The Photoisomerization of 1,2,3,4,5-Pentamethyl-5-vinyl-1,3-cyclopentadiene

Article abstract of DOI:10.1002/hlca.19840670805

The title compound 1 is shown to give, both upon direct irradiation at 254 nm and upon acetophenone-sensitized photolysis at 300 nm, the syn-vinyl-pentamethylhousene 5, which spontaneously rearranges in a <3,3>-sigmatropic process to give the bicyclo<3.2.0>heptadiene skeleton 2.Based on the photochemical behaviour of selectively deuterated starting material, the suggestion is made that the direct photolysis produces the vinylhousene skeleton by a classic electrocyclization, whereas the sensitized reaction reaches the same target via a di-?-methane rearrangement.The bicyclo<3.2.0>heptadiene derivative 2 gives pentamethylhomoprismane 3 upon prolonged irradiation at 254 nm.