Recent Approaches to Chiral 1,4-Dihydropyridines and their Fused Analogues

: The purpose of this review is to highlight recent developments in the synthesis of chiral 1,4-dihydropyridines and their fused analogues. 1,4-Dihydropyridines are among the most active calcium antagonists that are used for the treatment of hypertension. Enantiomers of unsymmetrical 1,4-dihydropyridines often show di ﬀ erent biological activities and may have even an opposite action proﬁle. Hantzsch synthesis usually produces racemic mixtures of unsymmetrical 1,4-dihydropyridines. Therefore, the development of stereoselective synthesis of 1,4-dihydropyridines is one of the priorities of medicinal chemistry. Over the years, numerous methodologies have been developed for the production of enantiopure 1,4-dihydropyridines, such as stereoselective synthesis using chiral auxiliaries and chiral cyclocondensation partners, chromatographical methods, resolution of diastereomeric 1,4-dihydropyridine salts, enzyme catalysed kinetic resolution, or asymmetrisation of ester groups of 1,4-dihydropyridines. These approaches have been studied in detail and are relatively well established. The catalytic asymmetric approach holds the greatest promise in delivering the most practical and widely applicable methods. Substantial progress has been made toward the development of enantioselective organocatalytic methods for the construction of the chiral dihydropyridines. However, most of them do not provide a convenient way to pharmacologically important 1,4-dihydropyridine-3,5-dicarboxylates. Organocatalytic enantioselective desymmetrisation of prochiral 1,4-dihydropyridine-3,5-dicarbaldehydes also has great promise in the synthesis of pharmacologically important 1,4-dihydropyridine-3,5-dicarboxylates.


1,4-Dihydropyridines
belong to the most beneficial scaffolds with unprecedented biological properties that are investigated by pharmaceutical research providing medicines for the treatment of various diseases [1,2]. It is worth underlining that, according to Triggle, 1,4-DHP is a privileged structure that can interact at diverse receptors and ion channels and receptors of the G-protein class, when scaffold is properly substituted [3]. 4-Aryl-1,4-DHP derivatives are among the most active calcium antagonists [4]. The intensive investigations of 1,4-DHPs encouraged by successful introduction of nifedipine in early 1970s [5] by Bayer AG led to the development of several generations of calcium antagonists, possessing longer lasting antihypertensive activity, better tissue selectivity, The analysis of the structures shows that most of them are the unsymmetrical ones. When substituents on the left side differ from those on the right side of a 1,4-DHP, the molecule becomes chiral, with C4 being the stereogenic centre [9]. Enantiomers of unsymmetrical 1,4-DHP often show different biological activities and they could have even an opposite action profile. For example, it was established that (−)-S-amlodipine [10], (+)-S-manidipine, and [11] (−)-S-nitrendipine [11,12] were more potent calcium channel blockers than the respective opposite enantiomers (Figure 2). The same occurrence was described for barnidipine, where the most active was (+)-S,S-isomer ( Figure 1) [13]. The analysis of the structures shows that most of them are the unsymmetrical ones. When substituents on the left side differ from those on the right side of a 1,4-DHP, the molecule becomes chiral, with C4 being the stereogenic centre [9]. Enantiomers of unsymmetrical 1,4-DHP often show different biological activities and they could have even an opposite action profile. For example, it was established that (−)-S-amlodipine [10], (+)-S-manidipine, and [11] (−)-S-nitrendipine [11,12] were more potent calcium channel blockers than the respective opposite enantiomers (Figure 2). The same occurrence was described for barnidipine, where the most active was (+)-S,S-isomer ( Figure 1) [13]. L-type voltage operated calcium channels are well-known for their involvement in electrical current generation; therefore, predominantly found in "excitable" cells, such as cardiomyocytes, muscle cells or neurons. In addition to the well-known role of 1,4-DHPs on the treatment of cardiovascular system disorders, as efficient agents in the management of hypertension, their potential activity on the cells from other tissues and organs is increasingly being revealed. L-type voltage operated calcium channels are also abundantly expressed in a range of "non-excitable" cells, including mesenchymal stem cells, osteoblasts, and chondrocytes, and they seem to have a range of activities, including mechanotransduction [17,18]. Alterations in intracellular Ca 2+ concentrations may initiate the downstream response of chondrocytes to mechanical stress via mechanosensitive ion-channels [19]. Therefore, the potential in regulation of chondrogenesis processes through the regulation of ion channels increasingly gain attention for stimulation of cartilage regeneration [20]. Mechanical loads trigger anabolic and catabolic responses in chondrocytes [21]. Noteworthy, the analysis of downstream signalling effects