The intramolecular photochemical [2+ 21 cycloadditions of a number of 4'mbstituted (3'- butenyloxy)-cyclooctenones have been studied. Two classes of substrates were found. One class (phenyl-and vinyl-substituted) forms [2+ 21 adducts in an efficient reaction, while the rest of the compounds form photoproducts slowly and in low yield. The behavior of a carbon analogue shows that rotational relaxation of the cyclooctenone triplet is faster than [2+ 21 ...
