Indolizines. III. Studies on the acylative cyclization of stabilized Methylene-1,2-dihydropyridines. A novel synthesis of 3-phenyl-1,2-phthaloylindolizines

Abstract

Cyclization of l-benzyl-2-ethoxycarbonylmethylene-1,2-dihydropyridine in acetic anhydride is shown to depend on the basicity of the reaction medium. The reaction between activated 2-methylpyridinium salts and ethyl chloroformate in aqueous base has provided a novel synthesis of 2-ethoxycarbonyl-methylene-l,2-dihydropyridines. Nass spectra of four 2-methylene-l,2-dihydropyridines are presented together with N.M.R. evidence for their 2-E configuration. Pyrophthalone (15) reacted with benzyl chloride in boiling nitrobenzene to give 2-benzylpyrophthalone. With phenacyl bromide, pyrophthalone gave 3-phenyl-1,2-phthaloylindolizine (6-phenylbenzo[f]pyrido[1,2-a]isoindole-7,12-dione) rather than the 3-benzoylindolizine (18). Quinophthalone reacted similarly, and the formation of both phthaloylindolizines has been attributed to a novel rearrangement of the 2-phenacylindan-l,3-dione system in the intermediate, alkylated products. Mass spectra of both phthalones and three phthaloylindolizines are presented.