Zum Mechanismus der α-Alkinon-Cyclisierung: Synthese und Thermolyse von 1-(1-Methylcyclopentyl)[3-13C]prop-2-inon
✍ Scribed by Manuel Koller; Martin Karpf; André S. Dreiding
- Publisher
- John Wiley and Sons
- Year
- 1983
- Tongue
- German
- Weight
- 618 KB
- Volume
- 66
- Category
- Article
- ISSN
- 0018-019X
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✦ Synopsis
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.