Recent developments in organocatalytic dynamic kinetic resolution

Tetrahedron xxx (2016) 1e18

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Tetrahedron report XXX

Recent developments in organocatalytic dynamic kinetic resolution
le ne Pellissier * He Aix-Marseille Universit
e, Centrale Marseille, CNRS, iSm2 UMR 7313, 13397, Marseille, France

a r t i c l e i n f o Article history: Received 8 January 2016 Received in revised form 11 April 2016 Accepted 20 April 2016 Available online xxx Keywords: Dynamic kinetic resolution Organocatalysis Asymmetric catalysis

Contents 1. 2.

3. 4. 5. 6. 7. 8.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aminocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Proline-derived catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Tetramizole catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N-Heterocyclic carbene catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogen-bonding catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brønsted acid catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brønsted base catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lewis base catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References and notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biographical sketch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction

Abbreviations: Ar, aryl; BINOL, 1,1’-bi-2-naphthol; Bn, benzyl; Boc, tert-butoxycarbonyl; Bz, benzyl; Cbz, benzyloxycarbonyl; Cy, cyclohexyl; DABCO, 1,4-diazabicyclo[2.2.2]octane; DCE, dichloroethane; de, diastereomeric excess; dkr, dynamic kinetic resolution; DMF, dimethylformamide; Dmpe, 1,2-bis(dimethylphosphino)-ethane; DMSO, dimethylsulfoxide; dr, diastereomeric ratio; ee, enantiomeric excess; HATU, 1-[bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b] pyridinium 3-oxid hexafluorophosphate; HBTM, homobenzotetramizole; Hept, heptyl; Hex, hexyl; HIV, human immunodeficiency virus; HMPA, hexamethylphosphoramide; MOM, methoxymethyl; MS, molecular sieves; Naph, naphthyl; NCS, N-chlorosuccinimide; NHC, N-heterocyclic carbene; Non, nonyl; Oct, octyl; Pent, pentyl; Phth, phthalimido; Piv, pivaloyl; PMB, p-methoxybenzyl; r.t., room temperature; TBME, t-butyl methyl ether; TBS, tert-butyldimethylsilyl; THF, tetrahydrofuran; TIPS, triisopropylsilyl; TMS, trimethylsilyl; Tol, tolyl; Ts, 4-toluenesulfonyl (tosyl). * Tel.: þ33 4 91 28 27 65; e-mail address: [email protected].

In spite of the important advances achieved in asymmetric synthesis and especially asymmetric catalysis, the resolution of racemic substrates is still the most prominent way to chiral compounds. A simple kinetic resolution is defined as a process where the two enantiomers of a racemate are transformed to products at different rates.1 If the kinetic resolution is efficient, one of the enantiomers of the racemic mixture is converted into the desired chiral product while the other is recovered unchanged (Fig. 1). However, this methodology presents the limitation of having a maximum theoretical yield of 50%. The wish of the chemical industry to reduce costs and waste in the production of chiral building blocks led chemists to develop novel resolution procedures of racemic mixtures that proceed

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Fig. 1. Classical kinetic resolution.

beyond this 50% limited theoretical yield. Many efforts have been devoted to overcome this limitation which has led to the evolution of classical kinetic resolution into dynamic kinetic resolution in which one can in principle obtain a quantitative yield of one of the enantiomers. Indeed, dynamic kinetic resolution combines the resolution step of kinetic resolution, with an in situ equilibration or racemisation of the chirally-labile substrate (Fig. 2). In this methodology, the enantiomers of a racemic substrate are induced to equilibrate at a rate that is faster than that of the slow-reacting enantiomer in reaction with the chiral reagent (CurtineHammett kinetics). If the enantioselectivity is sufficient, then isolation of a highly enriched non-racemic product is possible with a theoretical yield of 100% based on the racemic substrate. However, requirements have to be fulfilled in order to gain the complete set of advantages of dynamic kinetic resolution, such as the irreversibility of the resolution step, and the fact that no product racemisation should occur under the reaction conditions. In order to obtain products with high optical purity, the selectivity (kfast/kslow) of the resolution step should be 20. Furthermore, the rate constant for the racemisation process (kinv) should be faster than the rate constant of the resolution step (kfast). In a dynamic kinetic resolution process, all of the substrate can

Fig. 2. Dynamic kinetic resolution.

be converted into a single product isomer with a 100% theoretical yield. Racemisation of the substrate can be performed either chemically, biocatalytically or even spontaneously; with conditions chosen to avoid the racemisation of the product. The utility of dynamic kinetic resolution is not limited to a selective synthesis of an enantiomer; when the reaction occurs along with the creation of a new stereogenic centre, an enantioselective synthesis of a diastereoisomer is also possible. This powerful concept has been applied to either enzymatic or non-enzymatic reactions.2 One of the most important achievements in dynamic kinetic resolution recently developed deals with organocatalysed processes which have considerably expanded the synthetic scope of this methodology. While the end of the last century has been dominated by the use of metal catalysts,3 a change in perception occurred during the last 15 years when several reports confirmed that relatively simple organic molecules could be highly enantioselective catalysts in a myriad of transformations. This rediscovery has initiated an explosive growth of research activities in organocatalysis.4 Organocatalysts have several important advantages, since they are usually robust, inexpensive, readily available, and non-toxic.5,6 Their application in synthesis has permitted the preparation of a number of important chiral products with the exclusion of any trace of hazardous metals and with several advantages from an economical and environmental point of view. In recent years, the first examples of organocatalysed dynamic kinetic resolution processes have been described. Today, a wide number of chiral organocatalysts are available to achieve excellent levels of stereocontrol that could only previously be achieved using biocatalysts. Whilst the use of enzymes for the dynamic kinetic resolution of racemic substrates to

