1,2,3,4-Tetrahydro-1,6-naphthyridines. Part 2. Formation and unexpected reactions of 1,2,3,4-tetrahydro-7H-pyrano[4,3-b]pyridine-2, 7-diones

The nucleophilic substitution of the enol methoxy group of pyridones (1) by methyl cyanoacetate led to (Z)-5-cyano-6-cyanomethoxycarbonylmethylenepiperidones (6), which underwent cyclization in acid medium to 1,2,3,4-tetrahydro-7H-pyrano[4,3-b]pyridine-2,7-diones (7). Surprisingly, the treatment of 7 with ammonia yielded the 5-cyano-6-cyanomethyl substituted pyridones (11) which were not accessible by reaction of 1 with NaCH₂CN.