The reaction of homophthalic acid and some aza analogues with Vilsmeier reagent: a reinvestigation

Homophthalic acid and its pyrido and 8-methylquinolino analogs with DMF/phosphoryl chloride at 0° give the appropriate 4-(dimethylaminomethylene)isochroman-1,3-dione (2a, 2b, 2c, resp.). Under the literature conditions for conversion of 2a to 2-methyl-1-oxo-1,2-dihydroisoquinoline-4-carboxylic acid (3a), the aza analogs give instead 7-hydroxy-5-oxo-5H-pyrano[4,3-b]pyridine-8-carboxaldehyde (5b) and 3-hydroxy-6-methyl-1-oxo-1H-pyrano[4,3-b]quinoline-4-carboxaldehyde (5c), resp. Modified conditions were required to isolate analogs 6-methyl-5-oxo-5,6-dihydro-[1,6]naphthyridine-8-carboxylic acid (3b) and 2,6-dimethyl-1-oxo-1,2-dihydrobenzo[b][1,6]naphthyridine-4-carboxylic acid (3c). Further, while reaction of 2a with HCl in MeOH gave the known change to Me 1-oxo-1H-isochromene-4-carboxylate (4), 2b and 2c gave only products of oxa-ring cleavage. Me 2-(cis-2-hydroxyvinyl)-8-methylquinoline-3-carboxylate (8) was the main product from 2c, while a novel quinolizinium species 7-(3-carboxypyridin-2-yl)quinolizinium-1-carboxylate inner salt (11) was formed in good yield from 2b.