| Abstract: | 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‐pyrano4,3‐b]pyridine‐8‐carbox‐aldehyde ( 5b ) and 3‐hydroxy‐6‐methyl‐1‐oxo‐1H‐pyrano4,3‐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. |