and functions suggested that the activated mechanotransductive pathways are distinct in various loading modalities or electric stimuli [21,22]. Furthermore, the application of the same 1,4-DHP drug nifedipine generated different metabolic responses and inflammatory activity in different cell types, namely mesenchymal stem cells and   L-type voltage operated calcium channels are well-known for their involvement in electrical current generation; therefore, predominantly found in "excitable" cells, such as cardiomyocytes, muscle cells or neurons. In addition to the well-known role of 1,4-DHPs on the treatment of cardiovascular system disorders, as efficient agents in the management of hypertension, their potential activity on the cells from other tissues and organs is increasingly being revealed. L-type voltage operated calcium channels are also abundantly expressed in a range of "non-excitable" cells, including mesenchymal stem cells, osteoblasts, and chondrocytes, and they seem to have a range of activities, including mechanotransduction [17,18]. Alterations in intracellular Ca 2+ concentrations may initiate the downstream response of chondrocytes to mechanical stress via mechanosensitive ion-channels [19]. Therefore, the potential in regulation of chondrogenesis processes through the regulation of ion channels increasingly gain attention for stimulation of cartilage regeneration [20]. Mechanical loads trigger anabolic and catabolic responses in chondrocytes [21]. Noteworthy, the analysis of downstream signalling effects and functions suggested that the activated mechanotransductive pathways are distinct in various loading modalities or electric stimuli [21,22]. Furthermore, the application of the same 1,4-DHP drug nifedipine generated different metabolic responses and inflammatory activity in different cell types, namely mesenchymal stem cells and L-type voltage operated calcium channels are well-known for their involvement in electrical current generation; therefore, predominantly found in "excitable" cells, such as cardiomyocytes, muscle cells or neurons. In addition to the well-known role of 1,4-DHPs on the treatment of cardiovascular system disorders, as efficient agents in the management of hypertension, their potential activity on the cells from other tissues and organs is increasingly being revealed. L-type voltage operated calcium channels are also abundantly expressed in a range of "non-excitable" cells, including mesenchymal stem cells, osteoblasts, and chondrocytes, and they seem to have a range of activities, including mechanotransduction [17,18]. Alterations in intracellular Ca 2+ concentrations may initiate the downstream response of chondrocytes to mechanical stress via mechanosensitive ion-channels [19]. Therefore, the potential in regulation of chondrogenesis processes through the regulation of ion channels increasingly gain attention for stimulation of cartilage regeneration [20]. Mechanical loads trigger anabolic and catabolic responses in chondrocytes [21]. Noteworthy, the analysis of downstream signalling effects and functions suggested that the activated mechanotransductive pathways are distinct in various loading modalities or electric stimuli [21,22]. Furthermore, the application of the same 1,4-DHP drug nifedipine generated different metabolic responses and inflammatory activity in different Catalysts 2020, 10, 1019 4 of 21 cell types, namely mesenchymal stem cells and chondrocytes, while the effects of agonist BAYK8644 were also different, but not the opposite [18]. Those data further imply the diversity of regulatory mechanisms in VOCC L-type channels that emphasise the need of agents with different activities for each particular therapeutic application.
After synthesis, study and development of a class of calcium antagonists the interest is also growing towards other activities of 1,4-DHPs, such as neuroprotective [23], radioprotective [24], antimutagenic [25], antioxidative [26], anticancer [27], and antimicrobial [28][29][30]. Consequently, during the last decades, the development of new pyridinium moieties containing compounds based on a 1,4-DHP core has become an interesting area for medicinal chemistry research. Cationic 1,4-DHP amphiphiles having one or two cationic moieties and various length ester appendages, were found to be capable of transfecting pDNA into different cell lines in vitro [31,32], due to their self-assembling properties [33,34].

Stereoselective Synthesis of 1,4-Dihydropyridines
The synthesis of Hantzsch-type 1,4-DHPs remains to be an important field in organic chemistry. The most common way of synthesis of 1,4-DHPs is Hantzsch cyclisation and its modifications [35,36]. Information the scope and limitations of methods of synthesis and chemical properties of hydrogenated pyridine derivatives can be found in several good reviews and in citations therein [1,[37][38][39][40]. However, classical Hantzsch synthesis usually produces only racemic mixtures of unsymmetrical 1,4-DHPs. Therefore, the development of stereoselective synthetic methods for obtaining of therapeutic agents is one of the main priorities of medicinal chemistry.

