Zum Mechanismus der α‐Alkinon‐Cyclisierung: Synthese und Thermolyse von 1‐(1‐Methylcyclopentyl)[3‐ 13 C]prop‐2‐inon

On the Mechanism of the α‐Alkynone Cyclization: Synthesis and Thermolysis of 1‐(1‐Methylcyclopentyl)[3‐ 13 C]prop‐2‐ynone 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‐ 13 C]‐ 1 , specifically labeled with 13 C at the β‐acetylenic C‐atom C(3), was synthesized by an intramolecular Witting reaction (230–300°) of the diacylmethylidenephosphorane [ 13 C]‐ 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 [ 13 C]formic anhydride ([ 13 C]‐ 6. ) Upon thermolysis of [β‐ 13 C]‐ 1 , its label at C(β) was transferred almost exclusively to C(β) of the 2‐cyclopentenone moiety in the resulting cyclization product [ 13 C]‐ 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.