Enantioselective synthesis of chiral cyclopropane compounds through microbial transformations of trans-2-arylcyclopropanecarbonitriles

Enantioselective biotransformations of nitriles in organic synthesis

The hydration and hydrolysis of nitriles are valuable synthetic methods used to prepare carboxamides and carboxylic acids. However, chemical hydration and hydrolysis of nitriles involve harsh reaction conditions, have low selectivity, and generate large amounts of waste. Therefore, researchers have confined the scope of these reactions to simple nitrile substrates. However, biological transformations of nitriles are highly efficient, chemoselective, and environmentally benign, which has led synthetic organic chemists and biotechologists to study these reactions in detail over the last two decades. In nature, biological systems degrade nitriles via two distinct pathways: nitrilases catalyze the direct hydrolysis of nitriles to afford carboxylic acids with release of ammonia, and nitrile hydratases catalyze the conversion of nitriles into carboxamides, which then furnish carboxylic acids via hydrolysis in the presence of amidases. Researchers have subsequently developed biocatalytic methods into useful industrial processes for the manufacture of commodity chemicals, including acrylamide. Since the late 1990s, research by my group and others has led to enormous progress in the understanding and application of enantioselective biotransformations of nitriles in organic synthesis. In this Account, I summarize the important advances in enantioselective biotransformations of nitriles and amides, with a primary focus on research from my laboratory. I describe microbial whole-cell-catalyzed kinetic resolution of various functionalized nitriles, amino- and hydroxynitriles, and nitriles that contain small rings and the desymmetrization of prochiral and meso dinitriles and diamides. I also demonstrate how we can apply the biocatalytic protocol to synthesize natural products and bioactive compounds. These nitrile biotransformations offer an attractive and unique protocol for the enantioselective synthesis of polyfunctionalized organic compounds that are not readily obtainable by other methods. Nitrile substrates are readily available, and the mild reaction conditions are specific toward cyano and amido functional groups without interfering with other reactive functional groups. I anticipate that further advances in this field will lead to new and engineered nitrile-hydrolyzing enzymes or catalytic systems with improved activity and altered selectivity. These advances will broaden the scope of these transformations and their applications in organic synthesis.

Microbial transformation of nitriles to high-value acids or amides

Biotransformation of nitriles mediated by nitrile-amide converting enzymes has attracted considerable attention and developed tremendously in the recent years in China since it offers a valuable alternative to traditional chemical reaction which requires harsh conditions. As a result, an upsurge of these promising enzymes (including nitrile hydratase, nitrilase and amidase) has been taking place. This review aims at describing these enzymes in detail. A variety of microorganisms harboring nitrile-amide converting activities have been isolated and identified in China, some of which have already applied with moderate success. Currently, a wide range of high-value compounds such as aliphatic, alicyclic, aromatic and heterocyclic amides and their corresponding acids were provided by these nitrile-amide degrading organisms. Simultaneously, with the increasing demand of chiral substances, the enantioselectivity of the nitrilase superfamily is widely investigated and exploited in China, especially the bioconversion of optically active alpha-substituted phenylacetamides, acids and 2,2-dimethylcyclopropanecarboxamide and 2,2-dimethylcyclopropanecarboxylic acid by means of the catalysts exhibiting excellent stereoselectivity. Besides their synthetic value, the nitrile-amide converting enzymes also play an important role in environmental protection. In this context, cloning of the genes and expression of these enzymes are presented. In the near future in China, an increasing number of novel nitrile-amide converting organisms will be screened and their potential in the synthesis of useful acids and amides will be further exploited.

Highly efficient and enantioselective biotransformations of β-lactam carbonitriles and carboxamides and their synthetic applications

Catalyzed by Rhodococcus erythropolis AJ270, a nitrile hydratase and amidase containing microbial whole cell catalyst, a number of racemic 1-arylmethyl- and 1-allyl-4-oxoazetidine-2-carbonitriles and carboxamides underwent efficient transformations under very mild conditions to produce enantiopure functionalized S-amide and R-acid products in excellent yields. While the nitrile hydratase showed good enzyme activity but virtually no enantioselectivity, the amidase displayed high R-enantioselectivity against almost all amide substrates tested. The synthetic applications of the resulting functionalized chiral β-lactam derivatives were demonstrated by the facile preparation of β-lactam-fused heterocyclic compounds.

