Thermische Cyclisierung von α-Alkinonen zu 2-Cyclopentenonen

Thermal Cyclization of α-Alkynones to 2-Cyclopentenones Gas phase thermolysis at 600–740° of substituted 1-pentyn-3-ones (α-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 non-activated carbon atom is accompanied by a [1,2]-migration of one of the substituents at the triple bond. This novel ‘-alkynone cyclization’ reaction may be explained by postulating an alkylidene carbene intermediate which inserts into a C,H-bond five carbon atoms away at the non-acetylenic part of the ketone. Several examples demonstrate that the α-alkynone cyclization offers a simple tool for the preparation of certain monocyclic, bicyclic and spiro compounds containing a 2-cyclopentenone moiety.