Inter- and intra-molecular selectivity in the cyclisation of N-cinnamoyl-1-naphthamides in solid-state photochemistry and peri selectivity in their photocyclisation in solution

Abstract

The photocyclisation of N-cinnamoyl-1-naphthamides 1a–f was examined in the solid state and in solution. Three types of cyclisation, intramolecular 2π+ 2π and 4π+ 2π, and intermolecular 2π+ 2π cyclisations were observed in the solid state. The ratios of these products were largely dependent on the substituents at the para-position of the cinnamoyl moiety. Single-crystal X-ray crystallographic analysis of 1b showed that the intermolecular distance between the two double bonds was 4.34 Å. Amides with bulky substituents preferred intramolecular 2π+ 2π cyclisation. In contrast to the solid-state photochemistry, only intramolecular cyclisation was observed in solution. The ratios of 2π+ 2π and 4π+ 2π cyclisation products were irradiation time dependent. It was found that the 2π+ 2π cycloreversion and rearrangement from the 2π+ 2π product to the endo 4π+ 2π isomer were the cause of this dependency.