Hydroxy-steroids. Part XIV. The acid-catalysed ring opening of 5,6-epoxides in a largely non-aqueous medium

Abstract

5α,6α-Epoxides are converted rapidly (5 min at 20 °C) and quantitatively into 5α,6β-diols by perchloric acid in butan-2-one containing a very small amount of water. The reactions are catalysed by acid and retarded by added water, but the kinetic results do not fit either an A1 or an A2 mechanism. A borderline sequence is proposed in which protonation of the epoxide is followed by a slow stage. In the transition state of the latter, breaking of the O–C(6) bond is thought to be more advanced than bond making with the incoming water molecule.