Recent Advances in Divergent Synthetic Strategies for Indole-Based Natural Product Libraries

Considering the potential bioactivities of natural product and natural product-like compounds with highly complex and diverse structures, the screening of collections and small-molecule libraries for high-throughput screening (HTS) and high-content screening (HCS) has emerged as a powerful tool in the development of novel therapeutic agents. Herein, we review the recent advances in divergent synthetic approaches such as complexity-to-diversity (Ctd) and biomimetic strategies for the generation of structurally complex and diverse indole-based natural product and natural product-like small-molecule libraries.


Introduction
The inherent biological activities of natural products are induced by specific and selective interactions with macromolecules in living organisms, which makes them potential bioactive compounds and drug candidates [1]. To allow facile access to natural product-based small-molecule modulators and potential therapeutic agents, natural product screening collections are essential [2]. Extensive investigations have been conducted for the isolation and synthesis of such screening collections [3][4][5][6]. However, the available collections of natural products are insufficient for processes such as high-throughput screening (HTS) and high-content screening (HCS), as they require hundreds of thousands of compounds [7].
To address the unmet needs associated with the construction of libraries which have a large number of natural products and related compounds, two different approaches, namely, combinatorial chemistry and diversity-oriented synthesis (DOS), have been broadly applied [8]. Natural product-based combinatorial chemical libraries are usually constructed by adding various building blocks to natural products with bioactivity and high synthetic accessibility in order to reduce the time and cost of drug candidate development [9]. Combinatorial chemistry has significantly contributed to the discovery and optimization of therapeutic compounds. However, the deficiency of skeletal diversity in natural productbased combinatorial chemical libraries could limit the application of undruggable targets and unbiased phenotypic screening [10,11]. Consequently, substantial synthetic efforts have been devoted to the construction of natural product-based libraries with a high degree of molecular skeletal and stereochemical diversity and complexity through the DOS strategy.
A synthetic approach based on the DOS strategy was introduced by Schreiber et al., allowing the construction of small-molecule libraries containing natural product-like scaffolds from simple starting materials through rapid synthetic transformations in a highly In the typical total synthesis of natural products and the DOS approach for the generation of diverse natural products and related compounds, complex and intriguing structures of natural products are the goal and final target, not the starting point. However, the Ctd strategy considers the complexity and diversity in the synthetic scaffolds of the starting points. The Ctd strategy, introduced by Hergenrother et al. in 2013, employs systematic ring-distortion reactions for the construction of natural-product-like small-molecule collections using easily accessible complex natural products as starting points [30][31][32][33][34]. The Ctd approach achieves skeletal and stereochemical diversity via various chemoselective synthetic operations on the core ring region of the starting natural products while retaining their inherent stereochemistry and complexity. Biosynthetic pathways, with their delicate and efficient enzymatic routes which create complex and diverse natural products from common intermediates, inspired the development of the Ctd strategy. Moreover, owing to the structural complexity and uniqueness of the naturally-occurring skeletons in Ctd, diverse and discrete core structures can be easily generated through relatively simple chemical reactions such as ring expansion, contraction, breaking, and aromatization, as compared to the total synthesis of each natural product ( Figure 1). Therefore, this strategy allows for the exploration of new biologically relevant chemical spaces. Initial reports have demonstrated structurally complex and commercially available natural products from diverse structural classes, such as abietic acid, adrenosterone, gibberellic acid, and quinine, as starting points for the Ctd strategy. The diversified core skeletons obtained via chemo-, regio-, and stereoselective chemical transformations have distinct 3D structural features which could have potential selective interactions with biopolymers. Indole alkaloids have been considered as a target and starting point for the Ctd strategy as well.  Natural products as starting points in the complexity-to-diversity (Ctd) strategy along with representative compounds generated via ring-distortion reactions. (8) Yohimbine (8) is a complex pentacyclic indole alkaloid extracted from the bark of the Pausinystalia tree [35]. It can act as an α2-adrenoceptor antagonist and is used to treat erectile dysfunction in men [36]. In 2017, Huigens et al. selected yohimbine (8) as the starting point for Ctd after considering its high complexity and easy availability [37].

Ctd Strategy Based on Yohimbine
In order to systematically and efficiently alter the core ring of yohimbine (8), they devised a ring-distortion strategy based on two known synthetic transformations, namely, C-N bond cleavage [38] and oxidative rearrangement of tryptoline, a substructure of yo- Figure 1. Natural products as starting points in the complexity-to-diversity (Ctd) strategy along with representative compounds generated via ring-distortion reactions.

