Syntheses of heterocyclic compounds. Part XIII. Imidazoles from N-acylamino-naphthyl and -phenyl heterocycles by cyclisation with polyphosphoric acid

Abstract

Hot polyphosphoric acid cyclises 1-acylamino- and 2-acylamino-naphthalenes with a saturated N-heterocycle in the 2- and 1-position, respectively, to imidazoles with fission of the C–N bond in the heteroparaffinic ring. The alkyl chain set free by the ring opening becomes attached to the 3- or the peri-position of the naphthalene ring, respectively. With N-2-acylaminophenyl heterocycles of medium ring size (8 and 9) isomerisation in the methylene chain is observed during benzimidazole formation. Additional evidence of the cyclisation mechanism is obtained by trapping intermediates. Orientation of these new ring systems and some of their stereochemical features are discussed in terms of their nuclear magnetic resonance spectra.