Aminopentadiene imines from zincke salts of 3-alkylpyridines. Application to a synthesis of pyridinium salts from amino acids

The reaction of Zincke salts with primary amines to give pyridinium salts generally requires rather elevated temperature to go to completion (50-100 degrees C). It is shown that the addition of 1 equiv of a secondary amine allows formation, at ambient temperature, of intermediate aminopentadiene imine salts which can be isolated and were found to cyclize in acidic medium to give pyridinium salts at temperatures which do not exceed 50 degrees C. If this process has a tendency to give lower yields of pyridinium salt than the standard Zincke procedure, it can be advantageous in some cases, as illustrated by the synthesis of pyridinium salts from amino acids, a challenging reaction which does not work starting from Zincke salt in the absence of diethylamine. More generally, the reaction can be extended to primary amines featuring polar functions, as exemplified by a pyridinium salt synthesis (75 degrees C) in 55% yield from l-carnosine.