afford enantiopure compounds in high enantioselectivities and good yields has emerged as a popular strategy in synthesis,7 it is only relatively recently that the widespread application of nonenzymatic chiral catalysts for dynamic kinetic resolution has gained popularity within the synthetic community.1d The goal of the present review is to cover the advances in organocatalytic reactions evolving through dynamic kinetic resolution reported since the beginning of 2011, since this topic was previously reviewed in 2011.8 For the reader’s convenience, this review is divided into six sections, according to the different types of organocatalytic activation modes employed in these reactions, such as aminocatalysis, N-heterocyclic carbene catalysis, hydrogen-bonding catalysis, Brønsted acid catalysis, Brønsted base catalysis, and Lewis base catalysis. It must be noted that multicatalysed dynamic kinetic resolutions involving a combination of an organocatalyst with another catalyst are not included in this review. 2. Aminocatalysis 2.1. Proline-derived catalysts Asymmetric organocatalysis can follow different modes of activation which can be classified according to the covalent or noncovalent character of the substrateeorganocatalyst interaction and to the chemical nature of the catalyst (Lewis base, Lewis acid, Brønsted base, Brønsted acid). Furthermore, a wide range of organocatalysts can, however, interact with the substrate through both covalent and noncovalent interactions and/or display a dual acid/ base character (bifunctional organocatalysts). In the area of covalent organocatalysis, the enamine activation catalysis, based on the use of a chiral secondary amine as catalyst, has become one of the most applied organocatalytic modes of activation, allowing the enantioselective a-functionalisation of enolisable aldehydes and ketones with a wide variety of electrophiles. It began with the initial formation of an iminium ion by condensation of the aminocatalyst to the carbonyl group of aldehyde or ketone which evolves into an enamine intermediate that subsequently reacts with an electrophile to give the final product. The most employed catalyst for enaminetype reactions is the cheap, natural, simple, and readily available amino acid, L-proline. It can react with carbonyl groups to form iminium ions or enamines which constitute key synthetic intermediates in a number of asymmetric reactions. The high enantioselectivities generally observed in proline-mediated reactions can be rationalised by the capacity of this molecule to promote the formation of highly organised transition states with extensive hydrogen-bonding networks.9 There are several reasons why proline has become an important molecule in asymmetric catalysis, e.g., it is an abundant chiral molecule which is inexpensive and available in both enantiomeric forms. Since the first examples of prolinecatalysed enantioselective direct intermolecular aldol reactions reported by List et al. in 2000,10 these reactions have been extensively studied.11 However, despite the impressive stereoselectivity reached in many examples, a continuing limitation to synthetic applications of these processes has been the rather narrow substrate scope often limited to simple or aromatic aldehydes and few competent ketones. In this context, by extending their early methodology,12 Ward et al. have developed enantioselective direct aldol reactions of enolisable dioxolan-protected a-substituted b-ketoaldehydes with ketones which employed L-proline as organocatalyst.13 As shown in Scheme 1, the reactions of dioxolan-protected a-substituted b-ketoaldehydes 2 with cyclic ketones 1 afforded the corresponding aldol products 3 as almost single diastereomers (dr>20:1) in moderate yields (47e66%) and high enantiomeric excesses of 93 to >98% ee. Using an acyclic ketone such as acetone led to a better yield (72%), a good enantioselectivity of 90% ee, albeit a lower diastereoselectivity (dr¼10:1). Furthermore, when the reaction conditions were applied

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Scheme 1. Aldol reaction of a-substituted aldehydes with ketones.

to the reaction of a non enolisable (R3¼Ph) aldehyde with a cyclic ketone (R1,R2¼CH2SCH2), it provided the corresponding aldol product with a moderate diastereomeric ratio (dr¼14:1) albeit in excellent yield and enantiomeric excess of 92% and 96% ee, respectively (Scheme 1). The authors found that comparable results were reached when using 5-pyrrolidin-2-yltetrazole instead of proline as catalyst. Among other recently reported studies based on the use of proline-derived catalysts,14 Michael additions of malonates 4 to cyclic enones 5 evolving through dynamic kinetic resolution have been developed by Pandey et al.15 The additions of methyl and ethyl malonates to 6-substituted cyclohexenones without substituent at the 4-position (R2¼H) catalysed by chiral 5-pyrrolidin-2-yltetrazole 6 led to the corresponding chiral trans-2,5-dialkylcyclohexanones 7 in high to excellent yields (90e95%), good to excellent diastereomeric excesses (82e98% de), and moderate to good enantiomeric excesses of 58e82% ee. A much lower diastereomeric excess of 20% de was obtained with a 4,4-disubstituted cyclohexenone (R2,R2¼O(CH2)2O) bearing a sterically bulky TBS group on the alkyl chain at the 6-position. In addition, the authors have extended the scope of the process to other enones of different sizes using dimethylmalonate as the donor. However, even if good conversions (70e80%) were obtained for 5-allyl cyclopentenone and 8allyl octenone, the stereoselectivities of the reactions remained low (20e64% de, and 1e32% ee) whereas 7-allyl cycloheptenone did not react. To explain the results, the authors have proposed that the racemisation step required for dynamic kinetic resolution was rationalised by implicating an iminium/enamine tautomerisation, as shown in Scheme 2. In the presence of the chiral amine catalyst and piperidine as a base, the rate of the formation of the two iminium ions A and B could vary, leading to either one of the enriched iminium ion diastereomer preferentially (A), which on further conjugate addition with malonate produced stereoselectively the observed major chiral product.

Scheme 2. Michael addition of malonates to cyclic enones.

Tietze has defined a domino reaction as involving two or more bond-forming transformations which take place under the same reaction conditions, without adding additional reagents and catalysts, and in which the subsequent reactions result as a consequence of the functionality formed by bond formation or fragmentation in the previous step.16 Such single-step reactions allow the synthesis of a wide range of complex molecules, including natural products and biologically active compounds such as pharmaceuticals and agrochemicals.17,18 As a recent example of asymmetric domino reactions evolving through dynamic kinetic resolution, a domino DielseAlder/elimination reaction catalysed by chiral diphenylprolinol trimethylsilyl ether 8 was reported by Vicario et al., in 2012.19 The process occurred between racemic 5acyloxydihydropyranones 9 and enolisable a,b-unsaturated aldehydes 10 through dienamine intermediate. Indeed, the a,b-unsaturated aldehyde 10 reacted with the secondary amine catalyst 8 to give an intermediate dienamine C which subsequently reacted with the 5-acyloxydihydropyranone 9 to give the corresponding DielseAlder cycloadduct D. The latter then underwent an elimination reaction which regenerated the organocatalyst and delivered the final chiral substituted tetrahydro-1H-isochromane 11, as depicted in Scheme 3. A range of functionalised isochromanes 11 bearing three contiguous stereocentres were achieved in moderate to high yields (63e91%), general good diastereomeric excess of >82% de, and good to excellent enantiomeric excesses of up to 97% ee. The best enantioselectivities of 95e97% ee were reached in

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reactions of b-alkyl monosubstituted a,b-unsaturated aldehydes which gave moderate yields of 47e68% while g-aryl substituted a,b-unsaturated aldehydes resulted in higher yields (63e81%) combined with slightly lower enantioselectivities of 91e96% ee. These better yields could be explained in terms of the better ability of these aldehydes to form a more stable conjugated dienamine intermediate. Aldehydes containing functionalised side chains as well as b,b-disubstituted ones (R2sH) were also found tolerated by the reaction, giving satisfactory results (76e98% ee) while a significant drop in enantioselectivity (30e35% ee) was observed in the cases of aldehydes with R1¼H. Chiral diphenylprolinol trimethylsilyl ether 8 was selected as optimal catalyst among a range of diarylprolinol derivatives and an imidazolidinone. To explain the results, the authors assumed that one enantiomer of the starting racemic 5-acyloxydihydropyranone 9 reacted faster with the chiral dienamine intermediate C to give intermediate D, whereas the other enantiomer quickly racemised in the reaction medium before participating in the transformation (Scheme 3).

