Rearrangement photochimique de 4h-pyrannes et de dienones en 2h-pyrannes

The photoreactivity of the 4H-pyrans 1 was studied in detail by fluorescence and phosphorescence, by deactivation measurements for the singlet state and by the determination of the accessible quantum yields and rate constants. The photochemical isomerisation of the 4H-pyrans 1 into 2H-pyrans 2 occurs with concerted migration of the benzyl group, in a singlet state as a sigmatropic 1, 3-transposition. The 2H-pyrans 2 are also obtained by irradiation of the nonconjugated dienone isomers 3 of the 2H-pyrans 2.