Competitive thermal and photochemical processes in cyclopropylketone rearrangements. II. Thermal isomerization of spiro[n.2]alkan-2-ones

Conformationally-rigid methyl-substituted cyclopropyl ketones with a suitably disposed γ-hydrogen undergo thermal type II reactions efficiently. Comparison of yields of these rearrangements with their photochemical analogs suggest that photochemical type II reactions arise from excited vibrational states. In the absence of a suitably disposed γ-hydrogen, and if strain factors permit, such systems undergo oxovinylcyclopropane to dihydrofuran rearrangement. The inertness of these systems to undergo such thermal rearrangement increased on going from cyclohexanone to the cyclopentanone series.