Structures of the four possible [4 + 4] cycloaddition products formed on photodimerization of N-alkyl-2-pyridones

Abstract

Photodimerization of N-methyl-2-pyridone (1a) in water gave exclusively the four [4 + 4] cycloaddition products [trans,anti(5a)(51%), trans,syn(6a)(0.6%), cis,anti(7a)(11.2%), and cis,syn(8a)(6.8%)]. In contrast, the reaction in non-aqueous solvents (benzene or ethanol), gave only the anti-dimers. The four dimers (5a)–(8a) were photostable, but showed remarkable differences in their thermal stability: (5a) and (6a) were unchanged when heated under reflux in CHCl₃, but (7a) and (8a) rearranged to give an unsymmetrical [2 + 2] dimer (9), and symmetrical [2 + 2] dimers (10) and (11), respectively; thus, structures (7a) and (8a) possess Cope systems. Structures were assigned on the basis of spectral evidence. Variation of the N-alkyl groups and of reagent concentrations had no influence on the relative yields of the dimers.