Epoxide hydrolase-catalysed kinetic resolution of a spiroepoxide, a key building block of various 11-heterosteroids

Two microbial epoxide, hydrolases - i.e., Aspergillus niger (AnEH) and Rhodococcus erythropolis (the so-called "Limonene EH": LEH) were used to achieve, for the first time, the biocatalysed hydrolytic kinetic resolution (BHKR) of spiroepoxide rac-1. This compound is a strategic key building block allowing the synthesis of 11-heterosteroids. Interestingly enough, the two enzymes exhibited opposite and therefore complementary enantioselectivity allowing us to isolate the residual (R,R)-1 (from AnEH) and the residual (SS)-1 (frorn LEH) in nearly enantiopure forms (> 98%). Their absolute configurations were determined by X-ray crystallography. An opposite regioselectivity of the oxirane ring opening for both enantiomers of substrate 1, determined using (H2O)-O-18 labelling and chiral GC-MS analysis, was also observed, corresponding to an attack at the less substituted carbon atom using AnEH, and at the most substituted carbon atom using LEH. A chemical process-improving methodology was also developed. This allowed us to obtain both enantiomers of the substrate in high enantiomeric purity (99%) and optimised quantity. In the case of the AnEH, the use of a biphasic (water/isooctane) reaction medium allowed us to increase the global substrate concentration up to 200 g/L. The preparation of both enantiomers of 1 clearly paves the way to the preparative scale synthesis and biochemical evaluation of the corresponding 11-heterosteroid enantiomers.