PHOTOCHEMICAL REARRANGEMENTS OF QUINONE MONOKETALS

We have studied the photochemistry of over 15 quinone cyclic monoketal s, which were prepared by diol exchange from the dimethyl ketals, Swen ton oxidation/hydrolysis, or direct ketalization. Their reactions in a cidic media are generally explained by the classical mechanism for cyc lohexadienone photochemical (di-pi-methane) rearrangements: photocycli zation to a cyclopropane-oxyallylcation that is protonated, followed b y solvolysis. This reaction pathway provides, after hydrolysis, beta-c arboxy-substituted cyclopentenones. With a substituent at the beta-pos ition of the quinone monoketal, rearrangement selectivity is modestly in favor of the more substituted alkene product. With a substituent at the alpha-position of the quinone monoketal, rearrangement selectivit y is strongly in favor of the less substituted alkene product. Possibl e mechanistic reasoning to explain these observations is offered. (C) 1996 Elsevier Science Ltd.