Remarkable Electronic and Steric Effects in the Nitrile Biotransformations for the Preparation of Enantiopure Functionalized Carboxylic Acids and Amides: Implication for an Unsaturated Carbon−Carbon Bond Binding Domain of the Amidase

Biotransformations of various functionalized racemic nitriles catalyzed by Rhodococcus erythropolis AJ270, a nitrile hydratase/amidase-containing microbial whole-cell catalyst, were studied. While the nitrile hydratase exhibits high catalytic efficiency but very low enantioselectivity against almost all nitrile substrates examined, the amidase is very sensitive toward the structure of the amides. The release of the steric crowdedness around the stereocenter of the substrates and the introduction of an unsaturated carbon-carbon bond into the substrates led to the significant acceleration of the reaction rate and the dramatic enhancement of the enantioselectivity. Nitrile biotransformations provide a unique and high-yielding synthetic route to highly enantiopure carboxylic acids and amides functionalized with an allyl, propargyl, allenyl, or vinyl group. The synthetic applications have been demonstrated by the synthesis of enantiopure heterocyclic compounds including iodoenol gamma-lactone, gamma-lactam, and 3-allyl-1-phenyl-3,4-dihydro-1H-quinolin-2-one derivatives.

Nitrile Biotransformations for the Efficient Synthesis of Highly Enantiopure 1-Arylaziridine-2-carboxylic Acid Derivatives and Their Stereoselective Ring-Opening Reactions

Catalyzed by the Rhodococcus erythropolis AJ270 whole cell catalyst under very mild conditions, biotransformations of racemic 1-arylaziridine-2-carbonitriles proceeded efficiently and enantioselectively to produce highly enantiopure S-1-arylaziridine-2-carboxamides and R-1-arylaziridine-2-carboxylic acids in excellent yields. Although the nitrile hydratase exhibits no selectivity against all nitrile substrates, the amidase is highly R-enantioselective towards 1-arylaziridine-2-carboxamides. When treated with benzyl bromide, 1-phenylaziridine-2S-carboxamide underwent a highly regioselective and enantiospecific ring-opening reaction to afford an almost quantitative yield of R-beta-[(benzyl)phenylamino]-alpha-bromopropanamide (C-2 attack) and R-alpha-[(benzyl)phenylamino]-beta-bromopropanamide (C-3 attack) in a 10.5:1 ratio. Further treatment of the resulting ring-opening products with an N-nucleophilic reagent such as amine and azide led to, through most probably the aziridinium intermediate, the formation of S-alpha-substituted-beta-[(benzyl)phenylamino]propanamides in good chemical yields with high enantiomeric purity.

Asymmetric Synthesis of Cyclopropylamines Starting from N-Sulfinyl α-Chloro Ketimines

Treatment of novel chiral N-sulfinyl alpha-chloro ketimines with Grignard reagents resulted in the synthesis of chiral N-(1-substituted cyclopropyl)-tert-butanesulfinamides in acceptable to good yields and diastereoselectivity via 1,3-dehydrohalogenation and subsequent addition of the Grignard reagent to the intermediate cyclopropylideneamine. Only in the case of allylmagnesium chloride did the reaction lead to aziridines in high yield. Further deprotection toward N-unprotected 1-substituted cyclopropylamines was established, and the absolute configuration was determined. [reaction: see text].

An Unusual β-Vinyl Effect Leading to High Efficiency and Enantioselectivity of the Amidase, Nitrile Biotransformations for the Preparation of Enantiopure 3-Arylpent-4-enoic Acids and Amides and Their Applications in Synthesis

Biotransformations of 3-arylpent-4-enenitriles catalyzed by Rhodococcus erythropolis AJ270, a nitrile hydratase/amidase-containing microbial whole-cell catalyst were studied, and an unusual beta-vinyl effect of the substrate on the biocatalytic efficiency and enantioselectivity of the amidase was observed. While 3-arylpent-4-enenitriles and 3-phenylpentanenitrile were efficiently hydrated by the action of the less R-enantioselective nitrile hydratase, the amidase showed greater activity and higher enantioselectivity against 3-arylpent-4-enoic acid amides than 3-arylpentanoic acid amides. Under very mild conditions, nitrile biotransformations provided an efficient synthesis of highly enantiopure (R)-3-arylpent-4-enoic acids and (S)-3-arylpent-4-enoic acid amides, and their applications were demonstrated by the synthesis of chiral gamma-amino acid, 2-pyrrolidinone, and 2-azepinone derivatives.

