Pyridinium salts ANRORC reaction

Katritzky converted pyrylium salts 44 (formed from the reaction of enone 42 and ethyl pyruvate 43) to the corresponding pyridiniums (45) via ANRORC reaction with a primary amines. This strategy serves as an alternative to the Zincke-Konig method for the formation of substituted pyridinium salts. 

A variation of the ANRORC reaction uses an intramolecular nucleophile to open the activated heterocycle. Vanderwal accomplished a short, formal synthesis of porothramycin A and B (not depicted) via opening of the pyridinium salt with the amide nitrogen found within 59. Product 60 contains the key functionalized pyrroline of the natural product, and the expelled Zincke aldehyde was further functionalized. 

The special effects of a mesoionic system as a substituent have been noted in the reactions with nucleophiles of 3-7V (4-chloro-3-nitrophenyl)sydnone. A synthesis, using two amino-debromination reactions, has been used to prepare phenothiazines analogous to methylene blue (23). An imusual susceptibihty to the nature of the counteranion has been observed in the kinetics of the reaction of iV-(2,4-dinitrophenyl)-4-dimethylaminopyridinium salts (24) with piperidine in acetonitrile, and may indicate participation of the anion in stabilising the intermediate. An ANRORC mechanism is imphcated in the reaction of A-(2,4-dinitrophenyl)-4-(4-pyridyl)pyridinium cations with arylamines. Ring-opening and ring-closure reactions... 

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