Scheme 4. Three-component domino a-chlorination/aldol reaction.

the electronic and steric features of the aldehyde. For example, in the series propanal/butanal/pentanal, the diastereomeric ratio of the resulting chlorohydrins increased from 2.5:1 to 3.8:1 to 6:1. As exemplified by the reaction of isovaleraldehyde, the incorporation of a branching position adjacent to the chloromethane stereocentre also led to increased diastereoselectivity (dr¼22:1) without significantly impacting the overall yield of the reaction. The highest enantioselectivities (96e98% ee) were achieved for silyl (TBS) protected 3-hydroxypropanal, phenylacetaldehyde, as well as aldehydes incorporating propargyl and benzyl groups. To illustrate the utility of this methodology, several of the formed chiral chlorohydrins were further converted through successive reduction and heating in methanol into various carbohydrate analogues, such as D-ribose and 2-deoxy-2-chloroaltrose derivatives. 2.2. Tetramizole catalysts A series of chiral amidine-based catalysts were investigated by Birman et al. for the alcoholysis of azlactones 15, resulting in the selection of chiral benzotetramizole (S)-BTM as the most efficient one, which provided the corresponding esters of a-amino acids 16 in moderate to excellent yields (46e97%) and enantiomeric excesses of up to 97% ee, as shown in Scheme 5.22 The best results were obtained in reactions of C4-aryl-substituted azlactones 15 with di(1-naphthyl)methanol (R2¼(1-Naph)2CH) as the

Scheme 3. Domino DielseAlder/elimination reaction.

Multicomponent reactions are defined as domino reactions involving at least three substrates and, consequently, constitute a subgroup of domino reactions.20 In 2013, Britton et al. reported an L-proline-catalysed three-component domino a-chlorination/aldol reaction evolving through dynamic kinetic resolution.21 The process was based on the dynamic kinetic resolution of the a-chloroaldehyde intermediates E in situ generated through chlorination of the corresponding starting (functionalised) aldehydes 12 with Nchlorosuccinimide (NCS). The latter were then submitted to an aldol condensation with 2,2-dimethyl-1,3-dioxan-5-one 13 to afford the corresponding syn-chlorohydrins 14 in good yields (62e84%), moderate to high diastereomeric excesses (34e92% de), and general excellent enantiomeric excesses of 92e98% ee (Scheme 4). It was found that the diastereoselectivity of the process was dependent on

Scheme 5. Alcoholysis of azlactones.

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nucleophile, which were remarkable given the fact that the highest enantiomeric excess previously reported for either enzymatic or non-enzymatic dynamic kinetic resolution of these substrates has been 75% ee. In this study, density functional theory (DFT) calculations were performed to investigate the origins of the enantioselectivity of the process. It was found that the transition states of the fast-reacting enantiomer were stabilised by electrostatic interactions between the amide carbonyl group and the benzoate anion bound to the nucleophile. The chiral catalyst confined the conformation of the a-carbon and the facial selectivity of the nucleophilic attack to promote such electrostatic attractions. a-(Arylthio)- and a-(alkylthio)alkanoic acids and their derivatives are widely used in drug design.23 Compounds of this class are also valuable as synthetic intermediates.24 In 2011, Birman and Yang reported the first examples of direct dynamic kinetic resolution of these products through enantioselective esterification catalysed by an organocatalyst.25 Two tetramizole derivatives, such as benzotetramizole (S)-BTM and homobenzotetramizole (S)-HBTM, were investigated in these reactions, and the optimal catalyst was found to be the latter. In the presence of benzoic anhydride as activator, a range of a-(arylthio)- and a-(alkylthio)alkanoic acids 17 reacted with bis(a-naphthyl)methanol 18 to give the corresponding esters 19 in generally good yields (62e93%) and enantiomeric excesses of 84e92% ee, as shown in Scheme 6. The role of benzoic anhydride was supposed to react with the carboxylic acid 17 to give a mixed-anhydride F, allowing the rapid racemisation of the carboxylic acid (Scheme 6). In only one case of substrate, a low yield of 13% combined with a moderate enantioselectivity of 66% ee was obtained when a secondary substituent (R1¼i-Pr, R2¼Ph) was introduced in the substrate. For a-(arylthio)alkanoic acids, it was found that electron-donating or electron-withdrawing substituents on the phenyl ring (R2) had no appreciable effect on the enantioselectivity (89e92% ee). However, replacing the phenyl group with a benzyl or an n-octyl group proved to be slightly detrimental (84e85% ee). In the case of substituent R1, replacing a methyl group with a primary alkyl group, such as ethyl or n-butyl, resulted in lower yields (62e71% instead of 85e93% with R2¼Me) while maintaining good enantioselectivity (86% ee vs 84e92% ee).

5

The synthesis of chiral carboxylic acids and esters is a very important topic due to the significant role that they play in the field of medicinal and pharmaceutical science.26 For instance; several aarylalkanoic acids, such as ibuprofen and naproxen, function as nonsteroidal anti-inflammatory drugs.27 In this context, Shiina et al. have recently investigated the esterification of a-arylalkanoic acids 20 with bis(a-naphthyl)methanol 18 by using only 5 mol % of (R)-BTM as organocatalyst, and pivalic anhydride as activator in DMF.28 The process involved the enantio-discriminating esterification of racemic a-arylalkanoic acids 20 with bis(a-naphthyl) methanol 18 and the rapid racemisation of the chiral a-arylalkanoic acids via the formation of the mixed-anhydrides generated from pivalic anhydride in the presence of i-Pr2NEt. In this process, the choice of the solvent was found to be very important, since the racemisation was drastically enhanced in a polar solvent such as DMF. As summarised in Scheme 7, the reaction provided a range of a-arylalkanoic esters 21 in generally high yields of up to 99% and good to excellent enantiomeric excesses of 73e97% ee. The synthetic utility of this methodology was demonstrated in the synthesis of pharmaceutically active (S)-enantiomers of nonsteroidal anti-inflammatory drugs, such as (S)-ibuprofen, by using (S)-BTM as catalyst. Indeed, after a subsequent hydrogenation reaction of the formed (S)-ibuprofen ester 21a obtained in 97% yield and 92% ee through the dynamic kinetic resolution process of 20a, the desired a-arylalkanoic acid (S)-ibuprofen was achieved in 92% ee and 98% yield (Scheme 7). In 2015, Ortiz et al. developed the asymmetric esterification of pyranose lactols 22 with isobutyric anhydride (R4¼i-Pr) performed in the presence of inexpensive and readily available organocatalyst levamisole (Scheme 8).29 The reaction evolved through dynamic kinetic resolution and afforded the corresponding esters 23 in high yields (90e96%) and moderate to good enantiomeric excesses of 55e88% ee. The scope of the process was extended to benzyl anhydride (R4¼Bn) which provided by reaction with unsubstituted pyranose lactol (R1¼R2¼R3¼H) the corresponding benzyl ester 23a in 95% yield and 79% ee. This product was used as starting material in a total highly efficient synthesis of a potent HIV inhibitor BMS986001 performed in five steps without chromatography and with 44% overall yield.

