Photochemical rearrangements of quinone monoketals

Article abstract of DOI:10.1016/0040-4020(96)00207-4

We have studied the photochemistry of over 15 quinone cyclic monoketals, which were prepared by diol exchange from the dimethyl ketals, Swanton oxidation/hydrolysis, or direct ketalization. Their reactions in acidic media are generally explained by the classical mechanism for cyclohexadienone photochemical (di-π-methane) rearrangements: photocyclization to a cyclopropane-oxyallylcation that is protonated, followed by sovolysis. This reaction pathway provides, after hydrolysis, β-carboxy-substituted cyclopentenones. With a substituent at the β-position of the quinone monoketal, rearrangement selectivity is modestly in favor of the more substituted alkene product. With a substituent at the α-position of the quinone monoketal, rearrangement selectivity is strongly in favor of the less substituted alkene product. Possible mechanistic reasoning to explain these observations is offered.