, enriching the chemical literature with examples of its versatility. If we take into consideration for an update the five years (2011)(2012)(2013)(2014)(2015) following the one-century birthday, we can highlight that the reaction is still active in the syntheses of biologically relevant benzoannulated nitrogen heterocycles.

I think it's time
To do an update (Roger Turner)
Molecules 2020, 25, x FOR PEER REVIEW 7 of 84 based on glycine donors including hydantoin Pr-a, (±)-Boc-α-phosphonoglycine trimethyl ester Prb, and (±)-Z-α-phosphonoglycine trimethyl ester Pr-c (Scheme 7). Following this approach, a focused library of N-arylacyl, N-arylalkyl, and bis-THIQ3CA analogs 34 was synthesized [32]. Potter and coworkers developed a new series of anticancer agents by translating the key elements of steroidal pharmacophores into alternate scaffolds. The authors synthesized 6benzyloxy-7-methoxy tetrahydroisoquinoline 36 via the key P-S reaction (Scheme 8, up), and then connected the steroid A/B ring mimicking THIQ core to monomethoxybenzyl ring through methylene [33,34], carbonyl [34], and sulfonyl linkers X [34], to provide the desired putative steroidomimetic 37. Finally, the optimization of the representative 37 trough conformational biasing delivered a new series of microtubule disruptors with a 10-fold gain in antiproliferative activity [33]. Linkage of the THIQ-based A/B-mimic 36 with the trimethoxybenzyl motif that is prevalent in colchicine disclosed a series of chimeric molecules 38 (Scheme 8, down), whose activities surpass those of parent steroid derivatives [34]. Potter and coworkers developed a new series of anticancer agents by translating the key elements of steroidal pharmacophores into alternate scaffolds. The authors synthesized 6-benzyloxy-7-methoxy tetrahydroisoquinoline 36 via the key P-S reaction (Scheme 8, up), and then connected the steroid A/B ring mimicking THIQ core to monomethoxybenzyl ring through methylene [33,34], carbonyl [34], and sulfonyl linkers X [34], to provide the desired putative steroidomimetic 37. Finally, the optimization of the representative 37 trough conformational biasing delivered a new series of microtubule disruptors with a 10-fold gain in antiproliferative activity [33]. Linkage of the THIQ-based A/B-mimic 36 with the trimethoxybenzyl motif that is prevalent in colchicine disclosed a series of chimeric molecules 38 (Scheme 8, down), whose activities surpass those of parent steroid derivatives [34].
Studies on the quantitative structure-activity relationship (QSAR) of N-benzenesulfonyl THIQ analogs 41 revealed the toxicity of several compounds against MOLT-3 cell lines (Scheme 9). The sulfonyl group was expected both to increase the electrophilicity of the iminium intermediate and to govern the bioactivity [35,36].
Conformationally locked allene-containing THBC (not shown), which was generated from the corresponding tryptophan derivative 21 via P-S cyclization, was subjected to cycloisomerization reaction to give tetrahydroindolizinoindole 44a. The functionalization of 44a with an alkyne and [2+2] cycloaddition of the allene-yne provided the tetrahydro-β-carbolinecyclobutanes 44b, while cyclocarbonylation afforded α-methylenecyclopentenones 46d (Scheme 11) [51]. 8  Studies on the quantitative structure-activity relationship (QSAR) of N-benzenesulfonyl THIQ analogs 41 revealed the toxicity of several compounds against MOLT-3 cell lines (Scheme 9). The sulfonyl group was expected both to increase the electrophilicity of the iminium intermediate and to govern the bioactivity [35,36].
A library of 34 THBC-containing compounds (46), including the N-tosyl derivatives 46b, was synthesized utilizing a skeletal diversification strategy. In silico screening directed these compounds to the appropriate biological targets [51].
A library of 34 THBC-containing compounds (46), including the N-tosyl derivatives 46b, was synthesized utilizing a skeletal diversification strategy. In silico screening directed these compounds to the appropriate biological targets [51]. A library of 34 THBC-containing compounds (46), including the N-tosyl derivatives 46b, was synthesized utilizing a skeletal diversification strategy. In silico screening directed these compounds to the appropriate biological targets [51].
A protocol for the P-S condensation between the methyl ester of L-DOPA 63a and various N-Boc protected 1H-indole-3-carbaldehydes 64a gave C-1 indol-3-yl substituted THIQs 66a as a mixture of isolable cis/trans diastereomers in good yield. Following a Type-A procedure, the THIQ products 65a can be in turn transformed into optically active diketopiperazine fused analogue 66a. Alternatively, compounds 66b were directly prepared drom L-DOPA derivative 63b by condensation with N-Boc protected 1H-indole-3-carbaldehydes 64b without isolation of the P-S precursors 65b (Scheme 17) [73]. The type C procedure foresees two simultaneous ring closures, as in the enantiospecific and stereo-selective P-S bis-cyclization between (R)-(−)-methyl-2-amino-3-(3,4-dimethoxyphenylpropanoate 67 and 4-chloro-1,1-dimethoxybutane 68 preferentially, which provided the cis-tricyclic Scheme 17. Synthesis of diketopiperazine fused THIQ derivatives through type A and C procedures. The type C procedure foresees two simultaneous ring closures, as in the enantiospecific and stereo-selective P-S bis-cyclization between (R)-(−)-methyl-2-amino-3-(3,4-dimethoxyphenyl-propanoate 67 and 4-chloro-1,1-dimethoxybutane 68 preferentially, which provided the cis-tricyclic adduct 69 precursor of the natural product (+)-crispine A (Scheme 18). The unnatural antipode (−)-crispine A was similarly prepared from the commercially available (S)-(+)-amino acid ester [81]. total syntheses of structurally diverse compounds, all being endowed with a host of biological activities, include in most cases a step overcome by P-S methodology. In Table 1, the target products of multi-step syntheses are summarized together with some information on the P-S cyclization as well as on the type of related natural products.  In Table 1, the target products of multi-step syntheses are summarized together with some information on the P-S cyclization as well as on the type of related natural products. Table 1. Pictet-Spengler reaction in total synthesis of natural products.
• in the total synthesis of (±)-hamayne ( Figure 3) the C-1 methylene was introduced via a P-S reaction at the end of 13 steps, just before deprotection of hydroxyl groups [86]; • the total synthesis of γ-licorane ( Figure 3) was also completed by a P-S ring closure [87]; and, • the asymmetric total synthesis of (−)-saframycin A (Scheme 20) from L-tyrosine involved stereoselective intramolecular and intermolecular P-S reactions to induce the correct stereochemistry at C-1 and C-11, respectively [88]. (2011) Cyclization of 3-arylidene-6-methylpiperazinedione 75 with 2,2-diethoxyethyl benzoate afforded in two steps the P-S-adduct 76 as a single isomer (Scheme 20). The lactam 76 was used to construct the pentacyclic key intermediate framework (not shown) [82] and achieve the total synthesis of cribrostatin 4 [83] and renieramycin G ( Figure 3) [84,85]. Scheme 20. Construction of the core ring system of cribrostatin 4 via P-S cyclization.


in the total synthesis of (±)-hamayne ( Figure 3) the C-1 methylene was introduced via a P-S reaction at the end of 13 steps, just before deprotection of hydroxyl groups [86];  the total synthesis of γ-licorane ( Figure 3) was also completed by a P-S ring closure [87]; and,  the asymmetric total synthesis of (−)-saframycin A (Scheme 21) from L-tyrosine involved stereoselective intramolecular and intermolecular P-S reactions to induce the correct stereochemistry at C-1 and C-11, respectively [88].

Scheme 21.
Construction of the core ring system of cribrostatin 4 via P-S cyclization.  containing the THBC ring of mitragynine, paynantheine, and  speciogynine (Scheme 22), was constructed via an enantioselective thiourea-catalyzed P-S cyclization involving the tryptamine derivative 79 and the aldehyde 80 [89].

•
A thermodynamically controlled P-S reaction for the formation of the tetrahydroisoquinoline skeleton is among the steps that lead to the efficient and convergent total synthesis of (−)-lemonomycin ( Figure 3) [91].
(2013) The tetracyclic core of lemonomycin ( Figure 3) was synthesized from a known substituted tyrosinol through a 16-step sequence, which involved the P-S reaction inter alia [92].
In the first total syntheses of C-3 epimeric natural products venenatine and alstovenine (Scheme 24), the stereochemistry at C-3 of the yohimbinoid skeleton was effectively controlled in a P-S cyclization utilizing an aminonitrile intermediate [93].

•
In the framework of the synthesis of indole alkaloids such as the monomers (+)-locknerine, (+)-spegatrine, and the dimer P-(+)-dispegatrine (Figure 3), the mixture of cis/trans products from the P-S reaction was converted by treatment with TFA into the desired trans isomer [96].   A thermodynamically controlled P-S reaction for the formation of the tetrahydroisoquinoline skeleton is among the steps that lead to the efficient and convergent total synthesis of (−)lemonomycin ( Figure 3) [91].
(2013) The tetracyclic core of lemonomycin ( Figure 3) was synthesized from a known substituted tyrosinol through a 16-step sequence, which involved the P-S reaction inter alia [92].
In the first total syntheses of C-3 epimeric natural products venenatine and alstovenine (Scheme 24), the stereochemistry at C-3 of the yohimbinoid skeleton was effectively controlled in a P-S cyclization utilizing an aminonitrile intermediate [93].  24 compounds with diversified 3-aryl acrylic amide side chains of the simplified saframycinecteinascidin pentacyclic skeleton ( Figure 3) were synthesized via a stereospecific route, starting from L-DOPA [94,95].  In the framework of the synthesis of indole alkaloids such as the monomers (+)-locknerine, (+)spegatrine, and the dimer P-(+)-dispegatrine (Figure 3), the mixture of cis/trans products from the P-S reaction was converted by treatment with TFA into the desired trans isomer [96].
(2013-2014) Three renieramycin type anticancer alkaloids, jorunnamycins A and C, and jorumycin, were synthesized by a new convergent approach, which couples for a highly regio-and stereo-selective P-S cyclization tryptamine 87a and tetrahydroisoquinoline 88 to provide the intermediate 89a as a single isomer (Scheme 25, up) [97]. (2013-2014) Three renieramycin type anticancer alkaloids, jorunnamycins A and C, and jorumycin, were synthesized by a new convergent approach, which couples for a highly regio-and stereo-selective P-S cyclization tryptamine 87a and tetrahydroisoquinoline 88 to provide the intermediate 89a as a single isomer (Scheme 25, up) [97]. Conversely, a temperature-dependent stereoselective P-S reaction of amino ester 87b and aldehyde 88 afforded the cyclization product 89b; the subsequent deprotection and the lactamization of this compound were the protagonists of a flexible protocol for the asymmetric synthesis of antitumor alkaloids (−)-jorunnamycin A and (−)-renieramycin G (Scheme 25, down) [95].
(2013) O-Triflation of indolyl derivative 90, followed by the addition of DIBAL-H and wet Et2O-Rochelle salt work-up, gave the crude hemiaminal lactol, which, after quick treatment with wet DCM, underwent triflic acid elimination and P-S cyclization to afford the pentacyclic vinylogous ester 91. The removal of the N-tosyl group and N-methylation of 91 ended a concise synthesis of the macroline-related alkaloid (±)-alstonerine (Scheme 26, up) [99]. Scheme 25. Synthesis of renieramycin-type alkaloids via stereoselective cyclization to P-S adducts.
Conversely, a temperature-dependent stereoselective P-S reaction of amino ester 87b and aldehyde 88 afforded the cyclization product 89b; the subsequent deprotection and the lactamization of this compound were the protagonists of a flexible protocol for the asymmetric synthesis of antitumor alkaloids (−)-jorunnamycin A and (−)-renieramycin G (Scheme 25, down) [95].
(2013) O-Triflation of indolyl derivative 90, followed by the addition of DIBAL-H and wet Et 2 O-Rochelle salt work-up, gave the crude hemiaminal lactol, which, after quick treatment with wet DCM, underwent triflic acid elimination and P-S cyclization to afford the pentacyclic vinylogous ester 91. The removal of the N-tosyl group and N-methylation of 91 ended a concise synthesis of the macroline-related alkaloid (±)-alstonerine (Scheme 26, up) [99]. (2014) The key steps of the synthesis of erysotramidine included two oxidative dearomatization processes that were mediated by a hypervalent iodine reagent, a novel tandem aza-Michael rearomatization, a P-S cyclization (9293) to produce the main tetracyclic system and the final stereoselective ketone reduction (Scheme 26, down) [100,101].
(2014) In a P-S type reaction, the electrophile 94 and the π-nucleophile 95 gave the tetracyclic intermediate 96, which can be converted into the pentacyclic precursor 97 of the total synthesis of the indole alkaloid (±)-actinophyllic acid (Scheme 27) [102]. Scheme 27. Lewis acid catalyzed cascade: key step in the total synthesis of (±)-actinophyllic acid.
(2014-2015) Saito and coworkers presented an alternative large-scale approach for the total synthesis of cribrostatin four analogs, as well as C3-C4 unsaturated bis-p-quinone derivatives, such as renieramycin I. According to the results of previous studies [103], the treatment of the readily Scheme 26. P-S cyclization as a key step in the synthesis of (±)-alstonerine (up) and erysotramidine (down).
(2014) The key steps of the synthesis of erysotramidine included two oxidative dearomatization processes that were mediated by a hypervalent iodine reagent, a novel tandem aza-Michael rearomatization, a P-S cyclization (92→93) to produce the main tetracyclic system and the final stereoselective ketone reduction (Scheme 26, down) [100,101].
(2014) In a P-S type reaction, the electrophile 94 and the π-nucleophile 95 gave the tetracyclic intermediate 96, which can be converted into the pentacyclic precursor 97 of the total synthesis of the indole alkaloid (±)-actinophyllic acid (Scheme 27) [102]. (2014) The key steps of the synthesis of erysotramidine included two oxidative dearomatization processes that were mediated by a hypervalent iodine reagent, a novel tandem aza-Michael rearomatization, a P-S cyclization (9293) to produce the main tetracyclic system and the final stereoselective ketone reduction (Scheme 26, down) [100,101].
(2014) In a P-S type reaction, the electrophile 94 and the π-nucleophile 95 gave the tetracyclic intermediate 96, which can be converted into the pentacyclic precursor 97 of the total synthesis of the indole alkaloid (±)-actinophyllic acid (Scheme 27) [102]. Scheme 27. Lewis acid catalyzed cascade: key step in the total synthesis of (±)-actinophyllic acid.
(2014-2015) Saito and coworkers presented an alternative large-scale approach for the total synthesis of cribrostatin four analogs, as well as C3-C4 unsaturated bis-p-quinone derivatives, such as renieramycin I. According to the results of previous studies [103], the treatment of the readily Scheme 27. Lewis acid catalyzed cascade: key step in the total synthesis of (±)-actinophyllic acid.