3. N-Heterocyclic carbene catalysis

Scheme 6. Esterification of a-(arylthio)- and a-(alkylthio)alkanoic acids.

The utility of N-heterocyclic carbenes (NHCs) as organocatalysts in domino reactions has received growing attention in the past few years.17e,30 They can react with aldehydes to generate catalytically competent (homo)enolate equivalents, which can be trapped with various electrophiles. Although significant progress has been made, examples of NHC-catalysed dynamic kinetic resolution reactions are still rare. In 2012, Scheidt et al. reported the first NHC-catalysed dynamic kinetic resolution of a-substituted-b-ketoesters 24 to furnish chiral substituted bicyclic b-lactones 25 (Scheme 9).31 The process was performed in the presence of cesium carbonate as a base in DCE as solvent and employed N-heterocyclic carbene 26 as catalyst. When aromatic ketones (R1¼aryl) were used as substrates, they led to the corresponding lactones in general remarkable enantiomeric excesses of 97e99% ee, combined with moderate to good yields (53e88%), and moderate to high diastereomeric excesses (66e90% de). On the other hand, a cyclopropyl ketone (R1¼Cy, R2¼CO2Bn) proceeded to yield the corresponding lactone in high diastereomeric excess of 90% de albeit with moderate enantiomeric excess (50% ee) and yield (51%). To explain the results, the authors proposed that addition of the NHC catalyst 26 to the enal 24 induced the formation of the corresponding extended Breslow intermediate. This homoenolate then underwent b-protonation to form the corresponding enol G which was resolved through dynamic kinetic

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Scheme 7. Esterification of a-arylalkanoic acids and synthesis of (S)-ibuprofen.

Scheme 8. Esterification of pyranose lactols and synthesis of HIV inhibitor BMS986001.

resolution evolving by epimerisation under the mildly basic reaction conditions. This enol subsequently reacted through intramolecular aldol condensation to lead to the corresponding cyclopentane intermediate H (Scheme 9). A following acylation of this intermediate afforded the final bicyclic lactone 25. Later, these authors applied this efficient methodology to a total synthesis of a benzopyran estrogen receptor b-agonist 27 starting from functionalised enal 24a (Scheme 10).32 Therefore, the key step of this synthesis was the formation of chiral bicyclic lactone 25a which, in this case, underwent an unusual room temperature b-

Scheme 9. Domino aldol/acylation reaction.

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Scheme 10. Synthesis of a benzopyran estrogen receptor b-agonist.

lactone decarboxylation to yield the corresponding chiral cyclopentene product 28 in 86% yield and 92% ee. This chiral compound was further converted into desired estrogen receptor benzopyran 27 in four steps. In 2014, Johnson and Goodman recently described the dynamic kinetic resolution of b-halo a-ketoesters 29 on the basis of an asymmetric cross-benzoin reaction catalysed by related chiral Nheterocyclic carbene 30 (Scheme 11).33 This organocatalyst promoted the umpolung addition of aldehydes 31 to racemic b-halo aketoesters 29, which provided the corresponding chiral fully substituted b-halo glycolic ester products 32 with high levels of enantio- and diastereomeric excesses of up to 90% de and 96% ee, respectively. One limitation of this method was the requirement of using aromatic aldehydes in order to achieve high enantioselectivity. This was highlighted by the use of isobutyraldehyde (R1¼i-Bu) which provided by reaction with a b-bromo a-ketoester (R2¼Bn, X¼Br) the corresponding product in only 12% ee. The high chemoselectivity observed in the reaction resulted from a greater electrophilicity of the a-ketoester toward the Breslow intermediate. It must be noted that this work represented the first stereoconvergent crossbenzoin reaction that used racemic electrophiles.

Scheme 11. Cross-benzoin addition of aldehydes to b-halo a-ketoesters.

In 2015, Wang et al. disclosed a novel NHC-catalysed dynamic kinetic resolution of hemiacetals, which generated enantioenriched pyranone precursors for the synthesis of carbohydrates or application in glycochemistry.34 As shown in Scheme 12, the reaction of 6-

Scheme 12. Esterification of pyranose lactols.

hydroxy-3-pyranones 22 with alkynals 33 performed in the presence of a novel indanol-derived triazolium catalyst 34 bearing a bulky 2,4,6-i-Pr3C6H2 substituent on the nitrogen atom led to the corresponding variously substituted chiral 3-pyranones 35 in good yields (72e94%) and good to excellent enantiomeric excesses of up to 98% ee. The substrate scope was wide since a range of alkynals with (hetero)aryl as well as alkyl substituents all gave good results. The best result was reached for alkynal with a para-methoxybenzyl (PMB) substituent which afforded the desired product in 90% yield and 98% ee. Even for a pyranose lactol bearing a spirocycle (R1,R2¼CH2OCH2) attached to the C2 position, the corresponding product was achieved in 77% yield and 92% ee. To demonstrate the synthetic utility of this novel dynamic kinetic resolution process, the authors converted some of the formed pyranones into carbohydrates, such as L-lyxose, D-lyxose, D-noviose, L-rhamnose, and L-mannose. 4. Hydrogen-bonding catalysis Asymmetric organocatalytic reactions are not limited to covalent amine catalysis. For example, significant contributions have been made in the fields of hydrogen-bonding and Brønsted acid catalysis among others, depending on the degree of proton transfer in the transition state.4g,35 In hydrogen-bonding activation catalysis, an acidic hydrogen of the organocatalyst is still bound to this catalyst while in Brønsted acid activation catalysis a complete proton transfer from the catalyst to the substrate occurs, but it must be recognised that several intermediate situations are also possible. Chiral thioureas, chiral guanidinium ions, chiral squaramides, and chiral diols constitute the most widely employed chiral hydrogenbond-donor catalysts while the field of Brønsted acid activation organocatalysis is clearly dominated by chiral BINOL-derived phosphoric acids. To summarise, in Brønsted acid activation catalysis, the catalyst transfers a proton to a basic centre in the substrate, making it more electrophilic. In hydrogen-bonding activation catalysis, the catalyst also enhances the electrophilicity of the substrate, but in this case through hydrogen-bonding to a heteroatom of the substrate. Early in 2005, Berkessel et al. reported the highly enantioselective alcoholytic dynamic kinetic resolution of azlactones catalysed by a thiourea-based bifunctional organocatalyst,

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providing a direct access to a wide range of protected natural and non-natural a-amino acids in high enantiomeric excesses.36 This methodology was further extended by the same group through the synthesis of a library of bifunctional (thio)urea-based organocatalysts and their screening in the dynamic kinetic resolution of azlactones, providing similar excellent results.37 However, thioureabased organocatalysts can form hydrogen-bonded aggregates, which result in a strong dependence of reactivity and enantioselectivity on concentration and temperature.38 Due to the selfassociation phenomena of this type of catalysts, in general, enantioselectivity dramatically decreases with increasing concentration, which can hamper their practical use. X-ray crystal structures of mono- and bifunctional thiourea derivatives show that they form aggregates through hydrogen bonding between the thiourea NH group and the thiourea sulfur atom in an intermolecular fashion.36b In 2012, Song et al. reported that C2-symmetric bis-Cinchona-alkaloid-based thiourea catalysts were self-association free since the steric bulk of their two alkaloid moieties prevented their selfaggregation, and, therefore, showed concentration-independent enantioselectivity in the dynamic kinetic resolution of azlactones with alcohols.39 As shown in Scheme 13, the alcoholysis of a range of aryl azlactones 15 performed in the presence of thiourea bis-HCDTU led to the corresponding chiral non-natural a-amino esters 36 in high yields (86e95%) and generally high enantiomeric excesses of 88e91% ee. A lower enantioselectivity (73% ee) was obtained in the case of the reaction between azlactone derived from m-tyrosine (R1¼CH2(m-MeOC6H4), Ar¼Ph) and ethanol. NMR spectroscopic studies and single crystal X-ray analysis confirmed that this bifunctional organocatalyst did not form H-bonded self-aggregates in either solution or solid state.

