The order of first bond formation in the intramolecular photocycloaddition of 2-(4'-pentenyl)-cyclohex-2-enones

Complete trapping of 1,4-diradical intermediates, formed in the photocycloaddition of 9, provide direct evidence that first bond formation takes place at either C(alpha) or C(beta) positions of the cyclic enone. No cleavage of the 1,4-diradical LO or endo-diradical intermediates 11 and 12 could be detected. However, partial cleavage of the exo-diradical intermediates 13 and 14 was measured. Unusual long-range aromatic radical substitution was obtained in a competing rate with the cyclopropyl carbinyl radical rearrangement. (C) 1997 Elsevier Science Ltd.