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

Homophthalic acid and its pyrido and 8‐methylquinolino analogues with dimethylformamide/phosphoryl chloride at 0° 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‐carbox‐aldehyde (5b) and 3‐hydroxy‐6‐methyl‐1‐oxo‐1H‐pyrano[4,3‐b]quinoline‐4‐carboxaldehyde (5c), respectively. Modified conditions were required to isolate analogues 3b and 3c. Further, while 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.