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

摘要

Homophthalic acid and its pyrido and 8-methylquinolino analogues with dimethylformamide/phosphoryl chloride at 0 deg give the appropriate 4-(dimethylaminomethylene)isochroman-1,3-dione (2a, 2b, 2c, respectively). Under the literature conditions for conversion of 2a to 2-methyl-1-oxo-1, 2-dihydroisoquinoline-4-carboxylic acid (3a), the aza analogues give instead 7-hydroxy-5-oxo-5H-pyrano[4,3-b]pyridine-8-carboxaldehyde (5b) and 3-hydroxy-6methyl-1-oxo-1H-pyrano[4,3-b)quinoline-4-carboxaldehyde (5c), respectively. Modified conditions were required to isolate analogues 3b and 3c. Further, wile reaction of 2a with hydrogen chloride in methanol gave the known change to methyl 1-oxo-1H-isochromene-4-carboxylate (4), 2b and 2c gave only products of oxa-ring cleavage. Methyl 2-(cis-2-hydroxyvinyl)-8-methylquinoline-3-carboxylate (8) was the main product from 2c, while a novel quinolizinium species (11) was formed in good yield from 2b.