2-[1-(timiethylsilyl)alkylidene]-4]cyclopentene-1,3-dione from lewis acid-catalyzed reaction of cyclobutenedion monoacetal with alkynylsilane: novel cationic 1,2-silyl migrative ring opening and subsequent 5-exo-trig ring closure

摘要

An ethoxycarbenium ion intermediate, which was produced by the catalytic action of a Lewis acid on a cyclobutenedione monoacetal, reacted with phenyl(trimethylsilyl)acetylene to give a normal electrophilic substitution product. In sharp contrast, the same catalytic reaction with bis- (trimethylsilyl)acetylene afforded a 2-metylene-4-cyclopentene-1,3-dione derivative as a ring expansion product instead of an alkynylation product.