Summary Gas phase thermolysis at 600-740" of substituted 1-pentyn-3-ones (a-alkynones), which are easily prepared by acylation of trimethylsilyl acetylenes, leads to substituted 2- cyclopentenones. The intramolecular formation of a new C, C-bond between an acetylenic and a nonactivated carbon atom is accompanied by a [1, 2]-migration of one of the substituents at the triple bond. This novel'a-alkynone cyclization'reaction may be ...