Hydroxyl-assisted epoxidation

Hydroxyl-assisted epoxidation using TS-1/H202 is chemo- and stereoselective (165). Thus, when cyclopent-2-en-l-ol or cyclohex-2-en-l-ol was treated with... 

As a further example of a hydroxyl-assisted epoxidation, geraniol and nerol bearing two isolated C=C double bonds were regioselectively epoxidized with TS-1 at the 2-position (near the OH group), as reported by Kumar et al. (795). On the basis of these results, Kumar et al. (195) proposed that the transition state of the epoxidation of allylic alcohols involves coordination of the alcoholic functional group to the Ti active site and that the double bond interacts with one of the peroxidic oxygen atoms, not with the titanium site (Scheme 9). 

The stereoselectivity of this reaction is probably related to the preference of 1 for adopting conformation A, in which steric interactions between the Cs-ethyl and the Ci-substituents are minimized. Hydroxyl-assisted epoxidation (5, 76) then leads to 2. 

However, suitably located hydroxyl and acetoxyl functions can assist the cisoid approach of the peracid reagent. While 4jS-acetoxycholest-5-ene gives a 9 1 ratio of the a- and jS-epoxides, the 4a-acetoxy-5-ene yields the a-epoxide exclusively. The directive effect can be used to prepare and /9-oxiranes from A -steroids as illustrated by the epoxidation of (16) and (18). 

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