Dressing with Fashionable Clothes a Classic Reaction
Not to be old fashioned I'll put something red (Emily Dickinson) In the second century on stage, the P-S cyclization maintains its peculiar chameleonic quality, but to remain at the limelight needs to join the forces with in-fashion allies, such as the support of solid phases, the push of new catalysts, and mainly the impulse of new habits, such as the cascade sequences and the multicomponent reactions (MCRs).

Dressing with Fashionable Clothes a Classic Reaction
Not to be old fashioned I'll put something red

(Emily Dickinson)
In the second century on stage, the P-S cyclization maintains its peculiar chameleonic quality, but to remain at the limelight needs to join the forces with in-fashion allies, such as the support of solid phases, the push of new catalysts, and mainly the impulse of new habits, such as the cascade sequences and the multicomponent reactions (MCRs).

A New Scenography: Updating the Solid Phase Strategy
Advances in support, protecting group strategies, and extensive optimization of chemical methodology have expanded the scope of solid phase chemistry from mere peptide preparation to the synthesis of pharmacologically relevant small molecules [110][111][112], as well as the total synthesis of natural products and their analogues [113,114]. Different versions of P-S reaction have found application in a procedure, where the solid-phase support can be chosen to modulate the reactivity of the three functional groups (aldehyde, amine, and aromatic nucleophile) [115].
In 2009, a paper from Nielsen et al. reviewed the methods used to generate N-acyliminium ion intermediates on solid support and gain access via the intra-or intermolecular P-S condensation products to diverse structures [116].
In the 2011-2015 year range, only two papers concerning an approach in solid phase of the intramolecular P-S reaction appeared [121,122]. In the first one by Chanda et al., polyethylene glycol-immobilized tryptophan ester 102 was combined with a variety of ketones by reflux in acidic chloroform, to give soluble polymer-supported THBCs in good yields [121]. Amination of the N b -chloroacetamide that was obtained by treatment with chloroacetyl chloride, followed by intramolecular cyclization and cleavage of the polymer, finally led to the costruction of a tetracyclic architecture. The reactions and the following synthesis of biologically promising diketopiperazine-fused THBC structures 103 are resumed in Scheme 29 [121]. of natural products and their analogues [113,114]. Different versions of P-S reaction have found application in a procedure, where the solid-phase support can be chosen to modulate the reactivity of the three functional groups (aldehyde, amine, and aromatic nucleophile) [115]. In 2009, a paper from Nielsen et al. reviewed the methods used to generate N-acyliminium ion intermediates on solid support and gain access via the intra-or intermolecular P-S condensation products to diverse structures [116].
In the 2011-2015 year range, only two papers concerning an approach in solid phase of the intramolecular P-S reaction appeared [121,122]. In the first one by Chanda et al., polyethylene glycol-immobilized tryptophan ester 102 was combined with a variety of ketones by reflux in acidic chloroform, to give soluble polymer-supported THBCs in good yields [121]. Amination of the Nbchloroacetamide that was obtained by treatment with chloroacetyl chloride, followed by intramolecular cyclization and cleavage of the polymer, finally led to the costruction of a tetracyclic architecture. The reactions and the following synthesis of biologically promising diketopiperazinefused THBC structures 103 are resumed in Scheme 29 [121]. Another process (by Nielsen et al., 2012) relies on an efficient ketone-amide condensation, starting from γ-ketocarboxylic acids, immobilized on a solid support as 104. Substrates, by treatment with a mild acid (HCOOH, the more efficient after a screening), generate the Nakyliminium ions, which undergo P-S type cyclization (Scheme 30) [122]. The cascade sequence afforded in a high stereoselective fashion pure products 106 after cleavage from the resin. The combination of various γ-ketoacid amides with a range of nucleophiles, including electron-rich aromatic (as 3,4-dimethoxy substitution for 106a) and heteroaromatic rings (as indole nucleus for 106b), led to a library of a range of pharmaceutically interesting heterocyclic scaffolds with exclusive diastereocontrol of the junction stereocenters [122]. Another process (by Nielsen et al., 2012) relies on an efficient ketone-amide condensation, starting from γ-ketocarboxylic acids, immobilized on a solid support as 104. Substrates, by treatment with a mild acid (HCOOH, the more efficient after a screening), generate the N-akyliminium ions, which undergo P-S type cyclization (Scheme 30) [122]. The cascade sequence afforded in a high stereoselective fashion pure products 106 after cleavage from the resin. The combination of various γ-ketoacid amides with a range of nucleophiles, including electron-rich aromatic (as 3,4-dimethoxy substitution for 106a) and heteroaromatic rings (as indole nucleus for 106b), led to a library of a range of pharmaceutically interesting heterocyclic scaffolds with exclusive diastereocontrol of the junction stereocenters [122].

Ruthenium-Catalyzed N-Acyliminium Cyclization
(2011) The Nielsen group developed the synthesis of indolizinoindoles 110, starting from the ruthenium alkylidene catalyzed tandem ring closing metathesis (RCM) of dienes 107 via Nacyliminium intermediates 109 (Scheme 31, up). In the case of indole substitution (107a) and fivemembered ring (n = 0), the corresponding unsaturated lactams 108β and 108α are formed after a RCM and subsequent isomerization, respectively. The successive protonation or reaction of 108α with Ru + gave the reactive N-acyliminium species 109, which were finally trapped by the tethered nucleophile to give the THBC tetracycle 110a (n = 0). The homologous indole-based substrates 107a (n = 1; n = 2) underwent RCM reactions, but not further conversions into THBCs, being required the conjugation of the double bond that formed in the RCM step with the lactam carbonyl. Hoveyda-Grubbs catalyst HG-I (at 5 mol%, in m-xylene at reflux) gave the cleanest and highest conversion 107a (n = 1, 2)  110a (95%; n = 1, 2). The trimethoxybenzene derivative 107c (n = 1) also underwent the tandem reaction sequence to provide the tetrahydroisoquinoline derivative 110c in good yield (64%, Scheme 31, down). By contrast, the conversion of substrate 107d or other substrates bearing a heterocycle moiety (not shown) required the successive addition of TFA (1 eq.) and further 2 h heating to give the tricycle product 110d (or the other corresponding cyclization products) in 98% yield and diastereomeric ratio > 20:1, or the other cyclization products [123].

Ruthenium-Catalyzed N-Acyliminium Cyclization
(2011) The Nielsen group developed the synthesis of indolizinoindoles 110, starting from the ruthenium alkylidene catalyzed tandem ring closing metathesis (RCM) of dienes 107 via N-acyliminium intermediates 109 (Scheme 31, up). In the case of indole substitution (107a) and five-membered ring (n = 0), the corresponding unsaturated lactams 108β and 108α are formed after a RCM and subsequent isomerization, respectively. The successive protonation or reaction of 108α with Ru + gave the reactive N-acyliminium species 109, which were finally trapped by the tethered nucleophile to give the THBC tetracycle 110a (n = 0). The homologous indole-based substrates 107a (n = 1; n = 2) underwent RCM reactions, but not further conversions into THBCs, being required the conjugation of the double bond that formed in the RCM step with the lactam carbonyl. Hoveyda-Grubbs catalyst HG-I (at 5 mol%, in m-xylene at reflux) gave the cleanest and highest conversion 107a (n = 1, 2)↓110a (95%; n = 1, 2). The trimethoxybenzene derivative 107c (n = 1) also underwent the tandem reaction sequence to provide the tetrahydroisoquinoline derivative 110c in good yield (64%, Scheme 31, down). By contrast, the conversion of substrate 107d or other substrates bearing a heterocycle moiety (not shown) required the successive addition of TFA (1 eq.) and further 2 h heating to give the tricycle product 110d (or the other corresponding cyclization products) in 98% yield and diastereomeric ratio > 20:1, or the other cyclization products [123]. (2012) As disclosed earlier, Nielsen and coworkers performed the metal-catalyzed isomerization of N-acyl-N-allyl tryptamines that were previously described (Scheme 32, down) [124]. Scheme 31. Ruthenium-catalyzed RCM/isomerization/N-acyliminium cyclization tandem sequence.
(2013) In a third step, ruthenium hydride RuHCl-(CO)(PPh 3 ) was found as an effective promoter of the isomerization 111β↓111α since the Wilkinson's catalyst [124] was no more efficient for the isomerization of the double bond of acylated allylamines 111. The combination of the ruthenium catalyst (10% mol) together with the chiral phosphoric acid (PhO) 2 PO 2 H (30% mol) proved to be the most efficient for the transformation of the allylic amides 111ab into THBCs 112ab, but high temperature (toluene at reflux) was needed for the completion of the reaction. Finally, the cyclic allylic amides 111cd, in optimized reaction conditions, gave the corresponding THBCs 112cd in 92% and 68% yields, respectively (Scheme 32) [125,126]. The treatment of other electron-rich aromatics, such as the cyclic allylic amides 113ab in the same conditions as before, afforded the corresponding tetrahydroisoquinoline derivatives 114ab, which were isolated in moderate yields (49-67%, Scheme 32) [125,126]. Notably, N b -benzyl substituents in L-tryptophan derivatives proved to be more important than the N b -acetyl groups for the diastereoselectivity in a substrate-controlled version of the above tandem sequence. (2013) In a third step, ruthenium hydride RuHCl-(CO)(PPh3) was found as an effective promoter of the isomerization 111β111α since the Wilkinson's catalyst [124] was no more efficient for the isomerization of the double bond of acylated allylamines 111. The combination of the ruthenium catalyst (10% mol) together with the chiral phosphoric acid (PhO)2PO2H (30% mol) proved to be the most efficient for the transformation of the allylic amides 111ab into THBCs 112ab, but high temperature (toluene at reflux) was needed for the completion of the reaction. Finally, the cyclic allylic amides 111cd, in optimized reaction conditions, gave the corresponding THBCs 112cd in 92% and 68% yields, respectively (Scheme 32) [125,126]. The treatment of other electron-rich aromatics, such as the cyclic allylic amides 113ab in the same conditions as before, afforded the corresponding tetrahydroisoquinoline derivatives 114ab, which were isolated in moderate yields (49-67%, Scheme 32) [125,126]. Notably, Nb-benzyl substituents in L-tryptophan derivatives proved to be more important than the Nb-acetyl groups for the diastereoselectivity in a substrate-controlled version of the above tandem sequence. (2012) A ruthenium alkylidene complex and chiral phosphoric acid catalyzed an enantioselective version of the RCM/isomerization/P-S cascade process (Scheme 33 up). You et al. showed that the steric bulk of the substituent adjacent to the nitrogen in the allylic system was crucial for the high enantioselectivity [125]. Subsequently (2013), the authors established the optimal conditions for the reaction of N-1-naphthylmethyl protected substrate 115b: benzene as solvent, 0.5 mol% Hoveyda-Grubbs II (HG-II) and 5 mol % SPINOL-derived phosphoric acid (R)-SPA-5 as binary catalyst, and 4Å molecular sieves as an additive. Consequently, the yield and the ee of THBCs 117 were considerably increased (Scheme 33) [126]. Toda and Terada described a similar enantioselective P-S type cyclization, catalyzed by ruthenium hydride complex and chiral phosphoric acid. Relay catalysis on protected arylethylamine 118 afforded the protected tetrahydroisoquinoline 119 (Scheme 34). The substitution pattern (R) of the aromatic ring, the bulky 9-anthranyl group of the chiral phosphoric acid catalyst, and the N-protecting group (PG) were surveyed and optimized [127].
Molecules 2020, 25, x FOR PEER REVIEW 32 of 84 Toda and Terada described a similar enantioselective P-S type cyclization, catalyzed by ruthenium hydride complex and chiral phosphoric acid. Relay catalysis on protected arylethylamine 118 afforded the protected tetrahydroisoquinoline 119 (Scheme 34). The substitution pattern (R) of the aromatic ring, the bulky 9-anthranyl group of the chiral phosphoric acid catalyst, and the N-protecting group (PG) were surveyed and optimized [127]. Scheme 34. Relay catalysis of protected arylethylamines.

All in One Pot
Bond formation via the intramolecular attack of N-acyliminium ion electrophiles by πnucleophiles is a popular method for the construction of nitrogen-containing ring systems [128][129][130][131][132][133]. When N-acyliminium ion cyclization reactions are incorporated into cascade sequences, which are powerful strategies for the one-pot production of nitrogen-containing polycyclic ring systems emerge. At the beginning of the XXI century, the usual procedure for the synthesis of organic compounds, i.e., the stepwise transformation of the individual bonds in the target molecule, was being substituted by more efficient processes, where the readily available reactants are converted in a one-pot fashion into complex molecules. By a strict definition, a cascade reaction is a process in which multiple bonds are formed in sequence without changing conditions, adding reagents, or isolating intermediates. Most of the reported cascade sequences employ a single starting material, containing functional groups that are strategically positioned along a chain ending with an alkene moiety. The reactions enable two or more bond-forming and/or -cleaving events to occur in one vessel, where subsequent operations result as the consequence of the functionalities that formed in the previous step. The definition includes the prerequisite intramolecular to distinguish this reaction type from a multi-component reaction (vide infra). The cascade approach might assume different synonyms, such as tandem, domino, or one-pot and one-flask sequence, and it is also defined by the features of the key event, assuming the nuance of nucleophilic, electrophilic, cationic, anionic, pericyclic, radical, transition-metal catalyzed, enzymatic reaction, as well as being described as Heck reaction.
The main advantages of a cascade reaction in organic synthesis are given by reduction of time, labor, waste and resources, atom economy and the cleanliness of environmental tolerable procedures. Moreover, the process does not involve the workup and isolation of many intermediates and increases in efficiency bearing much complexity in effectively one step [128][129][130][131][132][133].

