Photochemistry of Structural Analogues of Previtamin D₃:  Generality of the Wavelength-Dependent Triene Photocyclization

Two structural previtamin D 3 analogues (R = H, CH 3 ) cyclized photochemically with a 1.4−1.8-fold increase in quantum yields over a 3-nm change in excitation wavelength. The sudden increase in quantum yields is due to the participation and mixing of both 2A and 1B excited states. At λ ≤ 306 nm, the 1B state is initially excited and then (a) partitions between isomerization to the corresponding trans triene isomers, (b) decays to the 2A state to give the corresponding cyclohexadienes, and (c) decays to the ground state. At λ ≥ 309 nm, the 2A state is directly excited to give the corresponding cyclohexadienes and the relaxation path from 1B state to the ground state or isomerization in the 1B state is diminished.