Stereoselectivity in the Epoxide Hydrolase Catalyzed Hydrolysis of the Stereoisomeric 3-tert-Butyl-1,2-epoxycyclohexanes. Further Evidence for the Topology of the Enzyme Active Site

Article abstract of DOI:10.1021/jo00137a015

(+/-)-cis-3-tert-Butyl-1,2-epoxycyclohexane is converted by rabbit liver microsomal epoxide hydrolase exclusively into the diaxial diol.The 1S,2R,3S enantiomer reacts at a much faster rate to yield the 1R,2R,3S diol, which is isolated at least 96percent optically pure in the first stages of the reaction, up to almost 50percent conversion. (+/-)-trans-3-tert-Butyl-1,2-epoxycyclohexane is a poorer substrate than the cis isomer: only the 1S,2R,3R epoxide undergoes slow enzymatic hydrolysis to produce exclusively optically pure 1R,2R,3R diol, the product of diequatorial opening, in contrast with the acid-catalyzed hydrolysis of the same epoxide that yields both the diequatorial and the diaxial diols.The absolute configurations of the diols and epoxides were established by chiroptical methods on appropriate derivatives.The enantiomeric excesses in the diols were determined with chiral shift reagents.The present results confirm previous hypotheses on the topology of the hydrolase active site, involving a large hydrophobic pocket situated in such a way as to accommodate bulky substituents to the right of the oxirane ring in the ES complex.They also are consistent with and supplement previous evidence on a general-base catalysis in the enzymatic reaction mechanism.