All in One Pot
Bond formation via the intramolecular attack of N-acyliminium ion electrophiles by π-nucleophiles is a popular method for the construction of nitrogen-containing ring systems [128][129][130][131][132][133]. When N-acyliminium ion cyclization reactions are incorporated into cascade sequences, which are powerful strategies for the one-pot production of nitrogen-containing polycyclic ring systems emerge. At the beginning of the XXI century, the usual procedure for the synthesis of organic compounds, i.e., the stepwise transformation of the individual bonds in the target molecule, was being substituted by more efficient processes, where the readily available reactants are converted in a one-pot fashion into complex molecules. By a strict definition, a cascade reaction is a process in which multiple bonds are formed in sequence without changing conditions, adding reagents, or isolating intermediates. Most of the reported cascade sequences employ a single starting material, containing functional groups that are strategically positioned along a chain ending with an alkene moiety. The reactions enable two or more bond-forming and/or -cleaving events to occur in one vessel, where subsequent operations result as the consequence of the functionalities that formed in the previous step. The definition includes the prerequisite intramolecular to distinguish this reaction type from a multi-component reaction (vide infra). The cascade approach might assume different synonyms, such as tandem, domino, or one-pot and one-flask sequence, and it is also defined by the features of the key event, assuming the nuance of nucleophilic, electrophilic, cationic, anionic, pericyclic, radical, transition-metal catalyzed, enzymatic reaction, as well as being described as Heck reaction.
The main advantages of a cascade reaction in organic synthesis are given by reduction of time, labor, waste and resources, atom economy and the cleanliness of environmental tolerable procedures. Moreover, the process does not involve the workup and isolation of many intermediates and increases in efficiency bearing much complexity in effectively one step [128][129][130][131][132][133].

Au(I)-Catalyzed N-Acyliminium Cyclization Cascade
Historically, gold(I) complexes have emerged over the last decade as powerful tools for the synthesis of polyheterocyclic molecules. Au(I)-catalyzed reaction sequences have taken center stage due to the metal ion's ability to activate various functionalities under mild conditions and at low catalyst loading. Moreover, the gold tolerance to oxygen, moisture, and many functional groups is very high, and this makes this metal an ideal candidate for the development of tandem catalytic strategies. The chemistry of gold-catalysis has been reviewed during the years from several papers [142][143][144][145][146].

Au(I)-Catalyzed N-Acyliminium Cyclization Cascade
Historically, gold(I) complexes have emerged over the last decade as powerful tools for the synthesis of polyheterocyclic molecules. Au(I)-catalyzed reaction sequences have taken center stage due to the metal ion's ability to activate various functionalities under mild conditions and at low catalyst loading. Moreover, the gold tolerance to oxygen, moisture, and many functional groups is very high, and this makes this metal an ideal candidate for the development of tandem catalytic strategies. The chemistry of gold-catalysis has been reviewed during the years from several papers [142][143][144][145][146].
(2012) Liu and Zhang reported a gold catalysis-triggered cascade reaction, where the formamide 124a (indole, R = H) gave, in presence of iPrAuNTf 2 or BrettPhosAuNTf 2 (5 mol %, as catalyst) and TFA (as acid additive) in the optimized reaction conditions, the indole-fused hexahydroquinolizin-2-one 127a (indole as aromatic moiety, R 1 = H; 82% yield) after 24 h via the THBC intermediate 126a (Scheme 36). Electron-rich aromatic rings, such as methoxy-and methylenedioxy-benzene yielded instead benzene-fused hexahydroquinolizin-2-ones 127b (benzene as aromatic moiety, R = OMe, OCH 2 O) in synthetically serviceable yields. The new method was applied for the succinct and stereoselective synthesis of dihydrocorynantheol and a formal synthesis of yohimbine ( Figure 3) and β-yohimbine (β-17-OH) [147]. (2013) The Dixon group developed a high enantioselective N-sulfonyliminium cyclization cascade, which provided complex and unusual sulfonamide scaffolds in excellent yield [148]. Treatment with Echavarren catalyst (EC-1, 10 mol% [149,150]) of a mixture of sulfonamide 128a (R = H) and BPA-1 (10 mol%) in toluene at 60 °C afforded the cascade product 129a (R = H) in 84% yield and 88% ee (Scheme 37). The choice of sulfonamide over carboxylic acid amide was influenced by their abundance in medicinally relevant compounds and the lack of sulfonamide scaffolds via cyclization cascade. The versatility of the new methodology was verified on the chain extended amide analogue 130, which was given by reaction cascades with EC-1 (0.5-1 mol%) and (R)-BPA-1 (10 mol%) in toluene at reflux the desired δ-lactams 131 in 86% yield and 66% ee (Scheme 38) [148]. (2013) The Dixon group developed a high enantioselective N-sulfonyliminium cyclization cascade, which provided complex and unusual sulfonamide scaffolds in excellent yield [148]. Treatment with Echavarren catalyst (EC-1, 10 mol% [149,150]) of a mixture of sulfonamide 128a (R = H) and BPA-1 (10 mol%) in toluene at 60 • C afforded the cascade product 129a (R = H) in 84% yield and 88% ee (Scheme 37). The choice of sulfonamide over carboxylic acid amide was influenced by their abundance in medicinally relevant compounds and the lack of sulfonamide scaffolds via cyclization cascade. The versatility of the new methodology was verified on the chain extended amide analogue 130, which was given by reaction cascades with EC-1 (0.5-1 mol%) and (R)-BPA-1 (10 mol%) in toluene at reflux the desired δ-lactams 131 in 86% yield and 66% ee (Scheme 38) [148]. (2013) The Dixon group developed a high enantioselective N-sulfonyliminium cyclization cascade, which provided complex and unusual sulfonamide scaffolds in excellent yield [148]. Treatment with Echavarren catalyst (EC-1, 10 mol% [149,150]) of a mixture of sulfonamide 128a (R = H) and BPA-1 (10 mol%) in toluene at 60 °C afforded the cascade product 129a (R = H) in 84% yield and 88% ee (Scheme 37). The choice of sulfonamide over carboxylic acid amide was influenced by their abundance in medicinally relevant compounds and the lack of sulfonamide scaffolds via cyclization cascade. The versatility of the new methodology was verified on the chain extended amide analogue 130, which was given by reaction cascades with EC-1 (0.5-1 mol%) and (R)-BPA-1 (10 mol%) in toluene at reflux the desired δ-lactams 131 in 86% yield and 66% ee (Scheme 38) [148].  The treatment of substituted tryptamines 1 and 2-ethynylbenzoic acids 132a (n = 0) or 2-ethynylphenyl acetic acids 132b (n = 1) by an efficient, facile gold(I)-catalyzed one-pot cascade protocol featured the formation of polycyclic privileged structures 133 with high yield and broad substrate tolerance (Scheme 39). Selected target molecules 134, which were obtained after reduction, were validated as α 1 -adrenergic receptor antagonists [151]. The treatment of substituted tryptamines 1 and 2-ethynylbenzoic acids 132a (n = 0) or 2ethynylphenyl acetic acids 132b (n = 1) by an efficient, facile gold(I)-catalyzed one-pot cascade protocol featured the formation of polycyclic privileged structures 133 with high yield and broad substrate tolerance (Scheme 39). Selected target molecules 134, which were obtained after reduction, were validated as α1-adrenergic receptor antagonists [151]. A P-S-type reaction was invoked for the final aromatic 1,7-cyclization step in the proposed mechanisms of gold(I)-catalyzed [5 + 2] cycloaddition of propargyl esters or acetals with imines (not shown), leading to benzo-fused azepine derivatives [152].
(2015) Waldmann and coworkers proposed a reaction sequence, where, in a first step acetylenic aldehydes 135 and tryptamine, yielded in a P-S reaction THBCs endowed with an alkyne substituent, which could be given via a hydroamination reaction the indoloquinolizine (IQZ) scaffold. Actually, microwave heating (120 °C) of a mixture of tryptamine (1a, R = H) and o-2phenylethynyl benzaldehyde (135a, R1 = Ph) in DCM with 10 mol% of Yb(OTf)3 after the addition of the ionic liquid [bmim]Cl-AlCl3 provided the THBC 136 (R = H, R1 = Ph; 74% yield) within one hour (Scheme 40) [153]. However, the second step, i.e., the hydroamination reaction of the P-S adduct, did not occur in the presence of ytterbium complexes [154,155]. By contrast, after a survey of selected gold complexes, the catalyst EC-1 (10 mol%) (Scheme 37) gave, at room temperature, the desired IQZ 137a (R = H, R1 = Ph; 62%). The authors explored the utility of the two-step protocol for the synthesis of hexacyclic indoloquinolizines, where a spirooxindole ring system is fused to a THBC moiety. Tryptamines 1 and isatins 138 afforded the hexacyclic heterocycle 139 (Scheme 41) [153]. A P-S-type reaction was invoked for the final aromatic 1,7-cyclization step in the proposed mechanisms of gold(I)-catalyzed [5 + 2] cycloaddition of propargyl esters or acetals with imines (not shown), leading to benzo-fused azepine derivatives [152].
(2015) Waldmann and coworkers proposed a reaction sequence, where, in a first step acetylenic aldehydes 135 and tryptamine, yielded in a P-S reaction THBCs endowed with an alkyne substituent, which could be given via a hydroamination reaction the indoloquinolizine (IQZ) scaffold. Actually, microwave heating (120 • C) of a mixture of tryptamine (1a, R = H) and o-2-phenylethynyl benzaldehyde (135a, R 1 = Ph) in DCM with 10 mol% of Yb(OTf) 3 after the addition of the ionic liquid [bmim]Cl-AlCl 3 provided the THBC 136 (R = H, R 1 = Ph; 74% yield) within one hour (Scheme 40) [153]. However, the second step, i.e., the hydroamination reaction of the P-S adduct, did not occur in the presence of ytterbium complexes [154,155]. By contrast, after a survey of selected gold complexes, the catalyst EC-1 (10 mol%) (Scheme 37) gave, at room temperature, the desired IQZ 137a (R = H, R 1 = Ph; 62%). The authors explored the utility of the two-step protocol for the synthesis of hexacyclic indoloquinolizines, where a spirooxindole ring system is fused to a THBC moiety. Tryptamines 1 and isatins 138 afforded the hexacyclic heterocycle 139 (Scheme 41) [153]. Guinchard and coworkers used allenals 140 as bifunctional key building blocks for the synthesis of polycyclic chiral architectures 141 (Scheme 42) in a reaction that combines an asymmetric phosphoric acid catalyzed P-S reaction and a self-relay palladium catalyzed cyclization [56,57]. Guinchard and coworkers used allenals 140 as bifunctional key building blocks for the synthesis of polycyclic chiral architectures 141 (Scheme 42) in a reaction that combines an asymmetric phosphoric acid catalyzed P-S reaction and a self-relay palladium catalyzed cyclization [56,57]. Scheme 41. Two-step protocol for the synthesis of hexacyclic indoloquinolizines containing spirooxindole ring-system. Guinchard and coworkers used allenals 140 as bifunctional key building blocks for the synthesis of polycyclic chiral architectures 141 (Scheme 42) in a reaction that combines an asymmetric phosphoric acid catalyzed P-S reaction and a self-relay palladium catalyzed cyclization [56,57].
Inspired by Waldmann and Kumar [153], ending in pentacyclic derivatives 143 (Scheme 42) [158], the authors proposed a gold-catalyzed cascade, where the P-S reaction between N-allyl tryptamines 1h and O-alkynyl arylaldehydes 142 is followed by cyclization with a concomitant allyl transfer to give compounds 143 in good yield (Scheme 42). EC-1 (Scheme 37) was the sole catalyst for both of the reactions. Further optimization came from the use of the stable cationic catalyst [(Ph 3 P)Au(NTf 2 )] (EC-2), as well as molecular sieves (4Å) [158] The combination of a base-catalyzed intermolecular Michael addition reaction-featured by an α,β-unsaturated carbonyl compound and a suitable amide pronucleophile-with an acid-catalyzed intra-molecular N-acyliminium ion P-S cyclization of the resulting adduct, was a quite popular cascade sequence for building complex multiring heterocyclic molecules in one-pot and under mild conditions. The β-ketoester 144, cinnamic aldehyde 145a, the Michael adduct 146, and the final indoloquinolizidine product 148 feature the sequence (Scheme 43) [159]. Inspired by Waldmann and Kumar [153], ending in pentacyclic derivatives 143 (Scheme 42) [158], the authors proposed a gold-catalyzed cascade, where the P-S reaction between N-allyl tryptamines 1h and O-alkynyl arylaldehydes 142 is followed by cyclization with a concomitant allyl transfer to give compounds 143 in good yield (Scheme 42). EC-1 (Scheme 37) was the sole catalyst for both of the reactions. Further optimization came from the use of the stable cationic catalyst [(Ph3P)Au(NTf2)] (EC-2), as well as molecular sieves (4Å ) [158].
Although many examples of cascade sequences that were catalyzed by a single chemical entity have been reported [134,136], which involve more than one mutually compatible catalyst are much less common [168,169]. With the aim of overcoming the problem of annihilation (catalyst quenching), due to the simultaneous use of both strongly basic and strongly acidic reagents, the Dixon group employed site isolated base and acid (SIBA) catalysis [170,171]. The group developed cascade reactions that incolve the polymer supported 2-tert-butylimino-2-diethyl-amino-1, The assembly by Zhu et al. of medicinally important butyrolactam-fused indoloquinolizidines in a highly stereo-controlled organocatalytic one-pot Michael/P-S sequence has been previously described [66,167].
Although many examples of cascade sequences that were catalyzed by a single chemical entity have been reported [134,136], which involve more than one mutually compatible catalyst are much less common [168,169]. With the aim of overcoming the problem of annihilation (catalyst quenching), due to the simultaneous use of both strongly basic and strongly acidic reagents, the Dixon group employed site isolated base and acid (SIBA) catalysis [170,171]. The group developed cascade reactions that incolve the polymer supported 2-tert-butylimino-2-diethyl-amino-1,3-dimethyl-perhydro-1,3,2-diaza-phosphorine (PS-BEMP) and (R)-BPA or bulky derivatives, such as (R)-BPA-1 and (R)-[H 8 ]-BPA-1 [172]. The authors hypothesized that the system would provide the necessary site isolation of mutually destructive acidic and basic functional groups and a size exclusion (molecular sieving) phenomenon would operate between PS-BEMP and (R)-BPA-1, but not between the first one and diphenyl phosphate (DPP). The model reaction of pro-nucleophile malonamate (156a, R = H) with methyl vinyl ketone (MVK, 155a, R 1 = Me) required the following optimal conditions: PS-BEMP (10 mol%), (R)-BPA-1 (10 mol%), the addition of MVK (3 eq) at room temperature for 24 h, N-acyliminium cyclization at reflux in toluene for 24 h. The product 157 (R 1 = H) was obtained in 81% yield and 57% ee (Scheme 46). The teatment of a set of malonamate nucleophiles 156 with methyl vinyl ketone (MVK, 155a, R 1 = Me) and ethyl vinyl ketone (EVK, 155b, R 1 = Et) in the presence of PS-BEMP and (R)-[H 8 ]-BPA-1 investigated the scope of the reaction. In general, yields that range from 73 to 80% and ee from 56 to 76% were achieved [172].
Tryptamine-derived urea 162a (R 1 = R 2 = H) and methyl vinyl ketone 155a (MVK, R 3 = Me, R 4 = H) produced the best results in toluene with BINOL phosphoric acid when the heating was increased to 110 • C. Catalyst screening revealed that optimal enantiocontrol was associated with (R)-BPA-1 and/or (R)-[H 8 ]-BPA-1 (both at 10 mol%) and optimal concentrations of substrates 162 were 5 mM (162) in the presence of 5 eq of MVK 155a. Under these conditions, compound 165a (R = R 1 = R 2 = H, R 3 = Me) was obtained in 76% yield and 73% ee, through the intermediates 163 (Michael addition to the distal nitrogen) and 164 (condensation of the ketone with tryptamine N b ) (Scheme 48). The scope of the reaction was surveyed while using an array of substituted ureas 162 and enones 155 [174].
(2014) The first catalytic asymmetric construction of a new class of bispirooxindole scaffold that incorporates a THBC moiety was established via a BPA-catalyzed three-component Michael/P-S cascade sequence, which afforded the structurally complex and diverse target compounds in excellent stereoselectivities (dr > 95:5; e.r. up to 98:2) [179]. N-benzylisatin 177, N-methylisatin-derived 3-indolylmethanol 178, and diethyl-2-aminomalonate 179 in CHCl 3 at 45 • C in the presence of (S)-[H 8 ]-BPA-3 (with bulky 9-phenanthrenyl groups at 3,3 -positions; Scheme 33) afforded the desired bispirooxindoles 180 with good yield (72%) and stereoselectivity (ee 82%) (Scheme 51). 1,1,2,2-tetrachloroethane (TCE) and CHCl 3 were alternated as needed, while changing the molecular sieves (MS) from 3 to 4 Å greatly improved the yield, but decreased stereoselectivity. Conversely, lowering the temperature to 25 • C provided the highest stereoselectivity (ee 92%) with a relatively high yield (61%). The incorporated THBC moiety and bispirooxindole framework are both core structures of pharmaceutically important compounds [179]. alkyl propiolates 172, and 3-phenacylidene-oxindoles 175 in the presence of anhydrous ZnCl2 provided, instead of the expected compound 176a, the functionalized 2-pyrrolo-3′-yloxindoles 176b, which, in turn, can be converted into the corresponding 6,11-dihydro-5H-indolizino [8,7-b]indoles 176c via a CF3SO3H catalyzed P-S cyclization process (Scheme 50, down). By contrast, when arylamines replaced tryptamines 1, the one-pot domino reaction only afforded the corresponding 2pyrrolo-3′-yloxindoles (not shown) [178].  1,1,2,2-tetrachloroethane (TCE) and CHCl3 were alternated as needed, while changing the molecular sieves (MS) from 3 to 4 Å greatly improved the yield, but decreased stereoselectivity. Conversely, lowering the temperature to 25 °C provided the highest stereoselectivity (ee 92%) with a relatively high yield (61%). The incorporated THBC moiety and bispirooxindole framework are both core structures of pharmaceutically important compounds [179].     (2013) Singlet oxygen transforms simple furan substrates in the presence of arylethylamines into complex nitrogen-bearing aromatic poly-cycles with the structural features of important natural products, such as Erythrina alkaloids. The combination of monosubstituted furan substrates 189 with different arylethylamines 1 in TFA/DCM at room temperature triggers a cascade reaction sequence to generate in a novel way an N-acyliminium ion 191 precursor of tricyclic compounds 192 (Scheme 55) [183]. The method has been used to achieve a rapid and highly effective formal synthesis of erysotramidine, a dienoid-type member of Erythrina alkaloids, by a sequence (see Scheme 56) starting with singlet oxygen photoaddition to the furan 189a and terminating with P-S-type aromatic substitution to give the tetracycle 192a [184]. Hexacyclic indole alkaloids with a THBC motif were obtained by a stereoselective synthesis that is based on sequential P-S cyclization, N-acylation, and intramolecular Diels-Alder reactions. The synthetic route started with the installation of a diene motif in the THBC skeleton by a P-S reaction between 5-substituted furan 2-carbaldehydes 193 (R1 = Me, Et, Br) and L-tryptophan The method has been used to achieve a rapid and highly effective formal synthesis of erysotramidine, a dienoid-type member of Erythrina alkaloids, by a sequence (see Scheme 56) starting with singlet oxygen photoaddition to the furan 189a and terminating with P-S-type aromatic substitution to give the tetracycle 192a [184].
(2013) Singlet oxygen transforms simple furan substrates in the presence of arylethylamines into complex nitrogen-bearing aromatic poly-cycles with the structural features of important natural products, such as Erythrina alkaloids. The combination of monosubstituted furan substrates 189 with different arylethylamines 1 in TFA/DCM at room temperature triggers a cascade reaction sequence to generate in a novel way an N-acyliminium ion 191 precursor of tricyclic compounds 192 (Scheme 55) [183]. The method has been used to achieve a rapid and highly effective formal synthesis of erysotramidine, a dienoid-type member of Erythrina alkaloids, by a sequence (see Scheme 56) starting with singlet oxygen photoaddition to the furan 189a and terminating with P-S-type aromatic substitution to give the tetracycle 192a [184]. Hexacyclic indole alkaloids with a THBC motif were obtained by a stereoselective synthesis that is based on sequential P-S cyclization, N-acylation, and intramolecular Diels-Alder reactions. The synthetic route started with the installation of a diene motif in the THBC skeleton by a P-S reaction between 5-substituted furan 2-carbaldehydes 193 (R1 = Me, Et, Br) and L-tryptophan Hexacyclic indole alkaloids with a THBC motif were obtained by a stereoselective synthesis that is based on sequential P-S cyclization, N-acylation, and intramolecular Diels-Alder reactions. The synthetic route started with the installation of a diene motif in the THBC skeleton by a P-S reaction between 5-substituted furan 2-carbaldehydes 193 (R 1 = Me, Et, Br) and L-tryptophan methyl ester. The resulting product 194ab (as a mixture of cis/trans diastereomers) that was treated with acryloyl chlorides (DCM, Et 3 N at rt) afforded the α,β-unsaturated amides 195ab, which spontaneously underwent [4 + 2] Diels-Alder cycloaddition to give the bridged indole alkaloids 196b in the exo form, while the cis-isomer 195a remained unreacted (Scheme 57) [185].
(2014) The treatment of a novel 1,2-dinucleophile (bis-silyldienediolate 197) with two p-methoxyphenyl (PMP) imines (198a and 198b) in a sequential Mannich/Mannich/P-S tandem process provided complex hexahydropyrrolo[3,2-c]quinolines (Scheme 58). The α-keto ester 200, which were obtained after a vinylogous Mannich addition (with Yb(OTf) 3 in dry MeCN or DME) and a second Brønsted acid-catalyzed Mannich reaction, spontaneously cyclizes within 10 min. into the pyrroloquinolines 202 (as a mixture of diastereomers, optimized by screening of the solvents and acids) (Scheme 58). Notably, being the second step triggered by the addition of the Brønsted acid, two different imines 198a and 198b can be employed, which substantially broadens the scope of the transformation [186].
The tandem P-S/lactamization of L-phenylalanine methyl ester with methyl levulinate (206) produced the tricyclic pyrroloisoquinoline motif 207, which was utilized as a platform containing the pluripotent reaction site for the diversity-oriented synthesis (DOS) of a library of hybrid systems. The structural diversity was generated through the introduction on the main motif of Scheme 59. Diastereoselective synthesis of constrained 7,5 and 7,6 fused azabicycloalkanes.
The tandem P-S/lactamization of L-phenylalanine methyl ester with methyl levulinate (206) produced the tricyclic pyrroloisoquinoline motif 207, which was utilized as a platform containing the pluripotent reaction site for the diversity-oriented synthesis (DOS) of a library of hybrid systems. The structural diversity was generated through the introduction on the main motif of spiro-connected privileged scaffolds, such as oxoindole (a), quinolone (b), cyclopent-ane/ene (c/d), and pyrrolidinone (e) (Scheme 60) [188].

