Zum Mechanismus der α-Alkinon-Cyclisierung: Synthese und Thermolyse von 1-(1-Methylcyclopentyl)[3-13C]prop-2-inon |
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Authors: | Manuel Koller Martin Karpf Andr S Dreiding |
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Institution: | Manuel Koller,Martin Karpf,André S. Dreiding |
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Abstract: | On the Mechanism of the α-Alkynone Cyclization: Synthesis and Thermolysis of 1-(1-Methylcyclopentyl)3-13C]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-13C]- 1 , specifically labeled with 13C at the β-acetylenic C-atom C(3), was synthesized by an intramolecular Witting reaction (230–300°) 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. |
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