Resolution of Racemic Basic 1,4-Dihydropyridine Derivatives
Among all of the separation techniques, preparative chiral chromatography on stationary phases is the most widely used technique for the direct analysis of enantiomers and it remains to be important way for the obtaining of 1,4-DHP enantiomers in the analytical scale.
Initially, the state of art for the preparation of (−)-(S)-amlodipine was based on a procedure where the key-step was chromatographic separation of its and (+)-S-2-phenylethanol diastereoisomeric derivative [47], its (−)-(1S)-camphanic acid derivatives [10] and by the resolution of intermediate racemic azido acid cinchonidine salts [48]. In the last decade, resolution of racemic amlodipine base to (+)-R and (−)-S isomer was performed using tartaric acids in dimethylformamide (DMF)/water mixture. Thus, (+)-L-tartrate salt of the unwanted R-isomer of amlodipine was crystallised in DMF/water mixture, while the salt of the required S-form was provided in DMF/water. The addition of water (15%) to DMF, as shown in Scheme 1, significantly improved the efficiency of resolution (yield 71%, enantiomeric excess (ee) 99%) [49].

Lipase-catalysed Kinetic Resolution of Racemic Activated Esters of 1,4-Dihydropyridinecarboxylic Acid
Enzyme-catalysed approach to enantiopure 1,4-DHPs was pioneered by groups of Sih and Achiwa in 1991. Since then, this methodology was widely used for asymmetrisation or kinetic

Lipase-catalysed Kinetic Resolution of Racemic Activated Esters of 1,4-Dihydropyridinecarboxylic Acid
Enzyme-catalysed approach to enantiopure 1,4-DHPs was pioneered by groups of Sih and Achiwa in 1991. Since then, this methodology was widely used for asymmetrisation or kinetic resolution of enzymatically labile esters activated spacer groups [44][45][46]. In the last decade, this approach was successfully applied by Tores et al. to kinetic resolution of various 1,4-DHPs and dihydropyridone  (Table 2). Applying previously well-known [(2-methylpropanoyl) oxy]methyl ester as activating group [51][52][53] in the substrate and Candida rugosa (CRL) or Candida antarctica B (CAL-B) as enzyme good to excellent enantioselectivity was reached [54,55]. Interestingly, that besides frequently used as solvents wet ethers, CRL has been found very efficient in EtOAc, in spite of the fact that this solvent is also susceptible to be hydrolysed by the lipase. The best results of CRL catalysed kinetic resolutions in wet EtOAc were obtained when aryl substituent in the position four of 1,4-DHP rac-2 ring was 2-or 3-NO 2 -C 6 H 4 -, 2-Cl-5-NO 2 -C 6 H 3 -, naphthyl-in short reaction times not exceeding 2.5 h, with high enantioselectivity (E-value) ranging from 50 to >200. CAL-B has been also shown enantioselectivity toward similar substrates in the range of 11-63 (E-value), where the advantage of the use of EtOAc as reaction media was also proven. CAL-B shows better enantioselectivity toward substrates rac-2 having bromine or methoxy group in 4-aryl substituent in comparison with CRL. Thus, hydrolysis of 4-Br-C 6 H 4 -substituted 1,4-DHP rac-2 in methyl tert-butyl ether (MTBE)/water occluded with E 24-27, while in EtOAc E 34 was reached. The same occurrence was described for 3-CH 3 O-C 6 H 4substituted 1,4-DHP rac-2 where E 29-31 was in MTBE/water and E 63 was in EtOAc. resolution of enzymatically labile esters activated spacer groups [44][45][46]. In the last decade, this approach was successfully applied by Tores et al. to kinetic resolution of various 1,4-DHPs and dihydropyridone derivatives (Table 2). Applying previously well-known [(2-methylpropanoyl) oxy]methyl ester as activating group [51][52][53] in the substrate and Candida rugosa (CRL) or Candida antarctica B (CAL-B) as enzyme good to excellent enantioselectivity was reached [54,55]. Interestingly, that besides frequently used as solvents wet ethers, CRL has been found very efficient in EtOAc, in spite of the fact that this solvent is also susceptible to be hydrolysed by the lipase. The best results of CRL catalysed kinetic resolutions in wet EtOAc were