The Ring-a Ring of Multi-Component Reactions
Awake-awake! Now come and make A ring-a ring of roses (A.S. Stephens) Scheme 60. Synthesis of pyrroloisoquinoline scaffold as a platform for diversity-oriented synthesis (DOS) strategy.

The Ring-a Ring of Multi-Component Reactions
Awake-awake! Now come and make A ring-a ring of roses In the continuous search for an efficient synthesis of the basic skeleton of biologically active THIQs, THBCs, or polycyclic alkaloid systems embodying them, the technique of multi-component reaction (MCR) gained increasing attention from the chemists as an alternative to sequential multistep synthesis [189][190][191]. MCR chemistry enables the rapid construction of complex and diverse structures from readily accessible starting materials in a single operation under mild conditions. MCR is defined as a reaction, in which three or more starting materials combine in a single event to form a single product that contains features of all of the inputs, with the exception of condensation products, such as H 2 O, HCl, or MeOH [192]. In practical terms, the order of addition of the individual components does not matter, since multiple elements of diversity are being introduced in a single operation, regardless of the sequence in which they are added. MCRs mostly involve a number of equilibrium subreactions, which, like in a spiral frame culminate in the final step, an irreversible process, such as C-C bond formation or a rearrangement [193,194]. Several descriptive tags are regularly attached to MCRs: atom economic, the majority of the atoms of the reactants being incorporated in the products; efficient, the final product being formed in one-step instead of multiple sequential steps; convergent, several reactants combining in one reaction; endowed with very high bond-forming index (BFI), many non-hydrogen atom bonds being formed in one synthetic preparation [190,195]. Multicomponent reactions, in which all of the substrates are added at once, can also be considered to be domino processes; alternatively, MCRs can be performed by the sequential addition of reactants, without the isolation of intermediate species or a change of solvent. After the emergence of combinatorial chemistry and diversity-oriented synthesis, today MCRs play a central role in the development of modern synthetic methodology for pharmaceutical and drug discovery research [196,197].
Ugi fully recognized the huge potential of the MCR methodology, who postulated that the reaction was ideally suited to probe structure-activity relationships via the synthesis of "collections of compounds", nowadays called libraries. Since its original publication in 1959 [198], the Ugi reaction has emerged as the most well known and widely used MCR in organic synthesis. The classical Ugi four-component reaction (U-4CR) allows for simultaneous variation of four very common starting materials (educts): carbonyl compound (I), amine (II), isocyanide derivative (III) as a special guest and carboxylic acid (IV). Isocyanides (formerly known as isonitriles) are the only class of stable organic compounds with a formally bivalent carbon and a functional group fundamentally different from the others. The most synthetically important property of isocyanides is the α-addition of nucleophiles and electrophiles at the carbon atom. Scheme 61 shows a very simplified reaction mechanism with carboxylic acid as the acid component.  [199][200][201][202][203].

Ugi Meets Pictet-Spengler
In earlier two-component (2C) intramolecular variations of the P-S condensation, THIQ and THBC scaffolds were prepared by the reaction of a solid-phase bound amine with a carbonyl derivative, prior to the cyclization of the incipient iminium species [204][205][206]. Successively, Wang and Ganesan added the N-acyliminium P-S reaction to the repertoire of multicomponent reactions, adapting a previous total synthesis of demethoxyfumitremorgin C (215a) [207] to the solid phase: Fmoc-protected L-tryptophan 212, which was immobilized on polystyrene-Wang resin, was treated with senecialdehyde (R = -CH = CMe2) and trimethyl orthoformate to give the imine 213a. The addition of Fmoc-L-proline chloride gave rise to N-acyliminium P-S cyclization to 214, which, in turn, treated with piperidine undergoes deprotection, diketopiperazine ring closure, and concomitant resin cleavage. The natural product 215a was obtained, together with its trans epimer (Scheme 62). Analogous compounds 215 were also prepared varying the aldehyde and/or replacing the proline unit by other amino acids [208]. On the other hand, Bonnet and Ganesan obtained THBC hydantoins (Scheme 62) with a similar cis/trans ratio by a slightly modified strategy [110]. Finally, van Loeveijin et al. reported a similar approach for the construction of a 42-membered library of fumitremorgin-type indolyl-diketopiperazines (215), such as verruculogen (Scheme 62) [209].