Scheme 13. Alcoholysis of azlactones.

Another type of hydrogen-bond-donor organocatalysts was applied by Takemoto et al. to the first isomerisation of a-substituted alkynoates 37 into the corresponding chiral trisubstituted allenoates 38 through dynamic kinetic resolution.40 Indeed, using benzothiadiazine catalyst 39 depicted in Scheme 14 allowed a range of these chiral allenoates 38 to be achieved in both high yields and enantiomeric excesses of 86e95% and 87e94% ee, respectively. The dynamic kinetic resolution process involved the racemisation of the starting a-substituted alkynoates 37 through deprotonation promoted by the chiral catalyst and interconversion of the two enantiomers. The study of the substrate scope of the reaction showed that concerning the ester moiety of the substrate, the best enantioselectivities of up to 94% ee were reached with a bulky t-butyl ester while less-hindered ethyl and allyl esters provided slightly lower enantioselectivities of 86e87% ee. A range of aryl substituents in g-position of the substrate (Ar) were also investigated, showing that substrates bearing either an electron-donating or electron-

Scheme 14. Isomerisation of a-substituted alkynoates.

withdrawing substituent at the para-position could be equally used in the reaction. The positions of the substituents were found to slightly affect the enantioselectivity of the reaction, with the metaor ortho-substituted alkynoates (Ar¼m-ClC6H4, Ar¼o-ClC6H4) giving diminished enantioselectivities (82e85% ee vs 92% ee with Ar¼p-ClC6H4). The best enantioselectivity of 94% ee was reached in the case of a heteroaromatic alkynoate (Ar¼3-thienyl). Since enantioselective isomerisation of alkynoates constitutes the most atom-economical way to access chiral allenes which are key building blocks in synthetic organic chemistry,41 this novel methodology allowing the first asymmetric isomerisation of asubstituted alkynoates was an important advance. In 2013, Liu and Feng described a dynamic kinetic resolution of azlactones 15 evolving through oxyamination with N-sulfonyl oxaziridines 40.42 The process was catalysed by chiral bisguanidinium salt 41, providing the corresponding chiral oxazolin-4-one derivatives 42 with potential biological activity in moderate to high yields (36e67%), good to high syn/trans ratios (85:15 to 98:2) and moderate to high enantiomeric excesses of 43e92% ee, as shown in Scheme 15. Remarkably, the kinetic resolution of the oxaziridines 40 occurred simultaneously with the dynamic kinetic resolution of the azlactones 15, allowing the recovered oxaziridines 40 to be achieved in moderate yields (29e60%) and with moderate to excellent enantiomeric excesses of up to 99% ee. The mechanism of the reaction began with the attachment of the N-sulfonyl oxaziridine electrophile to the azlactone. Ring closure of the sulfonamide onto the carbonyl group then resulted in the ring opening of the azlactone in intermediate I to generate the final oxazolidin-4one 42 (Scheme 15). Variously N-protected oxaziridines 40, including a broad range of N-tosyl-substituted ones with different aryl substituents on the C atom, provided good to high enantioselectivities of 71e99% ee along with syn/trans ratios of 90:10 to 98:2 with the exception of an N-nosyl-substituted oxazolidin-4-one which afforded the corresponding product in both lower enantiomeric excess (43% ee) and syn/trans ratio (85:15). Moreover, the oxaziridines 40 could bear aromatic as well as aliphatic substituents (R2). Concerning the azlactone partners 15, a representative selection of azlactones with different aromatic substituents at the C2 position were tolerated. Furthermore, azlactones bearing free OH and NH groups derived from tyrosine (R3¼p-HOC6H4CH2) and tryptophan (R3¼3-indolylmethyl) also participated successfully in the process, providing the corresponding chiral oxazolin-4ones 42 in both high e enantiomeric excesses and syn/trans ratios of 92e88% ee, and 95:5 to 98:2, respectively. In 2015, a novel dynamic kinetic resolution of azlactones 15 was described by the same authors based on the use of oximes 43 as nucleophiles.43 The process represented a novel direct and powerful strategy to achieve chiral N-acyl amino acid oxime esters 44 in both high yields (80e99%) and enantiomeric excesses of 80e97% ee. These excellent results were obtained by promoting the reaction

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Scheme 15. Oxyamination of azlactones with oxaziridines.

with chiral bisguanidinium salt catalyst 45 which is depicted in Scheme 16. A range of aromatic aldoximes 43 were found compatible with the reaction. The process proved to be sensitive to the electronic properties and the position of the substituent on the aryl group of aldoximes. Therefore, aldoximes with electron-donating substituents gave higher enantiomeric excesses (93e97% ee) than those with electron-withdrawing substituents at the corresponding position (81e93% ee). A heteroaromatic aldoxime (Ar1¼3thienyl) provided the best enantioselectivity (97% ee). In the area of azlactones 15, those derived from racemic amino acids phenylalanine (R¼Bn, Ar2¼Ph), tryptophan (R¼3-indolylmethyl, Ar2¼Ph), 2-amino-4-phenylbutanoic acid (R¼CH2Bn, Ar2¼Ph), and methionine (R¼(CH2)2SMe, Ar2¼Ph) resulted in the formation of carboxylamine oxime esters in higher enantiomeric excesses (91e95% ee) than the ones derived from alanine (R¼Me, Ar2¼Ph), valine (R¼i-Pr, Ar2¼Ph), 2-aminobutanoic acid (R¼Et, Ar2¼Ph), and 2-amino-2cyclohexylacetic acid (R¼Cy, Ar2¼Ph) (82e87% ee). As an application, some of the formed chiral carboxylamine oxime esters could be further converted into dipeptide derivatives by reaction with D-

and L-serine methyl ester and glycine methyl ester in good yields (84e90%), and excellent diastereo- and enantiomeric excesses of up to 90% de and 99% ee, respectively. Squaramides constitute a class of efficient hydrogen-bonding organocatalysts and have been increasingly used in organocatalysis.44 In this context, Song et al. recently developed the dynamic kinetic resolution of azlactones 15 with EtOD based on the use of a squaramide-based dimeric Cinchona alkaloid catalyst (BisHQN-SQA).45 The steric bulk of the two alkaloid moieties in this molecule suppressed its self-aggregation, which is an intrinsic problem of acid-base bifunctional catalysts. As shown in Scheme 17, the process afforded a range of chiral N-acylated a-deuterated amino esters 46 in high yields (85e88%) and good enantiomeric excesses of 80e88% ee. In the reaction, EtOD was used as a nucleophile as well as a deuterium source, and, in most cases, the products were obtained with a deuterium content greater than 95%. The utility of this process was demonstrated by the conversion of the products into the corresponding optically active amino acids without loss of their deuterium labeling or optical purity.