obtained when aryl substituent in the position four of 1,4-DHP rac-2 ring was 2-or 3-NO2-C6H4-, 2-Cl-5-NO2-C6H3-, naphthyl-in short reaction times not exceeding 2.5 h, with high enantioselectivity (E-value) ranging from 50 to >200. CAL-B has been also shown enantioselectivity toward similar substrates in the range of 11-63 (E-value), where the advantage of the use of EtOAc as reaction media was also proven. CAL-B shows better enantioselectivity toward substrates rac-2 having bromine or methoxy group in 4-aryl substituent in comparison with CRL. Thus, hydrolysis of 4-Br-C6H4-substituted 1,4-DHP rac-2 in methyl tert-butyl ether (MTBE)/water occluded with E 24-27, while in EtOAc E 34 was reached. The same occurrence was described for 3-CH3O-C6H4-substituted 1,4-DHP rac-2 where E 29-31 was in MTBE/water and E 63 was in EtOAc. Enzyme-catalysed kinetic resolution of 6-methoxycarbonylethylsulfanyl-1,4-dihydropyridines rac-4 has been performed by Krauze group using Amano Acylase (Aspergillus mellus) and Candida antarctica lipase B (CAL-B, Novozyme 435 ® ) in wet diisopropyl ether (IPE) with dichloromethane (DCM) as an additive to improve the solubility of the substrate rac-4 [56]. Table 3 shows the most enantioselective examples. The enantioselectivity of CAL-B increased together with an increase of the temperature. Thus the best enantioselectivities of CAL-B were achieved at 45°C when the substituent at the position 4 of 1,4-DHP was substituted aryl ( Enzyme-catalysed kinetic resolution of 6-methoxycarbonylethylsulfanyl-1,4-dihydropyridines rac-4 has been performed by Krauze group using Amano Acylase (Aspergillus mellus) and Candida antarctica lipase B (CAL-B, Novozyme 435 ® ) in wet diisopropyl ether (IPE) with dichloromethane (DCM) as an additive to improve the solubility of the substrate rac-4 [56]. Table 3 shows the most enantioselective examples. The enantioselectivity of CAL-B increased together with an increase of the temperature. Thus the best enantioselectivities of CAL-B were achieved at 45 • C when the substituent at the position 4 of 1,4-DHP was substituted aryl (Table 3, entries 5-7). While Amano Acylase was less enantioselective toward the same substrate rac-4 showing no enantioselectivity at elevated temperatures (Table 3, entries 9-12). Table 3. Enzyme-catalysed kinetic resolution of 6-methoxycarbonylethylsulfanyl-1,4-dihydropyridines rac-4 [56].

Organocatalytic Enantioselective Synthesis of 1,4-Dihydropyridines
The above-mentioned approaches have been studied in the detail and they are relatively well established. The catalytic asymmetric approach holds the greatest promise in delivering the most practical and widely applicable methods. During the last decade, substantial progress has been made in this field toward development of enantioselective organocatalytic methods for the direct construction of the chiral DHPs. However, most of them do not provide a convenient way to pharmacologically important 1,4-DHP-3,5-dicarboxylates. On the other hand, recently reported organocatalytic enantioselective desymmetrisation of prochiral 1,4-dihydropyridine-3,5dicarbaldehydes also has great promise in the synthesis of pharmacologically important 1,4dihydropyridine-3,5-dicarboxylates.

Organocatalytic Enantioselective Synthesis of 1,4-Dihydropyridines
The above-mentioned approaches have been studied in the detail and they are relatively well established. The catalytic asymmetric approach holds the greatest promise in delivering the most practical and widely applicable methods. During the last decade, substantial progress has been made in this field toward development of enantioselective organocatalytic methods for the direct construction of the chiral DHPs. However, most of them do not provide a convenient way to pharmacologically important 1,4-DHP-3,5-dicarboxylates. On the other hand, recently reported organocatalytic enantioselective desymmetrisation of prochiral 1,4-dihydropyridine-3,5-dicarbaldehydes also has great promise in the synthesis of pharmacologically important 1,4-dihydropyridine-3,5-dicarboxylates.