Ugi Meets Pictet-Spengler
In earlier two-component (2C) intramolecular variations of the P-S condensation, THIQ and THBC scaffolds were prepared by the reaction of a solid-phase bound amine with a carbonyl derivative, prior to the cyclization of the incipient iminium species [204][205][206]. Successively, Wang and Ganesan added the N-acyliminium P-S reaction to the repertoire of multicomponent reactions, adapting a previous total synthesis of demethoxyfumitremorgin C (215a) [207] to the solid phase: Fmoc-protected L-tryptophan 212, which was immobilized on polystyrene-Wang resin, was treated with senecialdehyde (R = -CH = CMe 2 ) and trimethyl orthoformate to give the imine 213a. The addition of Fmoc-L-proline chloride gave rise to N-acyliminium P-S cyclization to 214, which, in turn, treated with piperidine undergoes deprotection, diketopiperazine ring closure, and concomitant resin cleavage. The natural product 215a was obtained, together with its trans epimer (Scheme 62). Analogous compounds 215 were also prepared varying the aldehyde and/or replacing the proline unit by other amino acids [208]. On the other hand, Bonnet and Ganesan obtained THBC hydantoins (Scheme 62) with a similar cis/trans ratio by a slightly modified strategy [110]. Finally, van Loeveijin et al. reported a similar approach for the construction of a 42-membered library of fumitremorgin-type indolyl-diketopiperazines (215), such as verruculogen (Scheme 62) [209].

Scheme 62. Indolyl diketopiperazines (215a-d) and THBC hydantoins synthesized by solid phase multicomponent (MCR)/P-S approach.
The P-S-adducts finally appeared at the edge of the ring-a-ring in the framework of a substantial body of work devoted to post condensation modifications of Ugi-4CR educts 211. Dömling and Ugi first reported the combination of Ugi and P-S reactions and included the amino acid tryptophan, phthalic aldehyde 216, and t-butyl isocyanide 217 as substrates of the synthesis of complex polycyclic products containing the THIQ and THBC motifs. The non-isolable Ugi product afforded the pentacyclic compound 218 after spontaneous P-S cyclization and the following oxidation, while other aromatic amino acids, such as phenylalanine (R = H), tyrosine (R = 4-OH), and DOPA (R = 3,4-di-OH), produced tricyclic compounds, such as 219 (Scheme 63) [199]. The P-S-adducts finally appeared at the edge of the ring-a-ring in the framework of a substantial body of work devoted to post condensation modifications of Ugi-4CR educts 211. Dömling and Ugi first reported the combination of Ugi and P-S reactions and included the amino acid tryptophan, phthalic aldehyde 216, and t-butyl isocyanide 217 as substrates of the synthesis of complex polycyclic products containing the THIQ and THBC motifs. The non-isolable Ugi product afforded the pentacyclic compound 218 after spontaneous P-S cyclization and the following oxidation, while other aromatic amino acids, such as phenylalanine (R = H), tyrosine (R = 4-OH), and DOPA (R = 3,4-di-OH), produced tricyclic compounds, such as 219 (Scheme 63) [199]. The P-S-adducts finally appeared at the edge of the ring-a-ring in the framework of a substantial body of work devoted to post condensation modifications of Ugi-4CR educts 211. Dömling and Ugi first reported the combination of Ugi and P-S reactions and included the amino acid tryptophan, phthalic aldehyde 216, and t-butyl isocyanide 217 as substrates of the synthesis of complex polycyclic products containing the THIQ and THBC motifs. The non-isolable Ugi product afforded the pentacyclic compound 218 after spontaneous P-S cyclization and the following oxidation, while other aromatic amino acids, such as phenylalanine (R = H), tyrosine (R = 4-OH), and DOPA (R = 3,4-di-OH), produced tricyclic compounds, such as 219 (Scheme 63) [199]. Dömling and coworkers reported an efficient and flexible 2-step procedure for the synthesis of complex multicyclic indole alkaloid-type compounds 227, which feature Ugi MCR and P-S reactions (Scheme 65). The final product is a highly complex molecule that contains six cycles in total, four heterocycles (pyrrole, piperazinone, hydropyridine, γ-lactam) and two carbocycles (benzene, cyclopentane) [212]. Other notable employments of Ugi/P-S sequence prior to 2011 can be found in the combination of tandem MAO desymmetrization/MCR/P-S cyclization for the asymmetric synthesis of alkaloidlike polycyclic compounds [213,214], as well as for the synthesis of cyanocycline A and bioxalomycin β2 [215]. The mature state and the use in the synthesis of biologically active compounds of this large group of reactions are reflected in the numerous preclinical and development drugs, for instance, almorexant (first-in-class orexin I antagonist) and retosiban (oxytocin receptor antagonist) [216,217]. Dömling and coworkers reported an efficient and flexible 2-step procedure for the synthesis of complex multicyclic indole alkaloid-type compounds 227, which feature Ugi MCR and P-S reactions (Scheme 65). The final product is a highly complex molecule that contains six cycles in total, four heterocycles (pyrrole, piperazinone, hydropyridine, γ-lactam) and two carbocycles (benzene, cyclopentane) [212]. Dömling and coworkers reported an efficient and flexible 2-step procedure for the synthesis of complex multicyclic indole alkaloid-type compounds 227, which feature Ugi MCR and P-S reactions (Scheme 65). The final product is a highly complex molecule that contains six cycles in total, four heterocycles (pyrrole, piperazinone, hydropyridine, γ-lactam) and two carbocycles (benzene, cyclopentane) [212]. Other notable employments of Ugi/P-S sequence prior to 2011 can be found in the combination of tandem MAO desymmetrization/MCR/P-S cyclization for the asymmetric synthesis of alkaloidlike polycyclic compounds [213,214], as well as for the synthesis of cyanocycline A and bioxalomycin β2 [215]. The mature state and the use in the synthesis of biologically active compounds of this large group of reactions are reflected in the numerous preclinical and development drugs, for instance, almorexant (first-in-class orexin I antagonist) and retosiban (oxytocin receptor antagonist) [216,217].

Update of Ugi/Pictet-Spengler Combinations
(2011) Dömling and coworkers elaborated on their previous Ugi-3CR/P-S reaction sequence (Scheme 65 [212]), the synthesis of a small focused library of polycyclic ring systems, based on phenylethylamine-derived isocyanides 224. All the suitable bifunctional oxocarboxylic acids 228 reacted satisfactorily with aminoaldehyde dimethyl acetal and the aryl ethyl isocyanides 224 to afford Ugi-adducts 229 with yields that range from 38 to 62% (Scheme 66). P-S reaction of Other notable employments of Ugi/P-S sequence prior to 2011 can be found in the combination of tandem MAO desymmetrization/MCR/P-S cyclization for the asymmetric synthesis of alkaloid-like polycyclic compounds [213,214], as well as for the synthesis of cyanocycline A and bioxalomycin β2 [215]. The mature state and the use in the synthesis of biologically active compounds of this large group of reactions are reflected in the numerous preclinical and development drugs, for instance, almorexant (first-in-class orexin I antagonist) and retosiban (oxytocin receptor antagonist) [216,217].

Update of Ugi/Pictet-Spengler Combinations
(2011) Dömling and coworkers elaborated on their previous Ugi-3CR/P-S reaction sequence (Scheme 65 [212]), the synthesis of a small focused library of polycyclic ring systems, based on phenylethylamine-derived isocyanides 224. All the suitable bifunctional oxocarboxylic acids 228 reacted satisfactorily with aminoaldehyde dimethyl acetal and the aryl ethyl isocyanides 224 to afford Ugi-adducts 229 with yields that range from 38 to 62% (Scheme 66). P-S reaction of intermediates 229 afforded polycyclic scaffolds 230, in the presence of formic acid at RT for the more reactive 229c and with methanesulfonic acid at 70 • C for the less reactive 229a and 229b, which also required longer reaction times [218].
Molecules 2020, 25, x FOR PEER REVIEW 51 of 84 intermediates 229 afforded polycyclic scaffolds 230, in the presence of formic acid at RT for the more reactive 229c and with methanesulfonic acid at 70 °C for the less reactive 229a and 229b, which also required longer reaction times [218].

Scheme 66.
Ugi-3CR/P-S ring-closure reaction sequence for the synthesis of tetra-and pentacyclic structures.
A sequential Ugi/P-S/reductive methylation reaction was used for the synthesis of the piperazinohydroisoquinoline ring system, starting from aminoacetaldehyde dimethyl acetal, (−)-(S)-N-Boc-3-(3,4-dimethoxyphenyl) alanine 231, and t-butyl isocyanide 217, in combination with several aldehydes 30 (Scheme 67). The Ugi-4CR/P-S-2CR reaction was completed in a straightforward manner within 1-2 h when the reaction was carried out at 50 °C while using microwave irradiation. The one-pot transformation of the Ugi-adducts 232 into the desired piperazinohydrosoquinolines 234 via iminium intermediates 233 was performed with comparable yields, simply by evaporating the solvent from the reaction flask after the completion of the Ugi reaction and then adding the reactive mixture CH2O/HCOOH [219]. Scheme 67. Synthesis of the piperazinohydroisoquinoline system while using a sequential Ugi/P-S/reductive methylation reaction protocol.
The Lesma group carried out the synthesis of a novel Phe-Ala dipeptidomimetic 235 (Figure 4), built up on a diazaspiro-cyclic lactam core. Molecular modeling, IR, NMR, and X-ray diffraction experiments agree on the presence of a strong intramolecular hydrogen bond supporting the ability of this spiro compound to act as type II′ β-turn inducer [220].

Scheme 66.
Ugi-3CR/P-S ring-closure reaction sequence for the synthesis of tetra-and pentacyclic structures.
A sequential Ugi/P-S/reductive methylation reaction was used for the synthesis of the piperazinohydroisoquinoline ring system, starting from aminoacetaldehyde dimethyl acetal, (−)-(S)-N-Boc-3-(3,4-dimethoxyphenyl) alanine 231, and t-butyl isocyanide 217, in combination with several aldehydes 30 (Scheme 67). The Ugi-4CR/P-S-2CR reaction was completed in a straightforward manner within 1-2 h when the reaction was carried out at 50 • C while using microwave irradiation. The one-pot transformation of the Ugi-adducts 232 into the desired piperazinohydrosoquinolines 234 via iminium intermediates 233 was performed with comparable yields, simply by evaporating the solvent from the reaction flask after the completion of the Ugi reaction and then adding the reactive mixture CH 2 O/HCOOH [219]. intermediates 229 afforded polycyclic scaffolds 230, in the presence of formic acid at RT for the more reactive 229c and with methanesulfonic acid at 70 °C for the less reactive 229a and 229b, which also required longer reaction times [218].

Scheme 66.
Ugi-3CR/P-S ring-closure reaction sequence for the synthesis of tetra-and pentacyclic structures.
A sequential Ugi/P-S/reductive methylation reaction was used for the synthesis of the piperazinohydroisoquinoline ring system, starting from aminoacetaldehyde dimethyl acetal, (−)-(S)-N-Boc-3-(3,4-dimethoxyphenyl) alanine 231, and t-butyl isocyanide 217, in combination with several aldehydes 30 (Scheme 67). The Ugi-4CR/P-S-2CR reaction was completed in a straightforward manner within 1-2 h when the reaction was carried out at 50 °C while using microwave irradiation. The one-pot transformation of the Ugi-adducts 232 into the desired piperazinohydrosoquinolines 234 via iminium intermediates 233 was performed with comparable yields, simply by evaporating the solvent from the reaction flask after the completion of the Ugi reaction and then adding the reactive mixture CH2O/HCOOH [219]. Scheme 67. Synthesis of the piperazinohydroisoquinoline system while using a sequential Ugi/P-S/reductive methylation reaction protocol.
The Lesma group carried out the synthesis of a novel Phe-Ala dipeptidomimetic 235 (Figure 4), built up on a diazaspiro-cyclic lactam core. Molecular modeling, IR, NMR, and X-ray diffraction experiments agree on the presence of a strong intramolecular hydrogen bond supporting the ability of this spiro compound to act as type II′ β-turn inducer [220]. Scheme 67. Synthesis of the piperazinohydroisoquinoline system while using a sequential Ugi/P-S/reductive methylation reaction protocol.
The Lesma group carried out the synthesis of a novel Phe-Ala dipeptidomimetic 235 (Figure 4), built up on a diazaspiro-cyclic lactam core. Molecular modeling, IR, NMR, and X-ray diffraction experiments agree on the presence of a strong intramolecular hydrogen bond supporting the ability of this spiro compound to act as type II β-turn inducer [220]. (2012) In the framework of a program focused on the identification of new peptidomimetic of potential interest in drug discovery, Lesma and coworkers reported a two-step efficient route for the synthesis of THBC-based compounds as privileged molecular targets in the design of potential reverse turn mimics [221]. The authors applied the Ugi/P-S sequence for a rapid assembly of peptidomimetic scaffolds of type 236. NMR and molecular modeling on the corresponding methyl carboxamide N-acetyl derivatives both confirmed a β-turn like conformation for the cis-isomer 236a and γ-turn for the trans-isomers 236b-c (Figure 4) [222].
Praziquantel (PZQ) is the only effective drug for the treatment of schistosomiasis, a high volume neglected tropical disease that affects more than 200 million people worldwide. Liu et al. developed a convergent and versatile synthetic method to prepare easily accessible and highly diversified PZQ derivatives for extensive structure-activity relationship studies. The approach includes Ugi-4CR, followed by a P-S ring closure in a sequential one-pot, two-step procedure (Scheme 68). Even though the products were found to be slightly less active than the mother drug PZQ, the Ugi/P-S reaction sequence remains the shortest and scalable approach towards a future bioactivity-guided optimization of PZQ analogous [223]. Scheme 68. General approach Ugi-4CR/P-S for the synthesis of praziquantel analogous.
A novel stereoselective Ugi-type reaction of the four highly variable starting materials α-amino acid (e.g., leucine), oxo component I (e.g., 2-fluorobenzaldehyde), isocyanide 224 (e.g., benzyl (2012) In the framework of a program focused on the identification of new peptidomimetic of potential interest in drug discovery, Lesma and coworkers reported a two-step efficient route for the synthesis of THBC-based compounds as privileged molecular targets in the design of potential reverse turn mimics [221]. The authors applied the Ugi/P-S sequence for a rapid assembly of peptidomimetic scaffolds of type 236. NMR and molecular modeling on the corresponding methyl carboxamide N-acetyl derivatives both confirmed a β-turn like conformation for the cis-isomer 236a and γ-turn for the trans-isomers 236b-c (Figure 4) [222].
Praziquantel (PZQ) is the only effective drug for the treatment of schistosomiasis, a high volume neglected tropical disease that affects more than 200 million people worldwide. Liu et al. developed a convergent and versatile synthetic method to prepare easily accessible and highly diversified PZQ derivatives for extensive structure-activity relationship studies. The approach includes Ugi-4CR, followed by a P-S ring closure in a sequential one-pot, two-step procedure (Scheme 68). Even though the products were found to be slightly less active than the mother drug PZQ, the Ugi/P-S reaction sequence remains the shortest and scalable approach towards a future bioactivity-guided optimization of PZQ analogous [223].
A novel stereoselective Ugi-type reaction of the four highly variable starting materials α-amino acid (e.g., leucine), oxo component I (e.g., 2-fluorobenzaldehyde), isocyanide 224 (e.g., benzyl isocyanide), and primary or secondary amine (e.g., morpholine), thus comprising a novel and true 4-CR, provided the iminodicarboxamide 239 (Scheme 69). The extensive optimization of the reaction included: the solvent, a mixture MeOH/H 2 O = 4:1, as a compromise in solubility for the different classes of starting materials; the temperature, RT instead of microwave conditions avoided the formation of undesired side products); the reaction time, three days, and the catalyst. The scope of the reaction was investigated while using representative starting material of each class. The use of bifunctional starting materials allowed for cyclizing the initially formed Ugi products, such as compound 240a, whih was obtained by P-S reaction and deprotective cyclization [224]. developed a convergent and versatile synthetic method to prepare easily accessible and highly diversified PZQ derivatives for extensive structure-activity relationship studies. The approach includes Ugi-4CR, followed by a P-S ring closure in a sequential one-pot, two-step procedure (Scheme 68). Even though the products were found to be slightly less active than the mother drug PZQ, the Ugi/P-S reaction sequence remains the shortest and scalable approach towards a future bioactivity-guided optimization of PZQ analogous [223]. A novel stereoselective Ugi-type reaction of the four highly variable starting materials α-amino acid (e.g., leucine), oxo component I (e.g., 2-fluorobenzaldehyde), isocyanide 224 (e.g., benzyl isocyanide), and primary or secondary amine (e.g., morpholine), thus comprising a novel and true 4-CR, provided the iminodicarboxamide 239 (Scheme 69). The extensive optimization of the Scheme 68. General approach Ugi-4CR/P-S for the synthesis of praziquantel analogous.
Molecules 2020, 25, x FOR PEER REVIEW 53 of 84 reaction included: the solvent, a mixture MeOH/H2O = 4:1, as a compromise in solubility for the different classes of starting materials; the temperature, RT instead of microwave conditions avoided the formation of undesired side products); the reaction time, three days, and the catalyst. The scope of the reaction was investigated while using representative starting material of each class. The use of bifunctional starting materials allowed for cyclizing the initially formed Ugi products, such as compound 240a, whih was obtained by P-S reaction and deprotective cyclization [224].