Synthesis of 1-Arylmethyl-2-(cyanomethyl)aziridines and Their Ring Transformation into Methyl N-(2-Cyanocyclopropyl)benzimidates

1-Arylmethyl-2-(cyanomethyl)aziridines were prepared in high yields from the corresponding 2-(bromomethyl)aziridines upon treatment with potassium cyanide in DMSO. Ring opening of the aziridine moiety with N-chlorosuccinimide in CCl4 and subsequent treatment of the thus formed 4-chloro-3-(N-chloro-N-(alpha,alpha-dichlorobenzyl)amino)butanenitriles with sodium methoxide in methanol resulted in novel methyl N-(2-chloro-1-(cyanomethyl)ethyl)benzimidates, although in low yields. The latter gamma-chloro nitriles were smoothly converted into methyl N-(2-cyanocyclopropyl)benzimidates as precursors of biologically relevant beta-ACC derivatives through a 1,3-cyclization protocol by reaction with potassium tert-butoxide in THF.

Nitrile Biotransformations for Highly Enantioselective Synthesis of Oxiranecarboxamides with Tertiary and Quaternary Stereocenters; Efficient Chemoenzymatic Approaches to Enantiopure α-Methylated Serine and Isoserine Derivatives

[reaction: see text] Biotransformations of a number of differently substituted and configured oxiranecarbonitriles using Rhodococcus sp. AJ270, a microbial whole-cell catalyst that contains nitrile hydratase/amidase, were studied. While almost all trans-configured 3-aryl-2-methyloxiranecarbonitriles and 2,3-dimethyl-3-phenyloxiranecarbonitrile were efficiently hydrated by the action of the less enantioselective nitrile hydratase, the amidase exhibited excellent 2S,3R-enantioselectivity against 2-methyl-3-(para-substituted-phenyl)oxiranecarboxamides. Under very mild conditions, biotransformations of nitriles provided an efficient and practical synthesis of 2R,3S-(-)-3-aryl-2-methyloxiranecarboxamides, electrophilic epoxides with tertiary and quaternary stereocenters, in excellent yield with enantiomeric excess greater than 99.5%. The synthetic applications of the resulting enantiomerically pure epoxides were demonstrated by convenient and straightforward syntheses of polyfunctionalized chiral molecules possessing a quaternary stereocenter such as R-(+)-2-hydroxy-2-methyl-3-phenylpropionic acid, 2R,3R-(-)-3-amino-2-hydroxy-2-methyl-3-phenylpropionic acid, and 2S,3S-(+)-2-amino-3-hydroxy-2-methyl-3-phenylpropionic acid, employing the regio- and stereospecific epoxide ring opening reactions of 2R,3S-(-)-2-methyl-3-phenyloxiranecarboxamide as the key steps.

Expression and purification of a recombinant enantioselective amidase

Microbacterium sp. AJ115 metabolises a wide range of nitriles using the two-step nitrile hydratase/amidase pathway. In this study, the amidase gene of Microbacterium sp. AJ115 has been inserted into the pCal-n-EK expression vector and expressed in Escherichia coli BL21(DE3)pLysS. The expressed protein is active in E. coli and expression of the amidase gene allows E. coli to grow on acetamide as sole carbon and/or nitrogen source. Expression of active amidase in E. coli was temperature dependent with high activity found when cultures were grown between 20 and 30 degrees C but no activity at 37 degrees C. On induction, the amidase represents 28% of the total soluble protein in E. coli. The expressed amidase has been purified in a single step from the crude lysate using the calmodulin-binding peptide (CBP) affinity tag. The V(max) and K(m) of the purified enzyme with acetamide (50 mM) were 4.4 micromol/min/mg protein and 4.5mM, respectively. The temperature optimum was found to be 50 degrees C. Purified enzyme demonstrated enantioselectivity with the ability to preferentially act on the S enantiomer of racemic (R,S)-2-phenylpropionamide. S-2-phenylpropionic acid is produced with an enantiomeric excess of >82% at 50% conversion of the parent amide.

An Expedient and Practical Method for the Synthesis of a Diverse Series of Cyclopropane α-Amino Acids and Amines

A practical synthesis for the preparation of a diverse series of cyclopropane alpha-amino acids is described. Nitrocyclopropane carboxylates can be readily prepared through treatment of alpha-nitroesters and iodobenzene diacetate or alpha-nitro-alpha-diazoesters with a Rh(II) catalyst and an olefin. Reduction of the nitro group using zinc/HCl in i-PrOH affords substituted cyclopropane alpha-amino esters in modest to high yields (54-99%). A "one-pot" procedure involving sequential cyclopropanation and reduction is described. The method can also be applied to the preparation of arylcyclopropyl amines (three examples).

