Photodehydrocyclisations of stilbene‐like compounds XVI: Photoreactions of α‐(9‐phenanthryl)stilbene and 1‐(9‐phenanthryl)‐1‐phenylethylene

The regular photocyclisation of α-(9-phenanthryl)stilbene (1) to 9-phenylbenzo[g]chrysene (2) is accompanied by formation of 4,5-diphenylacephenanthrylene (3). A similar photocyclisation, providing a five-membered ring, occurs on irradiation of 1-(9-phenanthryl)-1-phenylethylene (4) which yields a small amount of 5-phenylacephenanthrylene (5). The structures of 3 and 5 were confirmed by comparison of their spectral data with those of acephenanthrylene (10). This was synthesized by photolysis of 1-(o-iodobenzylidene)indane (8), followed by dehydrogenation, however, 10 cannot be obtained by irradiation of 1-(o-iodobenzylidene)indene (11). The different photoreactivity of 8 and 11 is discussed briefly.