Christian Orrenius

CHIRAL RECOGNITION OF ALCOHOL ENANTIOMERS IN ACYL TRANSFER REACTIONS CATALYSED BY CANDIDA ANTARCTICA LIPASE B

A description of the substrate-enzyme interactions involved in the discrimination of see-alcohol enantiomers in acyl transfer reactions catalysed by the highly enantioselective Candida antarctica lipase B is presented. Experimentally found activities and enantioselectivities from kinetic resolutions of a series of secondary alcohol substrates were used together with molecular modelling for the elucidation of the stereoselective substrate-enzyme interactions. Matching experimental and calculated results allowed conclusions regarding the orientation of the tetra-hedral intermediates in the active site. The finding, valid for substrates of a specific activity above 1 mol min-1 mg-1 protein, describes the origin of enantioselectivity as a combination of a binding site of limited size, the ”stereospecificity pocket“, and principally different productive orientations of the two enantiomers.

Molecular recognition of sec-alcohol enantiomers by Candida antarctica lipase B

Abstract A model to explain the enantioselectivity of Candida antarctica lipase B towards sec -alcohols based on structure activity and molecular modelling is presented. The origin of the enantioselectivity was found to be due to different modes of binding for the enantiomers. The fast enantiomer places its medium substituent in a site of limited size, the stereoselectivity pocket, whereas the slow enantiomer has to position the large substituent in that same pocket. Our model is in agreement with the 24 different substrates tested. Only substituents smaller than n -propyl can be accommodated by the stereoselectivity pocket. Moreover, important unfavourable electrostatic interactions are involved between this region and halogenated substituents. The former requirement entails a high enantiomeric ratio ( E ) for sec -alcohols with a medium group smaller than n -propyl and a large group larger than n -propyl. The latter requirement allows high E only for short chain vic -halogenated alcohols.

The Candida antarctica lipase B catalysed kinetic resolution of seudenol in non-aqueous media of controlled water activity

Abstract For further investigation of the kinetic resolutions in transesterification reactions with the highly enantioselective Candida antarctica lipase B, an easy to study model reaction with one typical substrate, the Douglas Fir Beetle pheromone 3-methyl-2-cyclohexen-1-ol (Seudenol), was developed. The influence of the nature of the solvent and thermodynamic water activity was studied. Initial rates showed constant or progressively increasing values with increasing water activity. Enantioselectivity depended on the choice of solvent and descended in most cases with increasing water activity. No general correlations of enantioselectivity or activity with physicochemical constants of the solvent were found. However, at a water activity of 0.11 a tendency toward optimum hydrophobicity (i.elog P ≈ for enantioselectivity was observed. Enantiomeric ratios were in the range 8–32.

Kinetic resolutions of amine and thiol analogues of secondary alcohols catalyzed by the Candida antarctica lipase B

Abstract Chiral amine and thiol analogues of secondary alcohols were tested as substrates for the Candida antarctica lipase B with emphasis on lipase enantioselectivity and substrate reactivity. Although the apparent enantiomeric ratios were lower for the amines than for the corresponding alcohols, the amines 2-octylamine and 1-phenyl-ethylamine were produced with high optical purity (ee-93–99%) in a lipase-catalyzed acyl transfer reaction with ethyl octanoate as the acyl donor. The initial rates were lower and the reaction times longer for the amines than for the corresponding alcohols. The thiols 2-octanethiol and 1-phenylethanethiol did not yield any thiol ester product in the analogous lipase-catalyzed acyl transfer reaction. By preparing the thiooctanoate of 1-phenyl-ethanethiol and employing it as acyl donor in a lipase-catalyzed reaction with 1-phenylethanol as acyl acceptor, however, 1-phenylethanethiol could be resolved with high enantiomeric excess of the ( R )-enantiomer (ee-95%) and moderate enantiomeric excess of the ( S )-enantiomer (ee-75%).

Candida antarctica lipase B catalysed kinetic resolutions: Substrate structure requirements for the preparation of enantiomerically enriched secondary alcanols

Abstract The lipase B of the Candida antarctica yeast displays high enantioselectivity in transesterification reactions with chiral secondary alcohols in non-aqueous media. This was exploited to resolve a series of racemates structurally related to 2-octanol, namely 3-hydroxy-1-undecyne, 3-hydroxy-1-nonene, 3-nonanol, 1-chloro-2-octanol, 2-methyl-3-nonanol, 2,2-dimethyl-3-nonanol. The substrates were designed to probe the alcohol binding part of the active site of the lipase. The first four racemates could be resolved to produce compounds of high enantiomeric purity. A lipase catalysed transesterification of 1-chloro-2-octanol was observed. 2-Methyl-3-nonanol and 2,2-dimethyl-3-nonanol did not form any detectable amounts of product ester. The kinetic resolutions of the alcohols were performed with S-ethyl thiooctanoate as the acyl donor.

Enantiomerically enriched bifunctional sec-alcohols prepared by Candida antarctica lipase B catalysis. Evidence of non-steric interactions

Abstract Transesterification catalysed by Candida antarctica lipase B was used for the preparation of enantiomerically enriched bifunctional sec -alcohols. The enantiomeric ratio, E , for 3-butyn-2-ol was increased from 1.3 to over 500 by adding an easily removable protecting group. Kinetic resolution of bromohydrins and chlorohydrins bearing a halogen on the large substituent showed high enantioselectivity ( E >80). On the other hand, halohydrins with the halogen on the medium group showed low E . Large differences in enantioselectivity were found by substituting the halogen atom of 1-halo-2-alkanols by a methyl group. These differences corresponded to more than 2 kcal mol −1 and were ascribed to non-steric interactions.

Preparation of 1-pyridinylethanols of high enantiomeric purity by lipase catalysed transesterifications

Abstract Component B lipase of the Candida antarctica yeast displays high enantioselectivity in catalysing transesterification reactions in non-aqeuous media with chiral secondary alcohols. This was exploited to resolve racemates of 1-(pyridinyl)-ethanols, 1- (6-bromopyridin-2-yl)ethanol, and 1-(6-bromopyridin-2-yl)-2,2-dimethylpropanol. The lipase esterified the ( R )-alcohols of the first four substrates in > -99% enantiomeric excess in less than three hours with 30–40% isolated yield. Remaining ( S )-enantiomers were isolated in similar yields and in 97–98% ee. 1-(6-Bromopyridin-2-yl)-2,2-dimethylpropanol did not form any detectable ester in one week.