Abstract
Upon photolysis, cyclohexadiene (1) gives easily rise to (Z)-hexa-1,3-5-triene (2) in conrotatory manner. The nitrogen-analogue 1,2-dihydropyridine shows more complicated photochemical behavior because of the labile properties of dihydropyridine itself and the photoproduct azahexatriene. N-Methoxycarbonyl-1,2-dihydropyridine (3) gives 2-azabicyclo[2.2.0]hexane, (Z)- and (E)-hexatriene, pentadienals, and polymers. In sharp contrast, N-phenoxycarbonyl derivative (3a) gives 1-phenoxypenta-2,4-dienyl isocyanate (5a) as a principal photoproduct, where the phenoxy-migration might be initiated on the (Z)-azahexatriene (4a). On the other hand, the irradiation of the 2-phenyl-1,2-dihydropyridines (3b and 3c) results in a predominant generation of (Z,Z)-6-phenyl-1-azahexa-1,3,5-trienes (6b and 6c) toward (Z,E)-hexatrienes (4b and 4c) in the ratio of 9:1 and 19:1, respectively. These (Z)-hexatrienes (4 and 6b) regenerates the starting dihydopyridines (3 and 3b) after the standing of photolysis mixtures (k= 3.9 x 10-3s for 4, k= 0.56 x 10-3s for 4b at 20 °C). The characteristic absorptions of isocyanates were observed at 2247 and 2252 cm-1 in the both case of N-phenoxycarbonyl derivatives (3a and 3c) in which no electronic spectra of azatrienes were observed at all. The NMR spectra of MeOH-adduct were partially analyzed in crude mixtures indicating the major formations of 8b (66%) and 8c (75%) in addition to the (Z,E)-, (E,Z)-, and (E,E)-isomers.
