Highly efficient and enantioselective synthesis of l-arylglycines and d-arylglycine amides from biotransformations of nitriles

The Application of Biocatalysis in the Preparation and Resolution of Morita-Baylis-Hillman Adducts and Their Derivatives

The Morita-Baylis-Hillman (MBH) reaction affords highly functionalised allylic alcohols containing a new stereogenic centre. These MBH adducts are very versatile and have been transformed into a large range of products, some of which have medicinal potential. Several examples of asymmetric syntheses of MBH adducts have been reported, although a generally applicable method remains to be developed. Biocatalytic approaches for the synthesis and enzymatic kinetic resolution of MBH adducts have been reported, and are discussed in detail in this review. Enzymes able to catalyse the asymmetric MBH reaction have been identified, but selectivity and efficiency have generally been low. Lipases, esterases and nitrile-converting enzymes have all been successfully applied in the resolution of MBH adducts, with excellent selectivity being realised in most cases.

Conversion of phenylglycinonitrile by recombinant Escherichia coli cells synthesizing variants of the arylacetonitrilase from Pseudomonas fluorescens EBC191

The conversion of phenylglycinonitrile (2-aminophenylacetonitrile) by Escherichia coli strains was studied, which recombinantly expressed the arylacetonitrilase (NitA) from Pseudomonas fluorescens EBC191 and different nitrilase variants with altered reaction specificities. The whole-cell catalysts which formed the wild-type nitrilase converted (R,S)-phenylglycinonitrile preferentially to (S)-phenylglycine with a low degree of enantioselectivity. A recombinant strain which formed a variant of NitA produced mainly (S)-phenylglycine amide from (R,S)-phenylglycinonitrile and a second variant showed an almost complete enantioconversion and produced (R)-phenylglycine and left (S)-phenylglycinonitrile. The microbial-produced (S)-phenylglycinonitrile was used to study the chemical racemisation of (S)-phenylglycinonitrile at alkaline pH values in order to establish a dynamic kinetic resolution of the substrate. Subsequently, the conversion of (R,S)-phenylglycinonitrile by the whole-cell catalysts was studied at a pH of 10.8 which allowed a sufficient racemisation rate of phenylglycinonitrile. Surprisingly, under these conditions, strongly increased amounts of (S)-phenylglycine were formed by the recombinant E. coli cells expressing the amide-forming nitrilase variant. The aminopeptidase PepA from E. coli was identified by the construction of a deletion mutant and subsequent complementation as responsible amidase activity, which converted (S)-phenylglycine amide to (S)-phenylglycine.

Ru-Catalyzed highly diastereoselective hydrogenation of N-tert-butylsulfinyl ketimines for the synthesis of aryl glycine derivatives

A Ru-Pincer catalyst enables the diastereoselective hydrogenation of N-tert-butylsulfinyl ketimino esters, affording a promising method for the synthesis of aryl glycine derivatives.

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.

Practical biocatalytic desymmetrization of meso-N-heterocyclic dicarboxamides and their application in the construction of aza-sugar containing nucleoside analogs

Amidase-catalyzed desymmetrization of meso-N-heterocyclic dicarboxamides under very mild conditions provided a highly efficient and practical method for the preparation of enantiomerically pure carbamoyl-substituted heterocyclic amino acids that were unique and versatile platforms for the construction of both antipodes of aza-sugar containing nucleoside analogs.

An Improved Method for the Conversion of Nitriles to Amides Using N,N-Disubstituted Hydroxylamine

An improved method for the selective hydration of nitriles to amides employing N,N-disubstituted hydroxylamine is described leading to a reduction in reaction time and improved yield. Amides having electron-donating groups, which were not reported using the original method, were obtained in excellent yields. The amount of N,N-disubstituted hydroxylamine was reduced from three equivalents to one equivalent and the use of water as the reaction medium is environmentally friendly.

Asymmetric Strecker synthesis of α-arylglycines

A practically simple three-component Strecker reaction for the asymmetric synthesis of enantiopure α-arylglycines has been developed. Addition of a range of aryl-aldehydes to a solution of sodium cyanide and (S)-1-(4-methoxyphenyl)ethylamine affords highly crystalline (S,S)-α-aminonitriles that are easily obtained in diastereomerically pure form. Heating the resultant (S,S)-α-aminonitriles in 6 M aqueous HCl at reflux resulted in cleavage of their chiral auxiliary fragments and concomitant hydrolysis of their nitrile groups to afford enantiopure (S)-α-arylglycines. The enantiopurities of these (S)-α-arylglycines were determined via derivatization of their corresponding methyl esters with 2-formylphenylboronic acid and (S)-BINOL, followed by (1)H NMR spectroscopic analysis of the resultant mixtures of diastereomeric iminoboronate esters.

Facile and highly selective conversion of nitriles to amides via indirect acid-catalyzed hydration using TFA or AcOH-H2SO4

Both aliphatic and aromatic nitriles are conveniently and selectively converted in a single step, via an indirect acid-catalyzed hydration, into the corresponding amides in 1-8 h using a TFA-H2SO4 mixture as a reagent system. Although the same reagent did not work for the sterically hindered nitriles such as mesitonitrile, the transformation could be accomplished by changing TFA to AcOH at higher temperatures (>90 degrees C).

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.

Fast enantioseparation of arylglycine amides by capillary electrophoresis with highly sulfated-beta-cyclodextrin as a chiral selector

Nine racemic arylglycine amides were synthesized and successfully enantioseparated by capillary electrophoresis (CE) using highly sulfated beta-cyclodextrin (HS-beta-CD) as a chiral selector. Baseline enantioseparation of the analytes was obtained around neutral pH but not in the acidic conditions that are commonly used. HS-beta-CD content, buffer pH, type and concentration, and organic modifier concentration were studied and optimized for fast and efficient separation. A chiral CE separation system composed of 1.5% (w/v) HS-beta-CD, 0 to 10% (v/v) methanol and 20 mM 3-(N-morpholino)propanesulfonic acid at pH 6.5 was shown suitable for baseline enantioseparation of the mentioned amides within 6 min, including simultaneous enantioseparation of three positional isomer series (methyl-, methoxyl or chloro-substituted). By using this system, D-enantiomers migrated ahead of the L-enantiomers and the enantiomeric resolution order of arylglycine amides was more or less parallel to the pK(a), order of the analytes.

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.