278. Zum Mechanismus der α-Alkinon-Cyclisierung: Synthese und Thermolyse von 1-(1-Methylcyclopentyl)prop-2-inon

Article abstract of DOI:10.1002/hlca.19830660844

The relative migratory aptitude of two acetylenic substituents in the α-alkynone cyclization, a thermal conversion of α-acetylenic ketones A to 2-cyclopentenones C, was investigated by isotope-labeling experiments.The α-alkynone <β-13C>-1, specifically labelled with 13C at the β-acetylenic C-atom C(3), was synthesized by an intramolecular Wittig reaction (230-300 deg) of the diacylmethylidenephosphorane <13C>-7.The latter resulted from acylation of methylidenetriphenylphosphorane with the acid chloride 4 to yield the acylmethylidenephosphorane 5, which in turn was formylated with acetic <13C>formic anhydride (<13C>-6).Upon thermolysis of <β-13C>-1, its label at C(β) was transferred almost exclusively to C(β) of the 2-cyclopentenone moiety in the resulting cyclization product <13C>-2.We conclude that there is a distinct preference for hydrogen migration in the acetylene -> alkylidene carbene isomerization (A -> B) which precedes the cyclization step (B -> C).No evidence was found for a fast reversibility of this isomerization (A <=> B) involving both acetylenic substituents.