Acylation of alkenes generated in situ by hydride transfer from isoalkanes. Synthesis of pentalenones, hydrindenones, and cyclopentenones

Article abstract of DOI:10.1021/ja00021a036

Acylation, in the presence of AlCl₃ and hydride acceptor, of methylcyclopentane, methylcyclohexane, and 2-methylbutane by ethylenic acyl chlorides, in CH₂Cl₂ solution, respectively, leads to tetrahydropentalenones, tetrahydroindenones, and cyclopentenones in good yields. Hydride acceptor may be either acetyl chloride or the alkenoyl chloride itself. Better results are performed in the presence of nitromethane and CuSO₄. Overall yields are better than those obtained by the two-step process involving acylation of alkenes by alkenoyl chlorides and subsequent Nazarov cyclization of the resulting divinylketones. Methyl 1,4-migration is observed during the acylation of 2-methylbutane by sorboyl chloride. The mechanism of these conversions is discussed on the basis of results observed with cyclohexane-d₁₂ and methylbutane-d₆ as well as stereochemical studies of the cyclization process.