Photodehydrocyclizations of stilbene-like compounds XIV† Photosynthesis and photoreactions of 1,2- and 1,14-diphenylpentahelicene derivatives

Two polycyclic aromatics having a phenyl-substituted pentahelicene moiety, 13,14-diphenyldibenzo[c,g]chrysene (19) and 14,15-diphenyldibenzo[f,j]picene (38) have been obtained by irradiation of the appropriate diarylethylenes (13 and 18). Further irradiation of 19 in benzene yields mainly dibenzo[a,g]coronene (21) by cyclodehydrogenation with simultaneous loss of a phenyl substituent. The side products of this reaction are probably formed via rearrangements similar to those previously found during the photoreaction which occurs on irradiation of 1,12-diphenyltriphenylene. Their most probable structures are 15-phenyldibenzo[a,g]coronene (28) and 15-phenyldibenzo[a,ghi]naphtho[2,1,8a,8-cde]perylene (31). Irradiation of 19 in ethyl acetate yields only 28 (main product) and 31. Photocyclization of 38 could not be effected. NMR data for the cis-trans isomers of the starting compounds (13 and 18) revealed that the cis isomers exist mainly in the overcrowded syn conformation.