Trapping of 1,8-biradical intermediates by molecular oxygen in photocycloaddition of naphthyl-N-(naphthylcarbonyl)carboxamides;: Formation of novel 1,8-epidioxides and evidence of stepwise aromatic cycloaddition

The photocycloaddition reaction of naphthyl-N-(naphthylcarhdnyl)carboxamides was examined under argon and oxygen atmospheres. In addition to the [2 + 2] and [4 + 4]cycloadducts, 3 and 4, respectively, novel 1,8-epidioxides (5) were formed a under oxygen atmosphere. We transient absorption at lambda (max) of 360 nm with the lifetime of 360 ns was observed by laser flash photolysis of Ic and was interpreted as the absorption of biradical intermediate 2. On the basis of the anti I stereochemistry of 5, which was different from that of the major [4 + 4] cycloadducts, syn-4, it was deduced that; equilibrium between biradical intermediates syn-2 and anti-2 ould exist. Retro [2 + 2] cycloaddition of 3 was responsible for the efficient trapping of the biradical intermediate with molecular oxygen. The photocycloaddition of the anthryl derivatives, 9-anthryl-N-(methylethyl)-N-(naphthylcarbonyl)carboxamides (7), afforded the [4 + 4] cycloadducts (8) exclusively in a quantitative yield even under oxygen atmosphere. The absence of trapping with molecular oxygen was interpreted to de due to the lack of retro [4 + 4] cycloaddition of 8.