Ctd Strategy Based on Yohimbine (8)
Yohimbine (8) is a complex pentacyclic indole alkaloid extracted from the bark of the Pausinystalia tree [35]. It can act as an α2-adrenoceptor antagonist and is used to treat erectile dysfunction in men [36]. In 2017, Huigens et al. selected yohimbine (8) as the starting point for Ctd after considering its high complexity and easy availability [37].
In order to systematically and efficiently alter the core ring of yohimbine (8), they devised a ring-distortion strategy based on two known synthetic transformations, namely, C-N bond cleavage [38] and oxidative rearrangement of tryptoline, a substructure of yohimbine [39] (Scheme 1). Chemoselective ring cleavage reactions are powerful tools for ring distortion. To this end, cyanogen bromide (CNBr) was used to activate the tertiary amine of yohimbine (8) to ammonium ion 9, which could undergo orthogonal ring cleavage by different nucleophile attacks. Selective ring cleavage (pathway A, Scheme 1) occurred only in N,N-dimethylformamide (DMF) solvent, followed by bromide attack on the less hindered carbon via the S N 2 pathway, resulting in the major product 10. In contrast, the ammonium ion intermediary 9 formed the diastereomeric mixture of 11 and 12 by simultaneous S N 1 and S N 2 processes (pathway B, Scheme 1) in a chloroform/alcohol solvent mixture. Interestingly, after the introduction of 2-iodobenzyl alcohol, the diastereomers (11 and 12) underwent a Cu(I)-catalyzed C-N coupling reaction to afford the ring fusion products 13 and 14, respectively. The ring rearrangement scaffold 15 was obtained via another tryptoline ring-distortion reaction involving oxidation and subsequent alkyl migration. Upon treatment with CNBr, spirooxindole 15 underwent selective C-N cleavage to afford compound 17. cleavage by different nucleophile attacks. Selective ring cleavage (pathway A, Scheme 1) occurred only in N,N-dimethylformamide (DMF) solvent, followed by bromide attack on the less hindered carbon via the SN2 pathway, resulting in the major product 10. In contrast, the ammonium ion intermediary 9 formed the diastereomeric mixture of 11 and 12 by simultaneous SN1 and SN2 processes (pathway B, Scheme 1) in a chloroform/alcohol solvent mixture. Interestingly, after the introduction of 2-iodobenzyl alcohol, the diastereomers (11 and 12) underwent a Cu(I)-catalyzed C-N coupling reaction to afford the ring fusion products 13 and 14, respectively. The ring rearrangement scaffold 15 was obtained via another tryptoline ring-distortion reaction involving oxidation and subsequent alkyl migration. Upon treatment with CNBr, spirooxindole 15 underwent selective C-N cleavage to afford compound 17. This Ctd strategy based on ring-distortion transformations of yohimbine (8) is effective for the rapid generation of new natural product-like scaffolds with high complexity. The subsequent typical diversification pathways, such as functional group introductions or click chemistry (often used in combinatorial chemistry), afforded as many as seventy products. The phenotype-based screening of this yohimbine-based library has successfully led to the identification of a new bioactive compound demonstrating antiproliferative activity against hypoxia-inducible factor (HIF)-dependent cancer cells [37]. In addition, compound 20 (Scheme 1) was recently found to be potent against chloroquine-re- This Ctd strategy based on ring-distortion transformations of yohimbine (8) is effective for the rapid generation of new natural product-like scaffolds with high complexity. The subsequent typical diversification pathways, such as functional group introductions or click chemistry (often used in combinatorial chemistry), afforded as many as seventy products. The phenotype-based screening of this yohimbine-based library has successfully led to the identification of a new bioactive compound demonstrating antiproliferative activity against hypoxia-inducible factor (HIF)-dependent cancer cells [37]. In addition, compound 20 (Scheme 1) was recently found to be potent against chloroquine-resistant Plasmodium falciparum Dd2 cells (EC 50 = 0.33 µM) [40]. (21) Vincamine (21) is a commercially available monoterpenoid indole alkaloid administered to patients to enhance global or regional blood flow [41][42][43]. In 2020, Huigens et al. developed a remarkable Ctd strategy for vincamine (21), synthesizing eight stereochemically and structurally complex natural product-like scaffolds via the systematic application of a ring-distortion pathway including ring cleavage, ring rearrangement, and ring fusion reactions (Scheme 2) [44]. First, two reported ring cleavage reactions were employed to afford two new natural product-like compounds, 22 and 23. Methyl propiolate was used to activate the tertiary amine of vincamine (21) to the ammonium ion, which was subsequently subjected to selective ring cleavage by methanol attack to form 23. Notably, 22 became another useful starting point for the Ctd strategy. The carboxylic acid of 22 could be converted to either the ester, 24, or the amide, 25. Stereo-and regioselective oxidation of C3 in the indole core ring of 24 by meta-chloroperbenzoic acid (m-CPBA) afforded the hydroxylated compound 26, which underwent base-promoted semipinacol rearrangement to yield the spirooxindole 27. In contrast, the amide 25 underwent unexpected ring fusion upon treatment with m-CPBA to afford the novel compound 29. It has been proposed that the selective oxidation of compound 25 to a 3-hydroxyindolenine intermediate followed by dehydration yields intermediate 28, which undergoes 5-exo-trig cyclization to furnish 29. In addition, CNBr-induced ring cleavage reactions yielded two new scaffolds, 30 and 31, from compound 24 and vincamine (21), respectively. Subsequently, a typical diversified synthetic method was applied to the newly synthesized eight natural product-like compounds, which produced an eighty-member small-molecule library with distinct natural product-like complex scaffolds. The robustness of this Ctd strategy for vincamine (21) has been successfully validated by the identification of a new bioactive compound, 33, which has antagonistic activity against hypocretin type 2 receptor (HCRTR2) and hence possesses therapeutic potential against opioid abuse [44].