Scheme 69.
Assembly of iminodicarboxamides by a truly Ugi-4CR followed by P-S cyclization.
(2013) The authors presented a paper containing full experimental detail on the synthesis of indole (240) and THIQ (242) derivatives that were obtained using conc. HCOOH at room temperature by the P-S reaction reported in the previous paper [224]. Notably, the cyclization was performed without purification of the initial Ugi product. The structure of the indolo annulated derivative 240b (R1 = CH2CH2OCHO; R2 = R3 = Me) was confirmed by single-crystal X-ray analysis. Structures 240 and 242 can be found in the architectures of potential natural-product targets, such as the antitumor quinocarcin or the insecticidal notoamide B (Scheme 70) [225].
(2013) The authors presented a paper containing full experimental detail on the synthesis of indole (240) and THIQ (242) derivatives that were obtained using conc. HCOOH at room temperature by the P-S reaction reported in the previous paper [224]. Notably, the cyclization was performed without purification of the initial Ugi product. The structure of the indolo annulated derivative 240b (R 1 = CH 2 CH 2 OCHO; R 2 = R 3 = Me) was confirmed by single-crystal X-ray analysis. Structures 240 and 242 can be found in the architectures of potential natural-product targets, such as the antitumor quinocarcin or the insecticidal notoamide B (Scheme 70) [225]. (2015) The authors designed novel bi-and tri-cyclic scaffolds based on the Ugi tetrazole synthesis. The reaction of propionaldehyde (243), 3,4-dimethoxyphenethylamine (181), isocyanoacetaldehyde dimethyl acetal (244), and trimethylsilyl azide (245) in MeOH at room temperature afforded after 18 h the classical Ugi product 246, which in turn gave the P-S cyclized product 247 (67% yield) by simple treatment with methanesulfonic acid for 18 h at room temperature (Scheme 71). The scope of the methodology was examined with different oxo components and various aryl ethyl amines that produce a large range of results. Diastereomeric mixtures were obtained, but a major stereoisomer was isolated in some cases [227]. (2013) The Ugi reaction of four suitably components and post-condensation reactions can provide four heterocyclic scaffolds: tetrahydroimidazo[1,2-a]pyrazine-2,6(3H,5H)dione, pyrrolidinedione, and isoindolone systems have been yielded via post-Ugi secondary cyclization, while the unique structures of the strained tricyclic 3,9-diazabicyclo[3.3.1.]nonane skeletons 240/242 were obtained by a P-S reaction, where the required functional groups, an electron rich aromatic ring and the oxo partner, were conveniently introduced via the α-amino acid and the primary amine component, respectively. Several analogous reactions show the potential of this transformation (not shown) [226].
(2015) The authors designed novel bi-and tri-cyclic scaffolds based on the Ugi tetrazole synthesis. The reaction of propionaldehyde (243), 3,4-dimethoxyphenethylamine (181), isocyanoacetaldehyde dimethyl acetal (244), and trimethylsilyl azide (245) in MeOH at room temperature afforded after 18 h the classical Ugi product 246, which in turn gave the P-S cyclized product 247 (67% yield) by simple treatment with methanesulfonic acid for 18 h at room temperature (Scheme 71). The scope of the methodology was examined with different oxo components and various aryl ethyl amines that produce a large range of results. Diastereomeric mixtures were obtained, but a major stereoisomer was isolated in some cases [227].

The Enzymatic Pictet-Spengler: Briefing and Update 2011-2015
We are star-crossed; cursed to walk divergent paths (Grace, crossroad poems) The connection of small metabolites to the gene that encodes them has sparked a renaissance in natural product research, which is primarily focused on the biosynthesis [228]. Most of the strength of P-S reaction in the field resides in the parenthood with analogous enzyme-catalyzed keytransformations in the biogenetic pathways of natural products [1,229].
Starting in the ancient world of natural products, the pathway of P-S reaction culminates in the modern technological enzymatic applications to biological systems.

Pictet-Spenglerases
Enzymes catalyzing the P-S condensation have been isolated from several plants and they have entered in the community, named "Pictet-Spenglerases". These are carbon-carbon bond-forming enzymes, which join two relatively simple molecules to form a nitrogen heterocycle with excellent stereochemical control of the resulting chiral center [230].

The Enzymatic Pictet-Spengler: Briefing and Update 2011-2015
We are star-crossed; cursed to walk divergent paths (Grace, crossroad poems) The connection of small metabolites to the gene that encodes them has sparked a renaissance in natural product research, which is primarily focused on the biosynthesis [228]. Most of the strength of P-S reaction in the field resides in the parenthood with analogous enzyme-catalyzed key-transformations in the biogenetic pathways of natural products [1,229].
Starting in the ancient world of natural products, the pathway of P-S reaction culminates in the modern technological enzymatic applications to biological systems.

Pictet-Spenglerases
Enzymes catalyzing the P-S condensation have been isolated from several plants and they have entered in the community, named "Pictet-Spenglerases". These are carbon-carbon bond-forming enzymes, which join two relatively simple molecules to form a nitrogen heterocycle with excellent stereochemical control of the resulting chiral center [230].
An enzyme mechanism for natural substrates was developed on the basis of the holoTfCNS crystal structure, featuring dopamine and a non-productive aldehyde. This "HPAA-first" mechanism suggests the binding of 4-hydroxyphenylacetylaldehyde (4-HPAA) to the enzyme prior to dopamine [241,242].
Molecules 2020, 25 (2012) Oikawa and coworkers demonstrated that the core scaffold of microbial THIQ antitumor antibiotic of the type bis-THIQ, such as saframycin A and ecteinascidin, is biosynthesized by a nonribosomal peptide synthetase (NRPS), which catalyzes a highly unusual seven step transformation while using a simple fatty acyl-dipeptidyl coenzyme A (248a) and a modified tyrosine analogue (249a). The authors proposed a biosynthetic pathway that involves multiple reductions (Red) of thioester intermediates (250a and 252a) and two rounds of P-S cyclization (Scheme 74) [246,247]. The group described a protocol for the biochemical characterization of saframycin NRPS SfmC and showed that the P-S reaction relies heavily on the chain length of the cryptic long acyl chain in the peptide substrates [248]. (2012) Oikawa and coworkers demonstrated that the core scaffold of microbial THIQ antitumor antibiotic of the type bis-THIQ, such as saframycin A and ecteinascidin, is biosynthesized by a nonribosomal peptide synthetase (NRPS), which catalyzes a highly unusual seven step transformation while using a simple fatty acyl-dipeptidyl coenzyme A (248a) and a modified tyrosine analogue (249a). The authors proposed a biosynthetic pathway that involves multiple reductions (Red) of thioester intermediates (250a and 252a) and two rounds of P-S cyclization (Scheme 74) [246,247]. The group described a protocol for the biochemical characterization of saframycin NRPS SfmC and showed that the P-S reaction relies heavily on the chain length of the cryptic long acyl chain in the peptide substrates [248].
(2013) The Oikawa group reported then the identification of the biosynthetic gene clusters of quinocarcin and the antitumor antibiotic SF-1739, which share a common type II tetracyclic tetrahydroisoquinoline (THIQ)-pyrrolidine core scaffold. The authors proposed a reaction pathway for the construction of the quinocarcin core scaffold (sketched in Scheme 75), with similar protagonists as for saframycin and involving one P-S and one intramolecular Mannich reaction [249].
Abe et al. confirmed that mcbB indeed catalyzes the PS condensation as well as the decarboxylation and oxidation reactions by crystallographic studies and biochemical characterization. The resolution of the crystal structure of mcbB complexed with tryptophan revealed a totally different structure from those of other P-S cyclization catalyzing enzymes from plants, such as strictosidine synthase or norcoclaurine synthase [251].
(2014) The crystal structure of mcbB was also analyzed and its catalytic mechanism discussed by another group [252]. (2013) The Oikawa group reported then the identification of the biosynthetic gene clusters of quinocarcin and the antitumor antibiotic SF-1739, which share a common type II tetracyclic tetrahydroisoquinoline (THIQ)-pyrrolidine core scaffold. The authors proposed a reaction pathway for the construction of the quinocarcin core scaffold (sketched in Scheme 75), with similar protagonists as for saframycin and involving one P-S and one intramolecular Mannich reaction [249]. Ju and coworkers identified three genes mcbA, mcbB, and mcbC as the solely responsible for scaffold construction of marinacarbolines (MCBs), 1-acetyl-β-carboline (255a), and 1-acetyl-3carboxy-β-carboline (255b), which were isolated from Marinactispora thermotolerans. In particular, mcbB was proposed to be a multifunctional enzyme that catalyzes the P-S reaction of tryptophan and oxaloacetaldehyde for the assembly of the tetrahydro-β-carboline (THBC) 254 skeleton, followed by decarboxylation and C-ring oxidation to β-carbolines (Scheme 76) [250]. Abe et al. confirmed that mcbB indeed catalyzes the PS condensation as well as the decarboxylation and oxidation reactions by crystallographic studies and biochemical characterization. The resolution of the crystal structure of mcbB complexed with tryptophan revealed a totally different structure from those of other P-S cyclization catalyzing enzymes from plants, such as strictosidine synthase or norcoclaurine synthase [251].
(2014) The crystal structure of mcbB was also analyzed and its catalytic mechanism discussed Scheme 76. Marinacarbolines biosynthetic pathway.

