Dimethyl sulfoxide–trimethylsilyl bromide–amine system as a bromonium ion source containing a potential internal nucleophile; unusual bromolactonisation of cyclohex-3-enecarboxylic acid derivatives

Abstract

Bromo lactones have been successfully obtained from unsaturated carboxylic acids by employing a dimethyl sulfoxide–trimethylsilyl bromide–amine system. The characteristic feature of this system is that unusual cis-addition and cyclisation of the sterically unfavourable carboxy group take place in the reaction of 6-unsubstituted and trans-6-phenylcyclohex-3-enecarboxylic acids 4 and 21b, which can be clearly explained by taking account of formation of the sulfonium intermediate 3 from the bromonium intermediate 2 and dimethyl sulfide generated in situ.