There is a good evidence'that the photochemical [3+ 21 cycloaddition between a, P- acetylenic ketones (1) and simple olefins proceeds through reaction of the ketone triplet with olefin to form biradical 2 that then closes to carbene 3. Products are formed from 3 by way of various hydrogen shifts or cyclization to the cy~ lopropene. l-~ Biradical 2 from the excited singlet of 1 closes in the [2+ 21 manner to form alkynyloxetane.'The closure of 2 to 3 ...
