THERMAL CYCLIZATION OF ALPHA-ALKYNONES TO 2-CYCLOPENTENONES

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. Copyright © 1979 Verlag GmbH & Co. KGaA, Weinheim