Synthesis of Chiral Epihalohydrins Using Haloalcohol Dehalogenase A from Arthrobacter Erithii H10a

Abstract

Investigation of the epoxide enantiomers formed by the action of the haloalcohol dehalogenase from Arthrobacter erithii H10a revealed that (r)-epichlorohydrin (ECH) was selectively produced from 1,3-dichloro-2-propanol (1,3-DCP). A maximum enantiomeric excess (e.e. > 95%) of (r)-ECH was obtained when dehalogenation of 1,3-DCP occurred in the presence of an excess of KBr. During the reverse reaction, (r)-ECH was stereoselectively halogenated to form 1,3-DCP if the halogen in the reaction mixture was chloride; however, if chloride was substituted by bromide, the (s)-isomer was halogenated preferentially, resulting in the accumulation of the (r)-isomer. (r)-epibromohydrin (EBH) was formed as the result of transhalogenation. If the starting substrates were EBH and KCl, the (r)-isomer was selectively chlorinated while the transhalogenation product was (s)-ECH.

Introduction

Concern over chiral ballast in pharmaceutical and agrochemical products has led to significant interest in the synthesis of small chiral synthons to be used in their manufacture. One such synthon is optically active epichlorohydrin, a versatile C₃ chiral building block, used in the synthesis of a range of chiral products such as β-adrenergic blockers, vitamins, pheromones, and new materials such as ferro-electric crystals.1

Chemical synthesis of optically active ECH from mannitol2 and glycidol (GDL) using the Sharpless epoxidation3 requires expensive starting materials, can be technically difficult, and the resultant enantiomeric excess (e.e.) of the isomer formed is often low (GDL 91% e.e. synthesized using the Sharpless method). Biotechnological routes have also been considered for such synthesis. Iruchijima et al.4 synthesized (r)-ECH via (s)-1-acetoxy-2,3-dichloropropane obtained by asymmetric hydrolysis of the racemate using pancreatin and steapsin from hog pancreas; however, the yields and optical purity of the epoxide were too low for the process to be considered commercially viable (yield 15.4%, > 90% e.e.). Optically pure (r)-ECH could be obtained using an alkene-degrading bacterium, Nocardia sp. H8 capable of stereoselectively degrading ECH.5 Habets-Crutzen et al.6 studied the synthesis of optically active ECH from allylchloride using alkene-assimilating bacteria; however, since (s)-ECH was toxic to the biocatalyst, only low concentrations were produced at low enantiomeric excess (80–98%).

Most of the methods reported using dehalogenating bacteria have been based on the enantioselective degradation of racemic mixtures. The process patented by ICI for the production of l-2-monochloropropionate,7 utilized a d-specific 2-haloacid dehalogenase to selectively remove the d enantiomer from the racemic mixture. Similarly, bacteria that assimilated stereospecifically either isomer of 2,3-dichloro-1-propanol (2,3-DCP)1., 8. or 3-chloro-1,2-propanediol (3-CPD)9., 10. have been used in the preparation of chiral haloalcohols. The unassimilated haloalcohol, accumulated in the growth medium, was subsequently converted chemically to the epoxide by addition of hydroxide. Using this approach, both isomers of ECH and GDL were obtained at high enantiomeric excess (> 99%).

A disadvantage of optical resolution methodology based on enantioselective biodegradation is that the yield of the desired enantiomer is less than 50%. From an industrial point of view, the production of optically active compounds by enantioselective biotransformation of prochiral starting materials is more attractive since a quantitative yield of the desired enantiomer may be obtained. Nakamura et al.11 studied such a transformation of 1,3-DCP to r-3-CPD by Corynebacterium sp. N-1074. Using this approach, a molar conversion yield of 97.3% was obtained but the enantiomeric excess of the r-3-CPD product (87.3%) again was too low for the process to be useful as an industrial route to the chiral synthon.

Arthrobacter erithii H10a was isolated as a member of a stable microbial consortium which utilized halohydrins as sources of carbon and energy.12 The biochemical characteristics of the purified haloalcohol dehalogenase A (DehA, haloalcohol hydrogen-halide lyase, EC 4.5.1) from A. erithii H10a have been described.13 This paper describes the stereospecificity of 1,3-DCP dehalogenation and epihalohydrin halogenation catalyzed by DehA.