Biocatalysis
Biotechnological approaches are gaining importance for the production of alkaloids that cannot be isolated from their natural sources in quantities that were comparable with the demand of modern medicine.
Recent advances in metabolic engineering have enabled the tailored production of plant secondary metabolites in microorganisms. A Japanese group, using selected enzyme to construct a tailor-made biosynthetic pathway, being produced the plant benzylisoquinoline alkaloid, (S)-reticuline (vide infra: Scheme 76) with a yield of 46.0 mg/L culture medium (Escherichia coli fermentation system) [253]. Analogously, O'Connor and coworkers suppressed tryptamine biosynthesis in Catharanthus roseus hairy root cultures, introduced an unnatural tryptamine analog to the silenced plant cells, and obtained a variety of novel products [254,255].
Enzymatic catalysis has been proven to be useful for a number of synthetic biotransformations [256,257], although natural product biosynthetic enzymes often have narrow substrate scope that limits their use as biocatalysts. Pictet-Spenglerases are expected to produce nitrogen heterocycles with excellent stereochemical control of the resulting chiral center. For instance, strictosidine synthase from Ophiorrhiza pumila can utilize a range of simple achiral aldehydes and substituted tryptamines to form highly enantioenriched (ee > 98%) THBCs via the P-S reaction. These findings represent the first example of aldehyde substrate promiscuity in the strictosidine synthase family of enzymes [258].
A recombinant norcoclaurine synthase (NCS) from E. coli was used to prepare (S)-norcoclaurine (R = H), starting from tyrosine and dopamine (R = H) as substrates in a one-pot, two-step process. Tyrosine was first chemically decarboxylated by stoichiometric amount of NaClO to generate the aldehyde species (4-HPAA), to which the enzyme and tyrosine were added (Scheme 77). The optimized process afforded (S)-norcoclaurine (R = H; ee 93%) in 81% yield and allowed for the recycling of the enzyme [259]. Successively, the same group achieved the fully enzymatic asymmetric synthesis of substituted tetrahydroisoquinolines analogs of norcoclaurine in two steps, starting from dopamine and a set of amine substrates by an oxidation performed with a diamine oxidase from plant Lathyrus cicero L., followed by the P-S reaction being catalyzed by the recombinant NCS from Thalictrum flavum. In the first step, various aliphatic and aromatic amines were transformed into the corresponding aldehydes by the broad specificity of diamine oxidase enzyme, while the second step afforded chiral substituted THIQs in good yield [260]. (2011) Pictet-Spenglerases (P-Sases) are highlighted among the enzymes that have been employed for C-C bond forming reaction on a preparative scale [261]. In concert with Oikawa model for saframycin [246], the presumed activity of a P-Sase was invoked in the biosynthetic pathway for anticancer agent ET-743 [262].
(2012) O'Connor and coworkers described the substrate scope and limitations of a NCS from Thalictrum flavum (TfNCS). Nineteen aldehyde analogs 256a-n, which were synthesized or commercially available, were treated (at 1 mM concentration) with dopamine and TfNCS (300 µ M) in TRIS buffer (100 µ M, pH 7) to give the corresponding norcoclaurine derivatives 257 in Successively, the same group achieved the fully enzymatic asymmetric synthesis of substituted tetrahydroisoquinolines analogs of norcoclaurine in two steps, starting from dopamine and a set of amine substrates by an oxidation performed with a diamine oxidase from plant Lathyrus cicero L., followed by the P-S reaction being catalyzed by the recombinant NCS from Thalictrum flavum. In the first step, various aliphatic and aromatic amines were transformed into the corresponding aldehydes by the broad specificity of diamine oxidase enzyme, while the second step afforded chiral substituted THIQs in good yield [260]. (2011) Pictet-Spenglerases (P-Sases) are highlighted among the enzymes that have been employed for C-C bond forming reaction on a preparative scale [261]. In concert with Oikawa model for saframycin [246], the presumed activity of a P-Sase was invoked in the biosynthetic pathway for anticancer agent ET-743 [262].
(2012) O'Connor and coworkers described the substrate scope and limitations of a NCS from Thalictrum flavum (Tf NCS). Nineteen aldehyde analogs 256a-n, which were synthesized or commercially available, were treated (at 1 mM concentration) with dopamine and Tf NCS (300 µM) in TRIS buffer (100 µM, pH 7) to give the corresponding norcoclaurine derivatives 257 in enantioselective fashion (S-form) (Scheme 78) [263]. Including for each reaction inactive (boiled) enzyme controlled the enzymatic background. As a result, Tf NCS proved to have exceptionally broad aldehyde substrate specificity, turning over aldehydes 256a-n. Only the THIQs corresponding to (the small) formaldehyde (HCHO) and acetaldehyde (CH 3 CHO), as well as benzaldehyde (C 6 H 5 CHO), could not be detected. By contrast, NCS showed a strict requirement for the amine substrate, which is dopamine. In fact, neither tryptamine, which is the natural substrate of the enzyme in the plant, nor commercially available 3,4-disubstituted phenylethylamines (not shown), provided any product with native 4-HPAA. This substrate specificity is consistent with earlier studies [238,239] that require the amine substrate to contain a hydroxy group at the C-3 position of the aromatic ring. In conclusion, these findings revealed that NCS could be used as a catalyst to yield a variety of substituted THIQs. Pesnot et al. investigated the versatility and potential of a NCS from Coptis japonica (CjNCS2), together with the development and application of a novel fluorescence-based high-throughput assay using several amines/aldehydes. The tetrahydroisoquinolines were formed as the (1S)-isomer in 95% ee. The exceptional tolerance of NCS towards aldehyde substrates was further supported by the proposed mechanism, in which the aldehydes protrude out of the enzymatic pocket [264].
A novel function of strictosidine synthase (STR1) allowed, for the first time, a simple enzymatic synthesis of the strictosidine analogues 259, harboring the piperazino[1,2-a]indole (PI) scaffold, starting from secologanin and the novel indole-like amine 258 (Scheme 79). STR1 provided exclusively access to products 259 and can generate by chemoenzymatic approach libraries of a novel class of alkaloids with potentially new biological activities [265]. (2014) Nakagawa et al. developed a production system of (R,S)-tetrahydropapaveroline (THP) by altering the reticuline synthetic pathway [253] that was previously constructed while using E. coli. L-tyrosine (obtained from glycerol via a tyrosine over-producing pathway) is oxidized by Pesnot et al. investigated the versatility and potential of a NCS from Coptis japonica (CjNCS2), together with the development and application of a novel fluorescence-based high-throughput assay using several amines/aldehydes. The tetrahydroisoquinolines were formed as the (1S)-isomer in 95% ee. The exceptional tolerance of NCS towards aldehyde substrates was further supported by the proposed mechanism, in which the aldehydes protrude out of the enzymatic pocket [264].
A novel function of strictosidine synthase (STR1) allowed, for the first time, a simple enzymatic synthesis of the strictosidine analogues 259, harboring the piperazino[1,2-a]indole (PI) scaffold, starting from secologanin and the novel indole-like amine 258 (Scheme 79). STR1 provided exclusively access to products 259 and can generate by chemoenzymatic approach libraries of a novel class of alkaloids with potentially new biological activities [265].
(2014) Nakagawa et al. developed a production system of (R,S)-tetrahydropapaveroline (THP) by altering the reticuline synthetic pathway [253] that was previously constructed while using E. coli. L-tyrosine (obtained from glycerol via a tyrosine over-producing pathway) is oxidized by tyrosinase to L-DOPA, which in turn is transformed into dopamine by DOPA decarboxylase (DODC). Finally MAO oxidizes dopamine to 3,4-DHPAA and these last two compounds are spontaneously converted to (R,S)-THP through a non-enzymatic P-S reaction (Scheme 80) [266]. In the synthetic pathway from glycerol to tyrosine to 3,4-DHPAA, previously reported [254], NCS was used instead for THP synthesis. the proposed mechanism, in which the aldehydes protrude out of the enzymatic pocket [264].
A novel function of strictosidine synthase (STR1) allowed, for the first time, a simple enzymatic synthesis of the strictosidine analogues 259, harboring the piperazino[1,2-a]indole (PI) scaffold, starting from secologanin and the novel indole-like amine 258 (Scheme 79). STR1 provided exclusively access to products 259 and can generate by chemoenzymatic approach libraries of a novel class of alkaloids with potentially new biological activities [265]. (2014) Nakagawa et al. developed a production system of (R,S)-tetrahydropapaveroline (THP) by altering the reticuline synthetic pathway [253] that was previously constructed while using E. coli. L-tyrosine (obtained from glycerol via a tyrosine over-producing pathway) is oxidized by tyrosinase to L-DOPA, which in turn is transformed into dopamine by DOPA decarboxylase (DODC). Finally MAO oxidizes dopamine to 3,4-DHPAA and these last two compounds are spontaneously converted to (R,S)-THP through a non-enzymatic P-S reaction (Scheme 80) [266]. In the synthetic pathway from glycerol to tyrosine to 3,4-DHPAA, previously reported [254], NCS was used instead for THP synthesis. Maresh et al. reported a convenient method for oxidative decarboxylation of α-amino acids and extended the enantioselective enzymatic synthesis of (S)-norcoclaurine (R = H, Scheme 72 [259,260]) to halogenated (R = Cl, Br, I) high purity derivatives. Phenylalanine and tryptophan were also successfully converted in the corresponding P-S products (not shown) [267]. Nishihachijo et al. showed that NCS is a promising catalyst for synthesizing non-natural, optically active THIQs. The authors examined the aldehyde substrate specificity of a NCS from Coptis japonica expressed in E. coli, by synthesizing 6,7-dihydroxy-1-phenethyl-and 6,7-dihydroxy-1-propyl-1,2,3,4tetrahydroisoquinolines. The two P-S products were obtained in yield of 86.0 and 99.6%, and in ee of 95.3 and 98.0%, respectively [268].(2015) Stöckigt and coworkers updated strategies and methods for exploring the applicability of STR to the formation of new alkaloids with unusual substitution pattern or (even) with novel scaffold while taking strictosidine synthase from Rauwolfia serpentina (RsSTR) and Catharanthus roseus (CrSTR) as representative models. The authors introduced the latest released complex structures of RsSTR with new substrates. The examples provided in the article pave the way to the construction of novel alkaloid libraries by chemoenzymatic approaches [269]. Ward et al. described and assessed two different mechanisms of NCS activity: The "HPAAfirst" mechanism (based on the holo X-ray crystal structure [239]) and the "dopamine-first" mechanism. The authors observed novel kinetic parameters that show NCS to operate with low catalytic efficiency. The amino acid substitution pattern L76A, which was located in the proposed "dopamine-first" aldehyde binding site, resulted in a modification of the aldehyde activity profile, strongly supporting the mechanism in question [270].

Chemistry and biology of Pictet-Spengler Reaction
Started in the old world of natural products, the P-S reaction pathway culminates in the modern technological applications of enzymes.
Aldehyde-and ketone-functionalized proteins are appealing substrates for the development of chemically modified biotherapeutics and protein-based materials.
The close tie of P-S reaction with biology materializes in the iso-Pictet-Spengler reaction as the basis of a bond that connects the orthogonal bioconjugation of a small molecule with a large protein. Maresh et al. reported a convenient method for oxidative decarboxylation of α-amino acids and extended the enantioselective enzymatic synthesis of (S)-norcoclaurine (R = H, Scheme 72 [259,260]) to halogenated (R = Cl, Br, I) high purity derivatives. Phenylalanine and tryptophan were also successfully converted in the corresponding P-S products (not shown) [267].
(2015) Stöckigt and coworkers updated strategies and methods for exploring the applicability of STR to the formation of new alkaloids with unusual substitution pattern or (even) with novel scaffold while taking strictosidine synthase from Rauwolfia serpentina (RsSTR) and Catharanthus roseus (CrSTR) as representative models. The authors introduced the latest released complex structures of RsSTR with new substrates. The examples provided in the article pave the way to the construction of novel alkaloid libraries by chemoenzymatic approaches [269]. Ward et al. described and assessed two different mechanisms of NCS activity: The "HPAA-first" mechanism (based on the holo X-ray crystal structure [239]) and the "dopamine-first" mechanism. The authors observed novel kinetic parameters that show NCS to operate with low catalytic efficiency. The amino acid substitution pattern L76A, which was located in the proposed "dopamine-first" aldehyde binding site, resulted in a modification of the aldehyde activity profile, strongly supporting the mechanism in question [270].

Chemistry and biology of Pictet-Spengler Reaction
Started in the old world of natural products, the P-S reaction pathway culminates in the modern technological applications of enzymes.
Aldehyde-and ketone-functionalized proteins are appealing substrates for the development of chemically modified biotherapeutics and protein-based materials.
The close tie of P-S reaction with biology materializes in the iso-Pictet-Spengler reaction as the basis of a bond that connects the orthogonal bioconjugation of a small molecule with a large protein.

Iso-Pictet-Spengler
Natural and synthetic compounds containing the THBC framework are endowed with an extraordinary range of biological activity [271]. The closely related tetrahydro-γ-carboline (THGC) scaffold is unknown among natural product structures, however, holds considerable potential as a template for drug discovery [272,273].
Klausen and Jacobsen reported that chiral thiourea derivatives in combination with benzoic acid promote catalytic asymmetric iso-P-S reactions of electronically and sterically diverse imines, providing unprotected THBCs in high ee and yield [274].
(2011) Through an approach founded on the above findings, Jacobsen and coworkers described a straightforward and direct route to enantiomerically enriched THGCs 261 via the one-pot condensation/cyclization of 2-substituted-indolyl-ethylamines (isotryptamines) 260 and aldehydes or ketones I (Scheme 81). In a reaction defined "iso-Pictet-Spengler", chiral thioureas TH-2, TH-3, or TH-4 (20 mol%) (see Scheme 81) and benzoic acid PhCO 2 H (10-20 mol%) were found as the more effective cocatalysts and gave the THGC 261a (X = H, R = i-Pr; R = H) with a 97% yield and 95% ee. [275]. Ketone substrates I were also successfully applied to the iso-P-S protocol. The authors explored also whether the THGC framework might undergo analogous transformations into structurally and stereochemically complex alkaloid scaffolds. They targeted the synthesis of the spirocyclic oxindole 261b (X = H; R = Me; R = 4-ClC 6 H 4 ) [276]. Klausen and Jacobsen reported that chiral thiourea derivatives in combination with benzoic acid promote catalytic asymmetric iso-P-S reactions of electronically and sterically diverse imines, providing unprotected THBCs in high ee and yield [274].

Orthogonal Bioconjugation
The bioorthogonal chemical reactions are the key for selective modifications of biological species and they involve the creation of non-biological molecules that exert an effect on or reveal new information about biological systems. The bioorthogonal ligation between a biomolecule and a reactive partner does not perturb other chemical functionality naturally found within the cell system [279]. Recently, McKay and Finn provided an update on the last developments in the selective chemical modification of biological molecules, the so-called bioorthogonal chemistry, and analyzed strategies and applications [280]. Aldehydes and ketones were among the functionalities amenable to incorporation into biomolecules for their synthetic accessibility and small size. Several chemical, enzymatic, and genetic methods have been developed to introduce aldehydes and ketones into protein sites specifically [279]. Historically, oximes and hydrazones have been used for ligation to carbonyls, which, as mild electrophiles are typically conjugated with α-effect nucleophiles, such as substituted hydrazines or alkoxyamines. However, the resulting C = N Scheme 83. One-pot domino cycloisomerization/iso-Pictet-Spengler reaction.

Orthogonal Bioconjugation
The bioorthogonal chemical reactions are the key for selective modifications of biological species and they involve the creation of non-biological molecules that exert an effect on or reveal new information about biological systems. The bioorthogonal ligation between a biomolecule and a reactive partner does not perturb other chemical functionality naturally found within the cell system [279]. Recently, McKay and Finn provided an update on the last developments in the selective chemical modification of biological molecules, the so-called bioorthogonal chemistry, and analyzed strategies and applications [280]. Aldehydes and ketones were among the functionalities amenable to incorporation into biomolecules for their synthetic accessibility and small size. Several chemical, enzymatic, and genetic methods have been developed to introduce aldehydes and ketones into protein sites specifically [279]. Historically, oximes and hydrazones have been used for ligation to carbonyls, which, as mild electrophiles are typically conjugated with α-effect Scheme 84. Synthesis of benzazepinoindoles via organocatalysis.

