Acid-catalyzed and enzymatic hydrolysis of trans- and cis-2-methyl-3,4-epoxytetrahydropyran

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Abstract
Whereas the acid-catalyzed hydrolysis of trans- and cis-3,4-epoxy-2-methyltetrahydropyran gives the corresponding diols by opening both at C-4 and C-3 (64% attack at C-4 in the trans-epoxide, 86% in the cis-epoxide), the hydrolysis of the same substrates catalyzed by rabbit liver microsomal epoxide hydrolase is entirely regiospecific and involves in both cases exclusive attack at C-4. The racemic cis-epoxide reacts at a faster rate than the trans. The 2R,3R,4S-enantiomer of the latter epoxide reacts at a much faster rate than its antipode to yield the (–)-(2R,3R,4R)-diol which is isolated at least 98% optically pure up to almost 50% conversion when starting from the racemic substrate. A reference sample of optically pure (+)-(2S,3S,4S)-diol was prepared from L-rhamnose. The enantiomeric excess of the (–)-(2R,3R,4R)-diol was also determined more precisely through g.l.c. analysis of diastereoisomeric MTPA esters. The present results confirm previous hypotheses on the topology of the hydrolase active site and emphasize the overriding importance of the orientation of a lipophilic substituent near the oxirane ring.