Internuclear cyclisation. Part XXXII. Dienol–benzene and related rearrangements of some spirocyclohexadiene-lactams

Abstract

The stereoisomeric 4′-hydroxy-2-methylisoindoline-1-spiro-1′-cyclohexadien-3-ones (1) rearrange with acid to give N-methylphenanthridinone (3) by migration of the aryl group and not of the nitrogen atom. However, similar reactions of the 2′-,3′-benzo- and 6′-hydroxy-4′-,5′-benzo-analogues [(21) and (7)] occur with predominant nitrogen migration. Brief exposure of compound (1) or (21) to methanolic acid results in the formation of cyclo-hexadienyl methyl ethers, presumably by way of intermediate cyclohexadienyl cations. It has not, however, been possible to identify a similar process in the acid-catalysed methanolysis of compound (7). The cyclohexadienyl ethers are also obtained when the corresponding cyclohexadienyl iodides are exposed to methanolic silver nitrate. Treatment of the stereoisomeric 4′-,5′-benzo-6′-iodo-compounds (34) with methanolic silver nitrate leads to one stereoisomer of the 6′-methyl ether (26) and to N-methylbenzo[c]phenanthridin-6-one (9). Stereospecific formation of the same isomer of (26) was observed when 2-(N-methyl-2-naphthylcarbamoyl) benzenediazonium sulphate was decomposed by heating in methanol. Possible factors controlling stereoselection in these reactions, and the competition between aryl and nitrogen migration, are briefly discussed.