Nitrile biotransformations for highly efficient and enantioselective syntheses of electrophilic oxiranecarboxamides

Catalyzed by a nitrile hydratase/amidase-containing microbial Rhodococcus sp. AJ270 whole-cell catalyst, a number of racemic trans-2,3-epoxy-3-arylpropanenitriles 1 underwent rapid and efficient hydrolysis under very mild conditions to afford 2R,3S-2-arylglycidamides 2 in excellent yield with enantiomeric excess higher than 99.5%. The overall enantioselectivity of the biotransformations originated from the combined effects of a dominantly high 2S-enantioselective amidase and low 2S-enantioselective nitrile hydratase involved in the cell. The influence of the substrates on both reaction efficiency and enantioselectivity was also discussed in terms of steric and electronic effects.

Nitrile biotransformation for highly enantioselective synthesis of 3-substituted 2,2-dimethylcyclopropanecarboxylic acids and amides

Biotransformations of differently configured 2,2-dimethyl-3-substitued-cyclopropanecarbonitriles were studied using a nitrile hydratase/amidase-containing Rhodococcus sp. AJ270 whole-cell catalyst under very mild conditions. Although all of the cis-3-aryl-2,2-dimethylcyclopropanecarbonitriles appeared inert toward the biocatalyst, a number of racemic trans-isomers efficiently underwent a highly enantioselective hydrolysis to produce (+)-(1R,3R)-3-aryl-2,2-dimethylcyclopropanecarboxylic acids and (-)-(1S,3S)-3-aryl-2,2-dimethylcyclopropanecarboxamides in high yields with excellent enantiomeric excesses in most cases. The overall enantioselectivity of the biotransformations of nitriles originated from the combined effects of 1R-enantioselective nitrile hydratase and amidase, with the later being a dominant factor. The influence of the substrates on both reaction efficiency and enantioselectivity was discussed in terms of steric and electronic effects. Coupled with chemical transformations, biotransformations of nitriles provided convenient syntheses of optically pure geminally dimethyl-substituted cyclopropanecarboxylic acids and amides, including chrysanthemic acids, in both enantiomeric forms.

Practical and convenient enzymatic synthesis of enantiopure alpha-amino acids and amides

Catalyzed by the nitrile hydratase and the amidease in Rhodococcus sp. AJ270 cells under very mild conditions, a number of alpha-aryl- and alpha-alkyl-substituted DL-glycine nitriles 1 rapidly underwent a highly enantioselective hydrolysis to afford D-(-)-alpha-amino acid amides 2 and L-(+)-alpha-amino acids 3 in high yields with excellent enantiomeric excesses in most cases. The overall enantioselectivity of the biotransformations of nitriles originated from the combined effects of a high L-enantioselective amidase and a low enantioselective nitrile hydratase. The influence of the substrates on both reaction efficiency and enantioselectivity was also discussed in terms of steric and electronic effects. Coupled with chemical hydrolysis of D-(-)-alpha-phenylglycine amide, biotransformation of DL-phenylglycine nitrile was applied in practical scale to produce both D- and L-phenylglycines in high optical purity.

Selective biotransformation of substituted alicyclic nitriles by Rhodococcus equi A4

Nitrile hydratase from Rhodococcus equi A4 discriminated between geometric isomers of substituted alicyclic nitriles. The enzyme transformed trans-4-benzoyloxycyclohexanecarbonitrile (trans-1a), cis-3-benzoyloxy cyclohexanecarbonitrile (cis-2a), trans-2-hydroxycyclohexanecarbonitrile (trans-3a), and trans-2-hydroxycyclo pentanecarbonitrile (trans-4a) into the corresponding amides. On the contrary, cis-2-hydroxycyclohexanecarbonitrile (cis-3a) and cis-2-hydroxycyclopentanecarbonitrile (cis-4a) were not converted to a significant extent. cis-4-Ben zoyl oxycyclohexanecarbonitrile (cis-1a) was also a substrate of the enzyme but reacted slowly. Diequatorial arrangement of the substituents in trans-1a, cis-2a, and trans-3a appears to positively influence the activity of the nitrile hydratase.