Chemoenzymatic approaches to the synthesis of the (1S,2R)-isomer of benzyl 2-hydroxycyclohexanecarboxylate

We examined ten strains of cultured whole-cell yeasts for the asymmetric reduction of commercially available ethyl 2-oxocyclohexanecarboxylate, and found that the (1S,2S)-stereoisomer of ethyl 2-hydroxycyclohexanecarboxylate was the major stereoisomer produced by Williopsis californica JCM 3600. The ethyl group of the ester was then substituted with a benzyl group with low volatility and increased hydrophobicity to facilitate the isolation of the expected product. Incubation with W. californica furnished benzyl (1S,2S)-2-hydroxycyclohexanecarboxylate (>99.9% ee) in 51.0% yield together with its (1R,2S)-isomer (>99.9% ee) in 35.4% yield. Upon treatment of the same substrate bearing the benzyl ester with a screening kit of purified overexpressed carbonyl reductases (Daicel Chiralscreen^® OH), two enzymes (E031, E078) furnished the (1R,2S)-isomer as the major product. With another enzyme (E007), the (1S,2R)-isomer was obtained, but its ee was very low (25.6%). The highly enantiomerically enriched (1S,2S)-isomer obtained by W. californica was transformed to the (1S,2R)-isomer (>99.9% ee), whose availability until now has been low, in 43.3% yield over two steps involving tosylation and subsequent inversive attack with tetrabutylammonium nitrite.