The behavior of the three medium-sized cycloalkenes cyclooctene (1 0), cyclodecene (17), and cyclododecene (21) on direct irradiation in pentane and methanol solution has been studied. The results are summarized in Tables 1-3. Irradiation of medium-sized cycloalkenes is a convenient procedure for the preparation of bicyclic products (cf. 13, 14, 19, 20, and 23) through transannular insertion reactions of carbene intermediates (cf. 11, 18, and 22) ...