Ctd Strategy Based on Nature-Inspired Indole Monoterpenoids
More recently, the Al-Tel group reported a new Ctd strategy for the nature-inspired indole monoterpenoid compounds 36 and 37 [45], prepared from compounds 34 and 35 [46] (Scheme 3). The compounds 36 and 37 have inherent complexity and transformable moieties such as cyclohexanone, piperidine, and indole rings, which makes them efficient starting points for the Ctd strategy. Using 36 and 37, novel compounds such as indole[2 ,3-a]quinolizines (42), canthine-type alkaloids (39), and arborescidine-type alkaloids (44) were generated. Access to canthine-type scaffolds (39) was efficiently achieved by an unexpected reaction pathway upon treatment with electron-withdrawing-group-bearing hydrazines (38) in acetic acid. The proposed reaction mechanism involved the condensation reaction of compound 36 with phenylhydrazine (38) to yield the hydrazone intermediate 40. Subsequent ring cleavage via the cascade reactions of tautomerization, ring-opening, and elimination of the methoxy group afforded the intermediate 41. The diastereoselective intramolecular aza-Michael addition produced the canthine-type scaffold 39. In contrast, the reaction of 36 with electron-donating-group-bearing hydrazines (38) proceeded via the regioselective [3,3] sigmatropic rearrangement of the hydrazone intermediates, thus generating octahydroindole[2,3-a]quinolizine scaffolds (42) via the typical Fischer-Indole synthetic pathway. When compound 37 was exposed to oxidation conditions, unexpected oxidative rearrangement occurred followed by the 7-exo-trig aza-Michael addition reaction, which afforded the new arborescidine-like indole alkaloid 44.

Ctd Strategy Based on Nature-Inspired Indole Monoterpenoids
More recently, the Al-Tel group reported a new Ctd strategy for the nature-inspired indole monoterpenoid compounds 36 and 37 [45], prepared from compounds 34 and 35 [46] (Scheme 3). The compounds 36 and 37 have inherent complexity and transformable moieties such as cyclohexanone, piperidine, and indole rings, which makes them efficient starting points for the Ctd strategy. Using 36 and 37, novel compounds such as indole [2,3-a]quinolizines (42), canthine-type alkaloids (39), and arborescidine-type alkaloids (44) were generated. Access to canthine-type scaffolds (39) was efficiently achieved by an unexpected reaction pathway upon treatment with electron-withdrawing-group-bearing hydrazines (38) in acetic acid. The proposed reaction mechanism involved the condensation reaction of compound 36 with phenylhydrazine (38) to yield the hydrazone intermediate 40.
Subsequent ring cleavage via the cascade reactions of tautomerization, ring-opening, and elimination of the methoxy group afforded the intermediate 41. The diastereoselective intramolecular aza-Michael addition produced the canthine-type scaffold 39. In contrast, the reaction of 36 with electron-donating-group-bearing hydrazines (38) proceeded via the regioselective [3,3] sigmatropic rearrangement of the hydrazone intermediates, thus generating octahydroindole[2,3-a]quinolizine scaffolds (42) via the typical Fischer-Indole synthetic pathway. When compound 37 was exposed to oxidation conditions, unexpected oxidative rearrangement occurred followed by the 7-exo-trig aza-Michael addition reaction, which afforded the new arborescidine-like indole alkaloid 44.