Scheme 16. Alcoholysis of azlactones with oximes.

Scheme 17. Alcoholysis of azlactones.

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More recently, Connon et al. also investigated the alcoholysis of aryl azlactones 15 in the presence of closely related C2-symmetric squaramide-based catalyst 47 depicted in Scheme 18.46 The reaction involved a serine-based alcohol 48 as nucleophile and provided a range of chiral O-coupled products 49 in good to high yields (68e99%) and high to excellent diastereomeric excesses of 86e98% de (Scheme 18, first equation). The scope of the process was extended to novel tetrachloroisopropoxycarbonyl-substituted azlactone 15a (Scheme 18, second equation). The chlorine atoms both improved the electrophilicity of the azlactone and also augmented the steric bulk at the point of attachment to the heterocycle to ensure high diastereocontrol. The reaction of this lactone 15a with various protected serines 50 led to the formation of the corresponding N-phthalimido amino acid esters 51 in good to excellent yields (87e99%) and excellent diastereocontrol (92e98% de). In this case, after dynamic kinetic resolution had taken place, the addition of a catalytic amount of DABCO ensured the clean and efficient onepot phthalimide formation.

azlactone enantiomer, with the catalyst capable of accelerating the rate of racemisation to a greater degree than the alcoholysis of the slow reacting azlactone enantiomer (Scheme 19). Among various BINOL phosphoric acids, 3,30 -bis-(9-anthryl) BINOL phosphoric acid 52 was selected as optimal organocatalyst in the reaction of azlactones 15 with 1-naphthylmethanol 53 which was found as most efficient alcohol. 4-Aryl-substituted azlactones (R¼aryl) gave rise to the corresponding chiral a-arylglycine derivatives 54 in 85e92% ee and 82e90% yields while 4-alkyl-substituted substrates (R¼alkyl) produced only moderate enantioselectivities of 29e59% ee. Later, Gong et al. applied another chiral phosphoric acid (R)-55 derived from BINOL to induce the alcoholysis of 4-aryl-substituted azlactones 15 with 2,2-diphenylbutanol 56.50 The dynamic kinetic resolution process proceeded smoothly to yield the ring-opening products 57 in both excellent yields (95e99%) and enantiomeric excesses of 90e96% ee. In this case, the best results were reached by performing the reactions in toluene at 30  C, as shown in Scheme 20. The variation of electronic properties of C4-aryl substituent had

Scheme 18. Alcoholysis of azlactones.

5. Brønsted acid catalysis As already mentioned in Part 4, while a hydrogen-bond catalyst is a rather weak acid which activates a substrate by hydrogen bond donation, a Brønsted acid is a fairly strong acid which causes protonation of a substrate. Indeed, depending on the degree of proton transfer in the transition state, one may distinguish between hydrogen-bonding activation catalysis and Brønsted acid activation catalysis, even if there is no clear borderline between the two of them with the possibility of several intermediate situations. The field of Brønsted acid catalysis is clearly dominated by chiral BINOL-derived phosphoric acids. In the last few years, a number of reactions evolving through dynamic kinetic resolution have been successfully promoted by this type of organocatalysts.47 In 2010s, Birman and Lu described the first example of enantioselective alcoholysis of azlactones 1548 catalysed by chiral phosphoric acids and resulting in their dynamic kinetic resolution.49 In this process, one enantiomer of the racemic azlactone 15 reacted preferentially in the presence of chiral catalyst with an alcohol. The acidity of the a-hydrogen of the azlactone allowed the continuous regeneration of the fast reacting

little effect on the reaction rate, but the azlactones with electronically rich C4-aryl group provided relatively higher enantioselectivities. It was found that the structure of alcohols played an important role in the stereocontrol of the process, with the best enantioselectivities achieved in the case of sterically hindered alcohols. In addition, another chiral phosphoric acid 58 derived from H8BINOL was recently applied by Zhou et al. to the asymmetric transfer hydrogenation of quinolones 59.51 The reaction employed Hantzsch ester 60 as hydrogen source and led to the corresponding chiral tetrahydroquinoline derivatives 61 bearing three contiguous stereogenic centres, as depicted in Scheme 21 (first equation). It was noteworthy that substrates with different aliphatic substituents at the 4-position (R1) worked well with good general yields (71e99%), excellent diastereoselectivity of >90% de, and good enantioselectivities of 82e89% ee. Among these results, substrates bearing methoxy- and fluoro-substituents on the phenyl ring (R2) afforded the highest enantioselectivities (87e89% ee). Moreover, quinolone 59a bearing a phenyl group at the 4-position (R1) was also found a suitable reaction partner when the reaction temperature was raised to 60  C and by using related chiral

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Scheme 19. Alcoholysis of azlactones.

6. Brønsted base catalysis

Scheme 20. Alcoholysis of azlactones.

phosphoric acid (S)-55. Under these conditions (Scheme 21, second equation), the corresponding tetrahydroquinoline 61a was obtained in moderate yield (49%) and diastereo- and enantiomeric excesses (66% de and 46% ee). The authors have proposed the mechanism depicted in Scheme 21 to explain the dynamic kinetic resolution process. It began with the interaction of the substrate 59 with the chiral phosphoric acid catalyst 58 to give an intermediary chiral ion pair. Then, the partially reduced intermediate J was formed through 1,4-hydride addition, followed by Brønsted acidcatalysed isomerisation to produce iminium intermediates K as four stereoisomers. Subsequently, a highly diastereoselective 1,2hydride addition gave the final product 61. The excellent diastereo- and enantioselectivities achieved in this transfer hydrogenation were attributed to the rapid Brønsted acid-catalysed isomerisation among intermediates J, K, and L (Scheme 21).

