Synthesis and reactions of lactam sulfonium salts with a sulfonio bridgehead. Part 1. 4,4a,5,6-Tetrahydro-5-oxo-1H-thiopyrano[1,2-a]-1,4-be nzothiazinium perchlorates

Abstract

Tricyclic benzothiazinium salts 3 are prepared by [2⁺+4] polar cycloaddition of thionium intermediates 2A, generated from the corresponding α-chloro sulfides 2, and dienes in the presence of silver perchlorate. Ring transformation of benzothiazinium salts 3 with a reducing agent such as Mg, NaBH₄ and Zn–AcOH or with a base, furnishes spiro-vinylcyclopropane derivatives 4 in moderate to high yields. Electrolysis of 3a at -1.4 V vs. SCE in acetonitrile also affords vinylcyclopropane 4a (60%). These results indicate that both ionic and radical mechanisms may account for the vinylcyclopropane formation, although it is unclear as to the nature of the radical intermediate. The stereochemistry of 4a was determined by X-ray analysis showing that sulfur and the vinyl group are cis-orientated. Ten-membered lactam sulfides 6 are obtained as the major product of SmI₂ reduction of 3.