Kurt Faber

Asymmetric microbial hydrolysis of epoxides

Abstract Kinetic resolution of 2-mono- and 2,2-disubstituted epoxides was accomplished using epoxide hydrolases from bacterial and fungal origin by employing lyophilized whole microbial cells. In all cases investigated, the biocatalytic hydrolysis was shown to proceed with retention of configuration at the stereogenic center leading to 1,2-diols and remaining epoxides. The selectivity of the reaction was dependent on the substrate structure and the strain used with E-values ranging from low or moderate (with 2-monosubstituted epoxides) to excellent (E >100, with 2,2-disubstituted oxiranes).

Prospects for the increased application of biocatalysts in organic transformations.

Predicting the future use of biocatalysts for the transformation of organic compounds--both natural substrates and other compounds--needs to take many factors into account. To date, relatively few biotransformations have been developed to the industrial scale, primarily because there has been little economic incentive to replace existing successful processes with biocatalysts, with their inherent problems of instability, lack of selectivity and narrow operational range. However, advances that improve biocatalyst performance, coupled with the increasing emphasis on chirotechnology, are driving the development of biocatalysis as a complementary, if not a rival technology to existing chemical approaches.

Sensitivity of microbial lipases to acetaldehyde formed by acyl-transfer reactions from vinyl esters

SummaryThe sensitivity of twenty six microbial lipases towards acetaldehyde (an unavoidable by-product in lipase-catalysed acyl transfer reactions with vinyl esters) was investigated. The sensitivity of an individual enzyme strongly depends on its properties such as microbial source, molecular weight and relative lysine content. Whereas the majority of enzymes (from Pseudomonas, Rhizopus, Chromobacterium, Mucor and Candida antarctica sp.) proved to be remarkably stable, lipases from Candida rugosa and Geotrichum candidum lost most of their activity when exposed to acetaldehyde.

Enzymatic acylation using acid anhydrides: Crucial removal of acid

Abstract An efficient enzymatic resolution of 7,7-disubstituted 1,4,5,6-tetrachlorobicyclo [2.2.1]hept-5-en-2-ols was accomplished by means of lipase AY-30 from Candida cylindracea in toluene. When acid anhydrides were used as acyl donors, the enantioselectivity was found to depend strongly on the reaction conditions: Whereas low selectivity (E 200). Alternatively, adsorption of the biocatalyst onto Celite was equally effective (E > 300). Complete specificity was obtained when vinyl acetate was used as acyl donor (E ∼ 1000).

Epoxide Hydrolases and Their Synthetic Applications

Chiral epoxides and 1,2-diols, which are central building blocks for the asymmetric synthesis of bioactive compounds, can be obtained by using enzymes--i.e. epoxide hydrolases--which catalyse the enantioselective hydrolysis of epoxides. These biocatalysis have recently been found to be more widely distributed in fungi and bacteria than previously expected. Sufficient sources from bacteria, such as Rhodococcus and Nocardia spp., or fungi, as for instance Aspergillus and Beauveria spp., have now been identified. The reaction proceeds via an SN2-specific opening of the epoxide, leading to the formation of the corresponding trans-configured 1,2-diol. For the resolution of racemic monosubstituted and 2,2- or 2,3-disubstituted substrates, various fungi and bacteria have been shown to possess excellent enantioselectivities. Additionally, different methods, which lead to the formation of the optically pure product diol in a chemical yield far beyond the 50% mark (which is intrinsic to classic kinetic resolutions), are discussed. In addition, the use of non-natural nucleophiles such as azides or amines provides access to enantiomerically enriched vicinal azido- and amino-alcohols. The synthetic potential of these enzymes for asymmetric synthesis is illustrated with recent examples, describing the preparation of some biologically active molecules.

Biocatalytic resolution of 2-methyl-2-(aryl)alkyloxiranes using novel bacterial epoxide hydrolases

Abstract Biocatalytic resolution of alkyl- and (arylalkyl)-oxiranes was accomplished by employing the epoxide hydrolase activity of lyophilized whole cells of seven bacterial strains belonging to the genera Rhodococcus, Mycobacterium and Nocardia . Whereas no suitable biocatalyst was found for 1-octene oxide, excellent selectivities were obtained with 2-methyl-2-substituted epoxides bearing an alkyl- or (aryl)alkyl side chain.

Selectivity-Enhancement of Hydrolase Reactions

This review pinpoints the strategies which can be employed to improve the enantio- and diasteroselectivity of hydrolytic enzymes, i.e. esterases, proteases, and lipases. The influence of variations of reactants, — enzyme and substrate — and conditions — kinetics, medium, temperature, pH — on the chiral recognition process of the enzyme is discussed with examples from the recent literature.

Selective transformation of nitriles into amides and carboxylic acids by an immobilized nitrilase

Abstract Using an immobilized nitrilase from Rhodococcus sp. mild and selective hydrolysis of nitriles can be achieved even in the presence of acid or base sensitive groups under neutral conditions. This method is applicable to a broad range of substrates as exemplified by aliphatic, alicyclic, heterocyclic and carbohydrate type nitriles.

A substrate model for the enzymatic resolution of esters of bicyclic alcohols by candida cylindracea lipase

Abstract By evaluation of results obtained from enzymatic resolution of twenty five structurally different esters of secondary alcohols possessing a bicyclo[2.2.1]heptane or bicyclo[2.2.2]octane framework a model was developed which is proposed as an aid for the design of substrates to obtain good acceptance and high enantioselection by Candida cylindracea lipase.

Molecular reasons for lipase-sensitivity against acetaldehyde

Abstract The molecular reasons for the sensitivity of microbial lipases towards acetaldehyde, emerging as unavoidable by-product from acyl transfer reactions employing vinyl esters as acyl donors, were shown to be associated with specific properties of lysine residues. Since the mechanism of deactivation involves the formation of Schiff bases at the lysine e-amino groups, the relative reactivity (i.e. nucleophilicity) of each residue was estimated by using an electronic (p K a value) and a steric parameter (accessible surface area of the side chain). Sensitive lipases, as from Candida rugosa and Geotrichum candidum , possess several lysine residues that have high p K a values (> 12) and are highly exposed to the solvent (surface areas of 210–220 A 2 ). In contrast, the lysine groups of stable lipases like from Rhizomucor miehei, Candida antarctica B and Pseudomonas glumae have moderate p K a values (up to 11.6) and are rather buried (surface areas of 130–150 A 2 ). A close investigation of Candida rugosa lipase revealed that the most exposed lysine residues are located in the lid region (Lys75 and Lys85). The data suggest that Lys75, which is involved in fixing the lid in its open conformation, is presumably the prime target for deactivation by acetaldehyde.