The Cinchona alkaloid derivatives are categorised as Brønsted bases which are not especially highly enantioselective, probably due to the rather loose nature of nonbonded interactions between extended organic anions and quaternary ammonium salts. On the other hand, the concept of bifunctional asymmetric catalysis, involving the synergistic action of both acidic and basic sites in the substrate, has received a considerable attention. Indeed, the asymmetric activation by bifunctional species containing a hydrogen-bond donor in addition to a Brønsted basic moiety has evolved into a general strategy. The most applied bifunctional hydrogen-bond donor/Brønsted base catalysts are Cinchona alkaloids containing a hydrogen-bond donor.52 This type of catalysts has been among the most employed organocatalysts in reactions evolving through dynamic kinetic resolution. Indeed, these tunable bifunctional catalysts have emerged in the last few years as robust and tunable bifunctional organocatalysts for a range of synthetically useful transformations, including various beautiful asymmetric reactions evolving through dynamic kinetic resolution.53 Early in 2008, Connon et al. reported the dynamic kinetic resolution of azlactones with allylic alcohol catalysed by a dihydroquinine-derived urea derivative, providing the corresponding chiral amino esters in excellent yields and good enantioselectivities of up to 88% ee.54 In the same study, to extend the broad applicability of these catalysts, the authors have investigated the use of thiol nucleophiles, such as CySH, in these processes, furnishing the corresponding enantio-enriched a-amino acid thioesters, of potential use in chemical biology,55 albeit in moderate enantioselectivity (64% ee). With the aim of improving the enantioselectivity of the dynamic kinetic resolution of azlactones with thiols, the same authors recently investigated other Cinchona alkaloid-based catalysts to promote these reactions with thiols.56 For example, studying the reaction of N-furyl-substituted

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Scheme 21. Hydrogenations of quinolones.

azlactones 62 with benzyl thiol 63, these authors obtained good results by using C-9 arylated Cinchona alkaloid catalysts. Indeed, the dynamic kinetic resolution process catalysed by catalyst 64 (Scheme 22) allowed chiral N-2-furoyl amino acid thioesters 65 derived from natural amino acids, such as alanine (R¼Me), phenylalanine (R¼Bn), methionine (R¼(CH2)2SMe), and leucine (R¼iBu), to be achieved in good to high yields (86e97%) and enantiomeric excesses of 66e73% ee. Furthermore, products arisen from non-natural amino acid-derived azlactones (R¼allyl, n-Bu, Et) led to comparable results with 80e95% yields combined with 65e72% ee.

On the other hand, the involvement of the more hindered valinederived azlactone (R¼i-Pr) resulted in a low level of enantiodiscrimination (23% ee). It must be noted that even if the levels of enantioselectivity reached in these reactions remained lower than those usually obtained in the corresponding alcoholysis reactions, this work represented the most enantioselective dynamic kinetic resolution of azlactones by thiolysis to 2012. Higher enantioselectivities of up to 92% ee were later achieved by the same authors in related asymmetric reactions.57 Indeed, although both the parent alkaloid quinine and known C-50 -urea

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Henry product. Mechanistic studies have demonstrated that the process proceeded through a facile catalyst-mediated racemisation of the b-bromo-a-ketoester, as shown in Scheme 24, which allowed dynamic kinetic resolution.

Scheme 22. Thiolysis of N-furyl-substituted azlactones.

derivatives failed to promote the reactions effectively, a new class of C(50 -hydroxylated catalysts, such as 66, proved capable of catalysing the dynamic kinetic resolution of a range of azlactones 15 derived from both unbranched- and, previously challenging, branched-chain amino acids in unprecedented enantioselectivities of 84e92% ee combined with moderate yields of up to 60% (Scheme 23). These homogeneous high enantioselectivities were obtained for a range of thioesters 67 stemming from various natural as well as non-natural amino acids. Indeed, remarkably, previously unobtainable tert-leucine-derived thioester (R¼t-Bu, Ar¼pCF3C6H4) could be formed in 90% ee albeit in combination with a low yield (26%). Generally, in every case of substrate in which a comparison was possible, this novel methodology provided thioester products with significantly higher levels of enantioselectivity, making it the first highly enantioselective dynamic kinetic resolution of azlactones by thiolysis.

Scheme 24. Henry reaction.

These authors also successfully developed the dynamic kinetic resolution of b-bromo-a-ketoesters 69 through related aldolisation with acetone as nucleophile.58 In this case, the best results were achieved by catalysing the reaction with another Cinchona alkaloid catalyst 71 depicted in Scheme 25. Almost single diastereomers (>90% de) of aldol products 72 were remarkably obtained in high yields (93e97%) and high enantiomeric excesses of 91e92% ee in all cases of aryl, heteroaryl, and linear alkyl substrates. As in the aldol reaction with nitromethane as nucleophile (Scheme 25), a substrate with a b-bromo-a-ketoester (R¼iPr) led to the corresponding chiral substituted a-glycolic acid Scheme 23. Thiolysis of azlactones.

In 2014, Johnson and Corbett reported the use of another Cinchona alkaloid catalyst 68 to promote the dynamic kinetic resolution of b-bromo-a-ketoesters 69 through aldolisation with nitromethane.58 As shown in Scheme 24, the corresponding Henry products 70 were obtained in remarkable yields (92e98%), good to high enantiomeric excesses of 74e92% ee, and high diastereomeric excesses of 88e90% de for a range of substrates bearing various aryl and heteroaryl groups at the g-position (R). Moreover, a series of linear alkyl substrates underwent the Henry reaction with a similarly efficiency (95e98%, 88e90% de, 86e89% ee), thus indicating that aromatic interactions between the substrate and the catalyst were not required for achieving high levels of selectivity. On the other hand, the increased steric requirements of g-branching (R¼iPr) resulted in low diastereoselectivity (66% de) and moderate enantioselectivity (74% ee) in the formation of the corresponding

Scheme 25. Aldolisation reaction.

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derivative in both lower diastereo- and enantiomeric excesses of 42% de and 80% ee, respectively. In 2015, quinine was employed as chiral organocatalyst by Tran and Eastgate to promote a diastereoselective phosphorylation of a complex nucleoside 73.59 As shown in Scheme 26, the reaction performed in the presence of 1-[bis(dimethylamino)methylene]1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxid hexafluorophosphate (HATU) as activator produced the corresponding highly functionalised nucleoside phosphoramidate 74 as a 7:1 mixture of diastereomers in 83% yield. This unusual process was shown to proceed through the dynamic kinetic resolution of a 1:1 mixture of

the intermediate activated phosphate ester diastereomers M arisen from the reaction occurring between the starting phosphoramidic acid 75 and HATU (Scheme 26). 7. Lewis base catalysis

b-Amino acids and derivatives are important building blocks in the synthesis of natural products, b-peptides, and pharmaceuticals.60 In 2014, Zhang et al. reported a novel and facile access to these products which was based on asymmetric organocatalytic hydrosilylation of a-substituted b-N-PMP-protected enamino ethyl

Scheme 26. Diastereoselective phosphorylation of a nucleoside.

