Enantioselectivity of the enzymatic hydrolysis of cyclohexene oxide and (±)-1-methylcyclohexene oxide: a comparison between microsomal and cytosolic epoxide hydrolases

Abstract

The hydrolysis of cyclohexene oxide and (±)-1-methylcyclohexene oxide by rabbit liver microsomal and cytosolic epoxide hydrolyse (mEH and cEH) has been investigated. Microsomal preparations hydrolysed the two epoxides at respective V_s of 17.0 and 3.5 nmol min^–1 mg^–1 protein, cytosolic preparations at V_s of 0.95 and 1.0 nmol min^–1 mg^–1 protein. (–)-(R,R)-Cyclohexane-trans-1,2-diol was formed in the mEH and cEH catalysed hydrolysis of cyclohexene oxide with 94% and 22% e.e. respectively. (–)-(R,R)-1-Methylcyclohexene-r-1,t-2-diol, whose absolute configuration was deduced by c.d. measurements of its bis(p-methoxybenzoate), was obtained by partial hydrolysis of (±)-1-methylcylohexene oxide by mEH. The e.e. of the enzymatically formed diol was 94% at 8% conversion and decreased to 56% around 28% conversion. Racemic 1-methylcyclohexane-r-1,t-2-diol was instead obtained in the cEH catalysed hydrolysis of (±)-1-methylcyclohexene oxide. The substrate enantioselectivity of the mEH catalysed hydrolysis of this trisubstituted epoxide is rationalized on the basis of a better stabilization of the transition state for the anti opening at the carbon with (S) configuration of the (1R, 2S)-enantiomer of the epoxide with (3,4 M) helicity, in agreement with the stereochemical course of the analogous reaction of unsubstituted cyclohexene oxide.