Aminopentadiene Imines from Zincke Salts of 3-Alkylpyridines. Application to a Synthesis of Pyridinium Salts from Amino Acids
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文摘
The reaction of Zincke salts with primary amines to givepyridinium salts generally requires rather elevated temperature to go to completion (50-100 C). It is shown that theaddition of 1 equiv of a secondary amine allows formation,at ambient temperature, of intermediate aminopentadieneimine salts which can be isolated and were found to cyclizein acidic medium to give pyridinium salts at temperatureswhich do not exceed 50 C. If this process has a tendencyto give lower yields of pyridinium salt than the standardZincke procedure, it can be advantageous in some cases, asillustrated by the synthesis of pyridinium salts from aminoacids, a challenging reaction which does not work startingfrom Zincke salt in the absence of diethylamine. Moregenerally, the reaction can be extended to primary aminesfeaturing polar functions, as exemplified by a pyridiniumsalt synthesis (75 C) in 55% yield from L-carnosine.

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