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esters 76 evolving through dynamic kinetic resolution.61 As shown in Scheme 27, the starting a-substituted b-N-PMP-protected enamino ethyl ester 77 isomerised to generate the corresponding (S)-imine N and (R)-imine N. In the presence of chiral Lewis base catalyst 78 derived from L-proline, one of these enantiomers could be hydrosilylated by HSiCl3 preferentially, and the other enantiomer isomerised back to starting enamine. A range of chiral b-amino esters 76 could be produced in general excellent yields (87e99%) and moderate enantiomeric excesses of up to 77% ee by applying this methodology. Generally, a-aryl substrates underwent reactions smoothly to afford the corresponding products in comparable enantiomeric excesses (63e77% ee), except those bearing bulky substituents (Cl, MeO) on ortho position of the phenyl group (31e58% ee). Moreover, a-2-thienyl-b-N-PMP-protected enamino ethyl ester and a-benzyl-b-N-PMP-protected enamino ethyl ester provided rather lower enantioselectivities (54e55% ee). The lowest enantioselectivity of 19% ee was obtained in the reaction of a-benzamido-b-N-PMP-protected enamino ethyl ester. On the other hand, no desired product was obtained with a-isopropyl-b-N-PMP-protected enamino ethyl ester because it was very unstable and decomposed quickly under the reaction conditions.

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enantioselectivity (83% ee) in the reaction of benzylimine 79b derived from a-methyltetralone to give product 81b when the reaction was performed in the presence of one equivalent of water as additive (Scheme 28).

Scheme 28. Hydrosilylation of cyclic ketimines.

Scheme 27. Hydrosilylation of a-substituted b-N-PMP-protected enamino ethyl esters.

Chiral amines are an important class of organic compounds, and are widely used as building blocks in synthetic chemistry. Actually, a large number of active pharmaceutical ingredients are amines or contain functional groups derived from amines, making up about one-third of the total chiral active pharmaceutical ingredients market. Consequently, many developments have targeted the synthesis of this class of molecules. Among them, a wide number of chiral organocatalysts have been applied to the asymmetric reduction of ketimines with trichlorosilane. Using ketimines bearing a stereogenic centre adjacent to the imine function is suitable to dynamic kinetic resolution. In 2014, Jones and Zhao investigated this process with 2-substituted cyclic imines 79a,b in the presence of tertiary alcohol catalyst 80.62 As shown in Scheme 28, the asymmetric reduction of benzylimine derived from a-methylcyclohexanone 79a with trichlorosilane afforded the corresponding chiral syn-amine 81a as major diastereomer in moderate diastereomeric excess (75% de), low yield (23%), and good enantiomeric excess of 82% ee. A much better diastereoselectivity (>99% de) was obtained in combination with comparable yield (25%) and

8. Conclusions In spite of the important progress achieved in asymmetric catalysis during the last two decades, the most prominent way to reach chiral products in industry is still the resolution of racemic mixtures; despite its major disadvantage that only a maximum of 50% product yield can be obtained. Consequently, it is not surprising that dynamic kinetic resolution, which solves the problem of the limitation in yield, has become a serious alternative to conventional methods for asymmetric synthesis. In the last two decades, the widespread application of non-enzymatic chiral catalysts for dynamic kinetic resolution has known an explosive and impressive growth, providing the access to numerous chiral natural products and biologically active compounds through a wide range of transformations including fascinating domino reactions. Asymmetric organocatalysis is today one of the most important areas in organic synthesis, applicable in a broad variety of reaction types, including those evolving through dynamic kinetic resolution. Even though transition-metal-catalysed enantioselective dynamic kinetic

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resolutions will certainly continue to play a central role in synthetic organic chemistry in the future, the last five years have, however, seen an increasing trend in the use of organocatalysts, which present advantages from an economical and environmental point of view. This review collects the works dealing with the combination of these two powerful concepts, dynamic kinetic resolution and organocatalysis, reported in the literature since 2011. It is divided into six sections, according to the different types of organocatalytic activation modes employed in these reactions, such as aminocatalysis, N-heterocyclic carbene catalysis, hydrogen-bonding catalysis, Brønsted acid catalysis, Brønsted base catalysis, and Lewis base catalysis. In the last few years, significant developments in the efficiency and scope of this powerful methodology have been described. For example, excellent enantioselectivities as high as 98% ee in combination with remarkable yields up to quantitative were independently reported by the groups of Gong, Connon, Song, and Birman for the synthesis of chiral a-amino acid derivatives through the ring-opening of azlactones with various alcohols induced by different classes of organocatalysts including phosphoric acids, squaramides, thioureas, and tetramizoles. Furthermore, these reactions were performed for the first time using oximes as nucleophiles by Liu, providing, when catalysed by a chiral bisguanidinium salt, enantioselectivities of up to 97% ee in combination with up to 99% yield. The enantioselectivity of the thiolysis of azlactones was also widely improved to 92% ee by Connon by employing Cinchona alkaloid catalysts. Even more importantly, a wide variety of highly enantioselective domino reactions evolving through dynamic kinetic resolution have been recently described by several groups. Among them, domino DielseAlder/elimination reactions yielding chiral isochromanes (Vicario) and three-component domino achlorination/aldol reactions (Zhang) both catalysed by proline derivatives have allowed enantioselectivities of up to 97e98% ee to be achieved. Even higher enantioselectivity of 99% ee was achieved by Scheidt in a domino aldol/cyclisation reaction catalysed by NHC catalysts to yield chiral functionalised bicyclic b-lactones. Furthermore, comparable excellent enantioselectivities were also reported for other types of asymmetric transformations. For example, Birman described enantioselectivities of up to 97% ee in esterifications of (aalkylthio)- and (a-arylthio)alkanoic acids by using a tetramizole catalyst while esterifications of pyranones with a NHC catalyst developed by Wang provided up to 98% ee. In another context, several aldol reactions reinvestigated by Ward with L-proline as catalyst and by Johnson and Corbett with Cinchona alkaloid catalysts respectively afforded >98 and 92% ee. In addition, an enantioselectivity of 96% ee was obtained by Johnson and Goodman in the first NHC-catalysed cross-benzoin addition of aldehydes to b-halo glycolic esters. Finally, the first asymmetric isomerisation of asubstituted alkynoates into chiral trisubstituted allenoates was described by Takemoto, providing an enantioselectivity of 94% ee when promoted by a benzothiadiazine catalyst. Undoubtedly, the future direction in this field is to continue expanding the scope of organocatalytic dynamic kinetic resolutions through the employment of already known but also novel organocatalysts to even more types of (novel) transformations, and apply these powerful strategies to the synthesis of biologically interesting molecules including natural products.

3. 4.

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H. Pellissier / Tetrahedron xxx (2016) 1e18

Biographical sketch


le ne Pellissier carried out her PhD under the supervision of Dr G. Gil in Marseille He (France) in 1987. The work was focused on the reactivity of isocyanides. In 1988, she entered the Centre National de la Recherche Scientifique (CNRS) as a researcher. After a postdoctoral period in Professor K. P. C. Vollhardt’s group at the University of California, Berkeley, she joined the group of Professor M. Santelli in Marseille in 1992, where she focused on the chemistry of 1,8-bis(trimethylsilyl)-2,6-octadiene and its application to the development of novel very short total syntheses of steroids starting from
e de recherche (CNRS) 1,3-butadiene and benzocyclobutenes. She is currently charge
. at Aix-Marseille Universite

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