Reaction between vicinal diols and hydrogen bromide in acetic acid; synthesis of chiral propylene oxide

Abstract

Several vicinal diols react rapidly with 6M-hydrogen bromide in acetic acid to afford vicinal acetoxy-bromides in excellent yield. The mechanism, scope, and limitations of this reaction have been explored. Its mechanism involves monoacetylation of the diol, cyclisation to a 1,3-dioxolan-2-ylium ion, and capture of this intermediate by bromide ion. This has been established by kinetic studies, determination of substrate stereospecificity, and observation of intermediate 1,3-dioxolan-2-ylium ions by n.m.r. spectroscopy. The preparative value of the reaction is illustrated by its application in a simple three-stage synthesis of optically pure (S)-propylene oxide from readily available (S)-ethyl lactate.