Ring opening of α-pinene epoxide

Abstract

The acid-catalysed opening of the epoxide ring of α-pinene epoxide (2α,3α-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 β-pinene epoxide (2β,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.