Biomimetic Synthetic Divergent Approaches to Indole-Based Natural Product and Natural Product-Like Small Molecules
Biologically active natural products are among the best targets for the introduction of biological assay systems to explore unknown biological pathways, which may lead to potential therapeutic candidates [3][4][5][6]. However, synthetic accessibility is an obstacle owing to the stereo-and skeletal-complexity of natural products, despite notable advances in total synthesis and methodology. In contrast, natural product biosynthesis in living organisms allows facile access to structurally complex and distinct skeletons from relatively simple common intermediates through enzyme-catalyzed processes. This tremendous efficiency and diversity-generating power of the biosynthetic pathways allows for the rapid generation of diverse natural products and related compounds, including indole-based compounds [45][46][47][48][49][50][51][52].
A biosynthetic reaction of tryptamine (45) and secologanin (46) yields the key intermediate and a biosynthetic branching point strictosidine (47), a precursor to other structurally diverse and biologically significant monoterpene indole alkaloids formed via the biosynthetic pathways of structural rearrangement, oxidation, and intramolecular cyclization [53]. Thus, strictosidine (47) makes an attractive biomimetic branching point to furnish structurally distinct and complex skeletons. Consequently, substantial research efforts were aimed at developing new biomimetic branching points. The Kozmin group developed a new pluripotent synthetic branching point, indoloquinolizidine (55), which was

Biomimetic Synthetic Divergent Approaches to Indole-Based Natural Product and Natural Product-Like Small Molecules
Biologically active natural products are among the best targets for the introduction of biological assay systems to explore unknown biological pathways, which may lead to potential therapeutic candidates [3][4][5][6]. However, synthetic accessibility is an obstacle owing to the stereo-and skeletal-complexity of natural products, despite notable advances in total synthesis and methodology. In contrast, natural product biosynthesis in living organisms allows facile access to structurally complex and distinct skeletons from relatively simple common intermediates through enzyme-catalyzed processes. This tremendous efficiency and diversity-generating power of the biosynthetic pathways allows for the rapid generation of diverse natural products and related compounds, including indole-based compounds [45][46][47][48][49][50][51][52].

