Co-Authors:
Dunkelblum, E., Department of Chemistry, Michigan State University, East Lansing, MI 48824, United States
Hart, H., Department of Chemistry, Michigan State University, East Lansing, MI 48824, United States
Suzuki, M., Department of Chemistry, Michigan State University, East Lansing, MI 48824, United States
Abstract:
Six different 2,4-cycloheptadienones in which the 2,3-double bond is incorporated in an aromatic ring (4, 11, 16, 29, 30, and 31) gave photoproducts with a trans-4,5 double bond, as shown by trapping with furan. However, these six dienones behaved quite differently from one another when irradiated without a trapping agent present. In cyclohexane, irradiation of 4 gave dimers 5 and 6, whereas 29 gave (among other products) the remarkable dimer 35. These dimers have structures which clearly show that they arise from (4π + 2π) cycloaddition of a trans intermediate to the styrene moiety of the starting ketone, followed by further rearrangement. The predominant products from the irradiation of 11, 16, and 29 in cyclohexane were head-to-head cyclobutane dimers in which one moiety was trans and the other cis (at the sites of the original C(4)-C(5) double bond). Under the same irradiation conditions, 30 and 31 were recovered unchanged. The unusual stereochemistry of the cyclobutane dimers may be accounted for by a concerted thermal dimerization of two ground state trans intermediates. Alternatively, the cyclobutane dimers may arise from rearrangement of an initial (4π + 2π) adduct of the trans intermediate to the styrene moiety of the starting ketone, a mechanism which would explain the lack of dimer formation from 30 and 31. Irradiation of 11 in methanol resulted in stereospecific transannular addition of methanol to a trans intermediate, to give 15.
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