Catalysts 2020, 10, x FOR PEER REVIEW 10 of 22 Recently, the synthesis of highly functionalised 1-benzamido-1,4-dihydropyridine derivatives 15 from hydrazones 14 and alkylidenemalononitrile 12b in the presence of β-isocupreidine as organocatalyst was reported with rather low enantioselectivity (up to 10-52% ee) by Herrera's group [61]. From ten different chiral organocatalysts, β-isocupreidine was chosen as the most enantioselective. Recently, the synthesis of highly functionalised 1-benzamido-1,4-dihydropyridine derivatives 15 from hydrazones 14 and alkylidenemalononitrile 12b in the presence of β-isocupreidine as organocatalyst was reported with rather low enantioselectivity (up to 10-52% ee) by Herrera's group [61]. From ten different chiral organocatalysts, β-isocupreidine was chosen as the most enantioselective.       The reaction of an enamines 15 with isatylidene malononitrile derivatives 16 was studied in the presence of various organocatalysts. It was found that only Takemoto's thiourea [63] catalysed the reaction with low degree of enantioselectivity. Additional screening of the catalyst loading, the variation of the substituents of starting enamine 15 and isatylidene malononitrile 16, and the reaction conditions led to some improvement of enantioselectivity of the reaction from 44% to 58% ee. Not looking at low enantioselectivity, this is one of very few examples of synthesis of fully substituted 1,4-DHP derivatives.
In 2007, by Renaud group was reported mechanistically different chiral Brønsted acids catalysed enantioselective synthesis of four substituted 1,4-DHPs 19 [64]. Primary screening of two component reaction of N-benzyl β-aminobutenoate 18 with cinnamaldehyde 6 have shown that BINOL-derived phosphoric acid derivatives proved to be capable of catalysing the reaction in up to 50% enantiomeric excess at −7 • C in DCM (Scheme 3). 8 30 CH2C6H5 3- H3CO-C6H4  H  65  30  9  30  Allyl  3-H3CO-C6H4  H  61  30  10  30  C2H5  3-H3CO-C6H4  H  49  32 The reaction of an enamines 15 with isatylidene malononitrile derivatives 16 was studied in the presence of various organocatalysts. It was found that only Takemoto's thiourea [63] catalysed the reaction with low degree of enantioselectivity. Additional screening of the catalyst loading, the variation of the substituents of starting enamine 15 and isatylidene malononitrile 16, and the reaction conditions led to some improvement of enantioselectivity of the reaction from 44% to 58% ee. Not looking at low enantioselectivity, this is one of very few examples of synthesis of fully substituted 1,4-DHP derivatives.
In 2007, by Renaud group was reported mechanistically different chiral Brønsted acids catalysed enantioselective synthesis of four substituted 1,4-DHPs 19 [64]. Primary screening of two component reaction of N-benzyl β-aminobutenoate 18 with cinnamaldehyde 6 have shown that BINOL-derived phosphoric acid derivatives proved to be capable of catalysing the reaction in up to 50% enantiomeric excess at -7°C in DCM (Scheme 3).  The mechanism of the reaction involves the formation of an α,β-unsaturated imine that is activated through hydrogen bonding interaction with the hydroxyl group of the organocatalyst and undergoes nucleophilic addition of β-ketoester. A wide range of cinnamaldehydes was employed, but contrary to the Jorgensen method the use of 2-hexenal led to a low enantioinduction. A serious limitation of these methods is the inability to introduce the substituents at 5-and 6-position of 1,4-DHP 8 ring. Catalysts 2020, 10, x FOR PEER REVIEW 13 of 22 Scheme 4. Chiral BINOL-derived phosphoric acid catalysed synthesis of 1,4-DHPs 8 [65].
The mechanism of the reaction involves the formation of an α,β-unsaturated imine that is activated through hydrogen bonding interaction with the hydroxyl group of the organocatalyst and undergoes nucleophilic addition of β-ketoester. A wide range of cinnamaldehydes was employed, but contrary to the Jorgensen method the use of 2-hexenal led to a low enantioinduction. A serious limitation of these methods is the inability to introduce the substituents at 5-and 6-position of 1,4-  Table 9). The change of BINOL scaffold to H8-BINOL gave better yields and enantioselectivities. Among the H8-BINOL-based imidodiphosphoric acids, the derivative that was substituted with four  The mechanism of the reaction involves the formation of an α,β-unsaturated imine that is activated through hydrogen bonding interaction with the hydroxyl group of the organocatalyst and undergoes nucleophilic addition of β-ketoester. A wide range of cinnamaldehydes was employed, but contrary to the Jorgensen method the use of 2-hexenal led to a low enantioinduction. A serious limitation of these methods is the inability to introduce the substituents at 5-and 6-position of 1,4-DHP 8 ring.