Orthogonal Bioconjugation
The bioorthogonal chemical reactions are the key for selective modifications of biological species and they involve the creation of non-biological molecules that exert an effect on or reveal new information about biological systems. The bioorthogonal ligation between a biomolecule and a reactive partner does not perturb other chemical functionality naturally found within the cell system [279]. Recently, McKay and Finn provided an update on the last developments in the selective chemical modification of biological molecules, the so-called bioorthogonal chemistry, and analyzed strategies and applications [280]. Aldehydes and ketones were among the functionalities amenable to incorporation into biomolecules for their synthetic accessibility and small size. Several chemical, enzymatic, and genetic methods have been developed to introduce aldehydes and ketones into protein sites specifically [279]. Historically, oximes and hydrazones have been used for ligation to carbonyls, which, as mild electrophiles are typically conjugated with α-effect nucleophiles, such as substituted hydrazines or alkoxyamines. However, the resulting C = N linkages are susceptible to hydrolysis under physiologically relevant conditions [279,280]. Moreover, oxime formation requires acidic conditions (pH 4.5) to proceed at any appreciable rate, while prolonged exposure to acid might damage sensitive biomolecules. A C-C bond was taken in consideration and the classic P-S cyclization was applied to the N-terminal labeling of horse heart myoglobin to overcome the instability of the oxime constructs [281][282][283], but to avoid harsh acidic conditions, which were not consistent with protein bioconjugation, the used the incubation of the protein and tryptophan or tryptamine in phosphate buffer (pH 6.5) at 37 • C for 18 h as reaction conditions.
(2013) Bertozzi and coworkers introduced a P-S ligation to prepare hydrolytically stable conjugates with glyoxyl-and formylglycine-modified proteins, including a monoclonal antibody [281]. The canonical aliphatic amine was replaced with alkoxyamine since kinetic studies of the classic P-S reaction suggested the formation of the iminium ion as the rate-limiting step. In addition, the functionality was moved to the 2-position, leaving the more nucleophilic 3-position to engage in the electrophilic substitution, such as in an "iso-Pictet-Spengler" reaction [276]. Finally, the aminooxy substituent was methylated to provide a more reactive oxyminium ion intermediate [281][282][283]. Compound 272a (R = CH 2 CH 2 CO 2 H) was prepared in few steps, starting from indole-2-methanol, and culminating in the rapid formation under acidic conditions of dihydroxy-β-oxa-γ-carboline 273a, a product that was hydrolytically more stable than the model oxime (Scheme 85). Compounds 273b and 273c were prepared to evaluate the iso-P-S ligation as a means to label aldehyde-functionalized proteins. The first one was obtained by treatment of Teoc protected 272a with amino-poly (ethylene glycol)-functionalized biotin, followed by deprotection with CsF, to give 272b, and by ligation to the N-terminal glyoxyl moiety of horse heart myoglobin (glyoxyl Mb). Analogously, the indole 272c was synthesized by a similar sequence of coupling 272a with Alexa Fluor 488 (AF488), while the oxacarboline 273c was obtained by ligation with the formylglicine-functionalized C-terminus of the six-residue peptide sequence LCTPSR of maltose-binding protein (FGly-MBP). The studies put in evidence that P-S ligation might enhance the metabolic, enzymatic, and chemical functionalization of proteins and other biomolecules. the electrophilic substitution, such as in an "iso-Pictet-Spengler" reaction [276]. Finally, the aminooxy substituent was methylated to provide a more reactive oxyminium ion intermediate [281][282][283]. Compound 272a (R′ = CH2CH2CO2H) was prepared in few steps, starting from indole-2methanol, and culminating in the rapid formation under acidic conditions of dihydroxy-β-oxa-γcarboline 273a, a product that was hydrolytically more stable than the model oxime (Scheme 85). Compounds 273b and 273c were prepared to evaluate the iso-P-S ligation as a means to label aldehyde-functionalized proteins. The first one was obtained by treatment of Teoc protected 272a with amino-poly (ethylene glycol)-functionalized biotin, followed by deprotection with CsF, to give 272b, and by ligation to the N-terminal glyoxyl moiety of horse heart myoglobin (glyoxyl Mb). Analogously, the indole 272c was synthesized by a similar sequence of coupling 272a with Alexa Fluor 488 (AF488), while the oxacarboline 273c was obtained by ligation with the formylglicinefunctionalized C-terminus of the six-residue peptide sequence LCTPSR of maltose-binding protein (FGly-MBP). The studies put in evidence that P-S ligation might enhance the metabolic, enzymatic, and chemical functionalization of proteins and other biomolecules.
Scheme 85. Synthesis of hydrolytically stable conjugates with glyoxyl-and formylglycine-modified proteins through a reactive oxyminium ion intermediate.
(2013) Rabuka and coworkers introduced a new reaction, the hydrazino-Pictet-Spengler (HIPS) ligation, which proceeds quickly around neutral pH and allows for one-step labeling of aldehydefunctionalized proteins under mild conditions. The HIPS ligation product is very stable (>5 days) in human plasma when compared to an oxime -linked conjugate (~1 day) [283]. Scheme 85. Synthesis of hydrolytically stable conjugates with glyoxyl-and formylglycine-modified proteins through a reactive oxyminium ion intermediate.
(2013) Rabuka and coworkers introduced a new reaction, the hydrazino-Pictet-Spengler (HIPS) ligation, which proceeds quickly around neutral pH and allows for one-step labeling of aldehyde-functionalized proteins under mild conditions. The HIPS ligation product is very stable (>5 days) in human plasma when compared to an oxime -linked conjugate (~1 day) [283].
(2015) The site-specific HIPS ligation was also used for labeling covalently proteins with a fluorophore [286].
A paper from Kudirka and Rabuka reviews the advance in site-specific ADCs for cancer therapy and highlights the chemistry of the reaction, dubbed iso-Pictet-Spengler ligation [287].

Unnatural Compounds from Fungal Pictet-Spengler Biosynthesis
The P-S reaction contributes greatly to framework diversification of important alkaloids by forming a piperidine ring condensed to aromatic ring or indole moiety with plant-derived Pictet-Spenglerases. Piperidine ring-containing secondary metabolites have also been found in bacteria (e.g., THIQs 274 from Streptomyces lavendulae [288] and THBCs 275 from Marinactinospora thermotolerans [250]), in sponges (e.g., marine natural product hyrtioreticulin F from the sponge Hirtios reticulatis [289]) and animals (e.g., THBC derivatives 276 from rat brain [290]) (resumed in Figure 5) and a Pictet-Spenglerase has been presumed to be involved in their biosynthetic pathways.
Molecules 2020, 25, x FOR PEER REVIEW 67 of 84 Figure 5) and a Pictet-Spenglerase has been presumed to be involved in their biosynthetic pathways. Fungi, especially the Ascomycota genus, have been reported as prolific producers of alkaloids containing one or more indole/indoline moieties [291], mostly endowed with potent biological activities [292]. By time nothing was known regardinf the P-S reaction in the fungal kingdom. Only more recently the availability of fungal genome sequences has significantly accelerated the identification of genes involved in the biosynthesis of secondary metabolites from fungi [293,294].
Tang and coworkers revealed that the different strategies to incorporate and derivatize indole moiety in pathways of fungal alkaloids are based on building blocks, such as L-tryptophan and the related 4-dimethylallyl-tryptophan (4-DMAT, by prenylation) and trypamine (by decarboxylation). However, biochemical evidences that confirm the direct incorporation of tryptamine as a precursor have not been reported [295]. Most, if not all, Chaetonium fungi in the Chaetomiaceae family produce L-tryptophan-derived alkaloid, but no P-S reaction-based secondary metabolite has been detected [296][297][298].
(2014) A comparative genomic analysis has clarified that C. globosum does have a fungal Pictet-Spengler (FPS) gene, which remains silent or poorly activated in laboratory cultivations. Therefore, Fungi, especially the Ascomycota genus, have been reported as prolific producers of alkaloids containing one or more indole/indoline moieties [291], mostly endowed with potent biological activities [292]. By time nothing was known regardinf the P-S reaction in the fungal kingdom. Only more recently the availability of fungal genome sequences has significantly accelerated the identification of genes involved in the biosynthesis of secondary metabolites from fungi [293,294].
Tang and coworkers revealed that the different strategies to incorporate and derivatize indole moiety in pathways of fungal alkaloids are based on building blocks, such as L-tryptophan and the related 4-dimethylallyl-tryptophan (4-DMAT, by prenylation) and trypamine (by decarboxylation). However, biochemical evidences that confirm the direct incorporation of tryptamine as a precursor have not been reported [295]. Most, if not all, Chaetonium fungi in the Chaetomiaceae family produce L-tryptophan-derived alkaloid, but no P-S reaction-based secondary metabolite has been detected [296][297][298].
(2014) A comparative genomic analysis has clarified that C. globosum does have a fungal Pictet-Spengler (FPS) gene, which remains silent or poorly activated in laboratory cultivations. Therefore, a C. globosum IC5I strain was adopted to test for the activation of its "unworking" P-S reaction-based biosynthetic machinery, and 1-methyl-L-tryptophan (1-MT) was demonstrated to be able to up-regulate the FPS expression and condense with the fungal aldehyde flavipin (3,4,5-trihydroxy-6-methylphthalaldehyde) to unexpectedly form a family of skeletally unprecedented alkaloids (Scheme 86), trivially named chaetoglines A-H. Chaetogline B and F have been found to have antibacterial activity comparable to that of tinidazole (a coassayed drug prescribed in clinic for bacterial infections) against pathogenic anaerobes Veillonella parvula, Bacteroides vulgatus, Streptococcus sp., and Peptostreptococcus sp., whereas chaetogline F was also shown to be a potent inhibitor of acetyl-cholinesterase (AChE) [299]. Kasanah and coworkers studied the in vitro activity of the β-carboline-containing manzamine alkaloids against Fusarium solani, Fusarium oxysporum, and Fusarium proliferatum [300]. The data of their bioassays demonstrated that Fusarium spp were resistant to the manzamine alkaloids, because the fungi were able to transform manzamines via hydrolysis, reduction, and a retro-Pictet-Spengler. A pathway that involves the reverse catalytic ring opening and the hydrolysis of the iminium Kasanah and coworkers studied the in vitro activity of the β-carboline-containing manzamine alkaloids against Fusarium solani, Fusarium oxysporum, and Fusarium proliferatum [300]. The data of their bioassays demonstrated that Fusarium spp were resistant to the manzamine alkaloids, because the fungi were able to transform manzamines via hydrolysis, reduction, and a retro-Pictet-Spengler. A pathway that involves the reverse catalytic ring opening and the hydrolysis of the iminium group, namely a retro-Pictet-Spengler reaction, was proposed for the mechanism of the acid catalyzed epimerization of reserpine [301]. A retro-P-S pathway was also suggested for the cis to trans epimerization of 1,2,3-trisubstituted-1,2,3,4-tetrahydro-β-carbolines, but it was ruled out on the basis of kinetic data [302]. According to the mechanism proposed for the activity of Fusarium solani on manzamine F, the THBC 278,

Conclusions: Drawing a Veil over Act II
In the five years following the centenary birthday of P-S cyclization, a mess of paper demonstrated that the reaction did not exit the stage, but came up again on the limelight with new features. In the lustro the chameleonic transformation maintained the role of a protagonist, in spite of a fierce competition of variant and complementary reactions. But the history of the venerable reaction is not yet complete fulfilled.
In the same interval (2011)(2012)(2013)(2014)(2015), the synthesis of scaffolds other than THIQ and THBC with potential biological and pharmaceutical application required the use of modified Pictet-Spengler reactions with structurally different substrates. For instance, in the oxa-Pictet-Spengler cyclization, that is an oxygen variation of the P-S reaction, an aromatic alcohol component, usually a β-arylethyl alcohol, reacts with a carbonyl compound (aldehyde, ketone, or their masked derivatives) to yield polysubstituted isochromans (Scheme 88) [303,304]. Recently a minireview on the catalytic enantioselective approaches to the oxa-P-S cyclization has been published by Zhu et al. [305]. On the other hand, arylamines linked to an activated aromatic nucleus, such as imidazole, might be treated with aldehydes to give imidazo-quinoxalines (n = 0) after DDQ oxidation [306] or triazabenzoazulenes (n = 1) [307] (Scheme 89). Several novel N-rich polycyclic skeletons have been synthesized by application of this P-S variation [308][309][310][311][312][313], for which we propose the name of aza-Pictet-Spengler. About these presences and more we may talk in the third Act. The curtain falls. Scheme 87. Enzymatic retro-Pictet-Spengler of fungi with manzamine alkaloids and the proposed mechanism.

Conclusions: Drawing a Veil over Act II
In the five years following the centenary birthday of P-S cyclization, a mess of paper demonstrated that the reaction did not exit the stage, but came up again on the limelight with new features. In the lustro the chameleonic transformation maintained the role of a protagonist, in spite of a fierce competition of variant and complementary reactions. But the history of the venerable reaction is not yet complete fulfilled.
In the same interval (2011)(2012)(2013)(2014)(2015), the synthesis of scaffolds other than THIQ and THBC with potential biological and pharmaceutical application required the use of modified Pictet-Spengler reactions with structurally different substrates. For instance, in the oxa-Pictet-Spengler cyclization, that is an oxygen variation of the P-S reaction, an aromatic alcohol component, usually a β-arylethyl alcohol, reacts with a carbonyl compound (aldehyde, ketone, or their masked derivatives) to yield polysubstituted isochromans (Scheme 88) [303,304]. Recently a minireview on the catalytic enantioselective approaches to the oxa-P-S cyclization has been published by Zhu et al. [305]. On the other hand, arylamines linked to an activated aromatic nucleus, such as imidazole, might be treated with aldehydes to give imidazo-quinoxalines (n = 0) after DDQ oxidation [306] or triazabenzoazulenes (n = 1) [307] (Scheme 89). Several novel N-rich polycyclic skeletons have