Pummerer reaction of 2-vinylcyclopropyl sulfoxides: generation and reactions of butadienylthionium ion intermediates

Abstract

Generation of butadienylthionium ions in the Pummerer reactions of 2-vinylcyclopropyl sulfoxides has been investigated. Although the Pummerer reactions of 2-vinylcyclopropyl sulfoxides 1 are complicated, benzothiazinone derivatives 10 smoothly react with trifluoroacetic anhydride to give 1,3-dienes in good yields. The reactions proceed via butadienylthionium ions by proton abstraction from the 2′-methyl group or the cyclopropane ring. Reactions of disubstituted benzothiazinones 10e–h provided cyclic dienes while treatment of mono- or un-substituted derivatives gave acyclic conjugated dienes 11a–d. 2-Vinylcyclopropyl sulfoxides 1 and 10 were prepared by MCPBA oxidation of the corresponding 2-vinylcyclopropyl sulfides 19 and 23, respectively, which were obtained by cyclopropanation of α-chloro sulfides with 1,3-dienes via the 5,6-dihydro-2H-thiopyranium intermediate 22.