Novel thermolytic transformations of n-benzoyl 2-aza 3-oxa bicyclo(2.2.1)hept-5-ene and n-benzoyl 2-aza 3-oxa bicyclo(2.2.1)heptane

Abstract

Novel thermolytic pathways were encountered in careful studies with N-benzoyl 2-aza 3-oxa bicyclo (2.2.1)heptene (1) and N-benzoyl 2-aza 3-oxa bicyclo(2.2.1)heptane (2). Compound 1 thermally fragments by four major pathways, namely, (3,3)-sigmatropic shift, (4 + 2) cycloreversion, N-O rupture and C-N homolysis. The (3,3)-sigmatropic shift provides a novel route to unusual, bicyclic heterocycles. Electron withdrawing aryl groupings tend to promote the (3,3)-sigmatropic shift pathway over others. The (4 + 2)-cycloreversion of 1 leads to nitroso carbonyl benzene and cyclopentadiene. The weak σ bonds of PhCONO undergo ready homolysis. The intermediate arising from N-O rupture leads to a cyclopentenone radical similar to that involved in the PG endoperoxide → PGE change. This radical either combines with benzoyl radical leading to 4-dibenzoylamino cyclopentenone or is transformed to an enamide by loss of hydrogen, which, in turn, undergoes (4 + 2)-cycloaddition with cyclopentadiene leading to a novel spiro adduct. The C-N rupture leads to the formation of benzanilide. Nitrosocarbonyl benzene is a powerful electrophile. With cyclohexene it forms a hydroximic ester, initiated by nucleophile acceptance at the CO oxygen. In contrast, it reacts with P(OMe)₃ leading to diphenylurea via nucleophile acceptance at N=O oxygen. Thermolysis of 2 proceeds largely by N-O rupture, similar to that normally observed in the PG endoperoxide → PGE change.