VANADIUM-CATALYZED EPOXIDATION OF CYCLIC ALLYLIC ALCOHOLS - STEREOSELECTIVITY AND STEREOCONTROL MECHANISM

The stereochemistry of vanadyl acetylacetonate catalyzed epoxidation of cyclic allylic alcohols with tert-butyl hydroperoxide is examined and compared with that of the m-chloroperoxybenzoic acid epoxidation. The opposite direction of stereoselectivity is found for medium-ring alcohols. Thus, the cis epoxides are preferentially obtained by the vanadium-catalyzed epoxidation of (Z)-cyclooct-2-en-l-ol (1b), (1SR, 2RS)-(E)-cyclooct-2-en-l-ol (1g) (Z, Z)-2, 6-cyclooctadien-l-ol (9), (Z, Z)-2, 4-cyclooctadien-l-ol (10), (E)-and (Z)-cyclonon-2-en-l-ols (1h, 1e), and (E)-cyclododec-2-en-1-ol (li), whereas mainly the trans epoxides are produced by the peroxy acid epoxidation of them. (1 RS, 2RS)-(E)-cyclooct-2-en-l-ol (If) is an exception, which on vanadium-catalyzed epoxidation gives the trans epoxide selectively. In the vanadium-catalyzed epoxidation of conformationally biased 5-tert;-butylcyclohex-2-en-l-ols (16, 17), higher cis stereoselectivity is observed for the quasi-axial (17) than for the quasi-equatorial one (16). In the latter case, the vanadium-catalyzed oxidation of the alcohol function to give the conjugated enone becomes the main reaction pathway. From the results, a transition state geometry model involving a quasi-axial hydroxyl conformation can be proposed for the vanadium-catalyzed stereoselective epoxidation. © 1979, American Chemical Society. All rights reserved.