Propensity of 4-methoxy-4-vinyl-2-cyclopentenones housed in tri- and tetracyclic frameworks for deep-seated photochemical rearrangement

The excited-state behavior of a select group of highly enantioenriched tri- and tetracyclic 2-cyclopentenones has been examined in two solvents. Prepa red by a convergent pathway involving the coupling of cyclopentenyl bromide 12 to several chiral ketones followed by desilylation, perruthenate oxidat ion, and ring-closing metathesis, the reactants were conveniently irradiate d through quartz. Following promotion to the triplet level, a cyclopropylca rbinyl biradical intermediate is presumably generated provided that the dou ble bond linked to C4 is not saturated. Four pathways have been observed to operate, only one of which has been observed previously under rather diffe rent conditions. The new photorearrangements include non-Norrish fragmentat ion to a ketene, generation of a spirocyclic intermediate, and ring expansi on to a cyclobutyl radical in advance of intramolecular fragmentation of th e four-membered ring. In the latter instance, a hydrozulenone derivative is formed. In the majority of the examples, product structure and absolute st ereochemistry were corroborated by means of X-ray diffraction analysis.