Development of a New Biomimetic Divergent Branching Point from Dehydrosecodine
In contrast, compound 74, possessing a Boc group at the indole N1 position, was transformed to the dehydrosecodine derivative 77 by Cu(I)-catalyzed dihydropyridine formation without a subsequent cycloaddition. It was suggested that the hydrogen at the indole N1 position forms a hydrogen bond with the carbonyl oxygen of the α,β-unsaturated methylester, resulting in the increased electrophilicity of the dienophile and fixed orientation, thus rendering the subsequent [4 + 2] cycloaddition reaction more pronounced. Notably, the dihydropyridine-vinylindole intermediate 76 was found to be capable of undergoing redox-mediated cyclization reactions as well as [4 + 2] cycloaddition. The ngouniensine-type alkaloid 85 was produced by microwave-assisted heating of the dihydropyridine-vinylindole intermediate 76 at 120 • C. Based on the experimental results of deuterium-labeled dihydropyridine-vinylindole intermediate 76 and subsequent structural confirmation, a mechanism was proposed to the effect that the hydride of the dihydropyridine-vinylindole intermediate 76 shifts to form the zwitterionic intermediate 84, which possesses a pyridinium cation and an enolate anion, and subsequently undergoes 7-endo cyclization to furnish the ngouniensine-type alkaloid 85. Furthermore, upon treatment with the photo-redox catalyst [Ru(bpy) 3 ](BF 4 ) 2 under visible-light irradiation at 0 • C, the dihydropyridine ring of 76 underwent single-electron oxidation and was converted to the radical species 86, which generated an unnatural tetracyclic scaffold, 87, via redoxmediated 8-endo cyclization (Pathway E). This study successfully validated the utility of dehydrosecodine derivatives acting as biomimetic branching points to generate natural product-like small-molecule libraries without structural simplification. To generate different monoterpene indole alkaloids, melodinine E (97) was used as another pluripotent branching point. To synthesize melodinine E (97), the azide of intermediate 90 was first converted to the acetamide of compound 94. Upon treatment with TFA, cyclization occurred to afford compound 95, which was then converted into leuconodine B (96) via an intramolecular aldol reaction. Leuconodine B (96) underwent dehydration to afford melodinine E (97), which yielded leuconoxine (98) upon catalytic hydrogenation. Chemoselective reduction of the five-membered lactam upon treatment with Meerwein's salt (Me3OBF4) and sodium cyanoborohydride furnished leuconodine D (99). In contrast, the addition of H2SO4 induced the rearrangement of melodinine E (97) to leuconolam (100), which on chemoselective DIBAL-H reduction of the 5-hydroxy-pyrrolone yielded rhazinilam (101). Notably, this study provided new insights into the potential biosynthetic pathways for terpene indole alkaloids and the feasibility of constructing indole-based natural products and natural product-like small-molecule libraries via the development of artificial pluripotent intermediates. To generate different monoterpene indole alkaloids, melodinine E (97) was used as another pluripotent branching point. To synthesize melodinine E (97), the azide of intermediate 90 was first converted to the acetamide of compound 94. Upon treatment with TFA, cyclization occurred to afford compound 95, which was then converted into leuconodine B (96) via an intramolecular aldol reaction. Leuconodine B (96) underwent dehydration to afford melodinine E (97), which yielded leuconoxine (98) upon catalytic hydrogenation. Chemoselective reduction of the five-membered lactam upon treatment with Meerwein's salt (Me 3 OBF 4 ) and sodium cyanoborohydride furnished leuconodine D (99). In contrast, the addition of H 2 SO 4 induced the rearrangement of melodinine E (97) to leuconolam (100), which on chemoselective DIBAL-H reduction of the 5-hydroxy-pyrrolone yielded rhazinilam (101). Notably, this study provided new insights into the potential biosynthetic pathways for terpene indole alkaloids and the feasibility of constructing indole-based natural products and natural product-like small-molecule libraries via the development of artificial pluripotent intermediates.

Biomimetic Divergent Synthesis to Generate Iboga and "Post-Iboga" Alkaloids
Iboga alkaloids are a large family of monoterpenoid indole alkaloids that contain indole, tetrahydroazepine, and isoquinuclidine scaffolds. Following the 1901 isolation of ibogaine, an iboga alkaloid possessing anti-addictive activity, considerable progress has been made in the identification of new ibogaine-type natural products over the past 100 years, leading to the identification of hundreds of iboga alkaloids and the development of remarkable synthetic pathways for them [59][60][61][62]. In addition, secondary metabolites produced from iboga alkaloids via biosynthetic pathways have sparked great interest. In 2019, inspired by the efficient biosynthesis of secondary metabolites with iboga alkaloids as the starting point, Han et al. developed a biomimetic method for the transformation of (+)-catharanthine (63) to various iboga alkaloids and secondary metabolites coined "post-iboga" alkaloids (Scheme 7) [63]. In that study, (+)-catharanthine (63), a starting material for the synthesis of the anticancer drug navelbine, was used as a biomimetic branching point with high accessibility.

Conclusions
Natural products with highly complex and diverse structures are desirable candidates for screening bioactivity owing to their remarkable specificity toward biomolecular targets. However, the current natural product screening libraries are insufficient for processes such as HTS and HCS. To address this issue, a DOS strategy has been used to generate natural products and drug-like compounds with high skeletal and stereochemical diversity. Despite significant advances in DOS strategies, efficient synthetic methods for the rapid and easy preparation of small-molecule libraries comprising complex and diverse natural products remain challenging. Herein, we briefly reviewed Ctd and biomimetic synthetic strategies for the efficient generation of indole-based natural product and related smallmolecule libraries. These two strategies enable high molecular diversity and structural complexity in the small-molecule libraries, thus distinguishing themselves as powerful synthetic tools. However, high demand for more starting points and branching points will always remain. The synthesis and identification of new pluripotent synthetic starting points could be a promising way to advance new complex natural products and related small-molecule libraries.

Conflicts of Interest:
The authors declare no conflict of interest.