NOVEL [3+2] PHOTOCYCLOADDITIONS OF 3-(1-ALKYNYL)-2-CYCLOALKEN-1-ONES WITH ALKENES

Photochemical cycloaddition of 3-alkynylcycloalkenones 3a,b,c and 4 with tetramethylethylene (8) at approximately 40-degrees-C leads to mixtures of 1:1 adducts that arise largely from 1,5 closure of the biradical intermediate (eq 1), providing examples of a novel [3 + 2] cycloaddition reaction. Similar reactions occur between these same ketones and 1,1-dimethoxy-2-methylpropene (9) and 1,1-dimethoxyethylene (10). In several cases, these reactions lead in a single step to complex and otherwise difficulty accessible systems such as 21 and 30. In contrast, at -60-degrees-C, normal 1,4 closure is favored in addition of 4 with 8. This and previously observed temperature effects on related rearrangements of triplet biradicals suggest that such biradical rearrangements occur prior to and in competition with intersystem crossing to the singlet.