Asymmetric photocyclization of diarylethene derivatives

Diarylethene derivatives, which have an optically active l- or d-menthyl group at the 2-position of benzo[b]thiophene ring, were synthesized. Irradiation with 450 nm light in solution led to the formation of diastereomer pairs of the closed-ring forms. The product ratio of the diastereomers was dependent on solvent polarity and temperature. In slightly polar (or polarizable) solvents, such as THF and toluene, an asymmetric photocyclization was observed. At -40 degrees C in toluene, a diastereomer excess as large as 86.6% was observed. The mechanism of the asymmetric photocyclization is discussed.