Internuclear cyclisation. Part XXX. The photolysis of 2-iodo-2′-, -3′-, and -4′-methoxy-N-alkylbenzanilides in benzene

Abstract

The title reactions give the expected variety of products derived from the radicals formed by loss of iodine. The initial reactions of these radicals are identified as cyclisation to γ- and δ-lactam intermediates, together with a minor amount of 1,5-hydrogen transfer from the N-alkyl group. The δ-lactam intermediates are oxidised to N-alkyl-phenanthridinones, and 1,5-hydrogen transfer results in dealkylation at the nitrogen atom. The γ-lactam intermediates react by several pathways. That from the 2′-methoxy-amide predominantly dimerises, whilst from the 3′-methoxy-amide a major product is a spirocyclohexenone γ-lactam, the result of demethylation at the oxygen atom (possibly catalysed by hydrogen iodide liberated during the reaction).

The γ-lactam intermediate from the 4′-methoxy-amide also suffers demethylation at the oxygen atom to give a spirocyclohexadienone γ-lactam; this is photoisomerised under the reaction conditions to the stereoisomeric cyclopentenones (23) and (24). Further products believed to be formed via this γ-lactam intermediate are 3-methoxy-N-methylphenanthridinone and 4′-methoxy-N-methylbiphenyl-2-carboxamide. The possibility is discussed that these two products may also arise by photoinduced rearrangement of a primary photoproduct.