Formation of Isoquinoline and 1-Azetine Derivatives via Novel Photocyclization of Substituted α-Dehydrophenylalanines

The irradiation of substituted N-acetyl α-dehydrophenylalanines in MeOH with Pyrex-filtered light was found to give isoquinoline and 1-azetine derivatives in relatively good yields, which may be formed via intramolecular cyclization reactions from the (Z)- and (E)-isomers, respectively. Solvent viscosity effects on the product distribution and composition strongly suggested the minor role of a radical pair mechanism. In the presence of benzophenone as a triplet sensitizer, the starting (Z)-isomer underwent an exclusive isomerization into the corresponding (E)-isomer without yielding any cyclization products, being consistent with the occurrence of the novel photocyclization reactions from the excited singlet-state isomers. On the other hand, the irradiation of N-substituted benzoyl α-dehydrophenylalanines in MeOH afforded selectively 1-azetine derivatives without forming any isoquinolines. It was suggested that the stereoelectronic effects of bulky aromatic acyl groups are responsible for the complete suppression of the cyclization reactions, which eventually afford isoquinoline derivatives via the excited singlet-state (Z)-isomers.