2,6‐Disubstituted 4‐Aminopyridines from 1,3‐Dialkoxy‐2‐azapropenylium Salts and N ‐Methyl‐4‐piperidone Enamines

1,3‐Dialkoxy‐2‐azapropenylium salts 1 react with enamines 2 of N ‐methyl‐4‐piperidone at room temperature to give 2,6‐disubstituted 4‐aminopyridines 4 , 5 in low to moderate yield after hydrolysis. Intermediates of the reaction of 1a with 2 are the bicyclic iminium salts 6 and 7 , which may be detected 1 H‐NMR spectroscopically prior to hydrolysis. Hydrolysis of the mixture obtained from the reaction of 1a with 2a under basic conditions furnishes the bicyclic ketone 3 as the major product. A “retro‐Mannich”‐type reaction is suggested to explain the degradation of the bicyclic intermediates 6 , 7 with the formation of the pyridines 4 , 5 .