Benzopyrones. Part III. Synthesis and mass spectra of some oxopyranobenzoxazolecarboxylic esters
The preparation of ethyl 8-amino-7-hydroxy-4-oxochromen-2-carboxylate (1) and two of its isomers from the corresponding nitro-compounds is described. Cyclization of the Schiff bases from these amino-phenols and various nitrobenzaldehydes gave the oxopyranobenzoxazoles (4)–(7). The amino-phenols (1) and (2) were also cyclized by use of phosgene or thiophosgene to give the corresponding oxo- or thioxo-oxazoles (8)–(11). The mass spectra of 2-phenylbenzoxazole (12) and benzoxazol-2(3H)-one (13) are interpreted. A similar study of the pyranobenzoxazoles (4)–(11) showed fragmentation patterns characteristic of chromones rather than of benzoxazoles.