RING-OPENING OF ALPHA-PINENE EPOXIDE

The acid-catalysed opening of the epoxide ring of alpha-pinene epoxide (2alpha,3alpha-epoxypinene) has been shown to proceed without participation of cyclobutane ring expansion in the rate-determining step. The reaction cannot proceed by initial C-O bond fission, as intermediates generated by either fission are on other reaction pathways. It is suggested that the reaction proceeds by fission of the C-C bond to give a heterocyclic intermediate, which can ring open, and finally expand the cyclobutane ring in the product-determining step. Reactions which could proceed by a similar mechanism are discussed. The corresponding reaction of beta-pinene epoxide (2beta,10-epoxypinene) in which the epoxide ring is not part of a strained bicyclic system, proceeds via this route to the extent of only 0.5%. The main reaction involves conventional C-O bond fission.