Ring Transformations of Heterocycles

The rearrangement of 3-nitropyridinium salts to 3-nitropyrroles

“…The reaction involves breaking the pyridine С3-С4 bond with formation of a cyclic ketoketene, which produces pyrrole-2-carboxamides by ammonolysis. 6,7 2-Pyridyl-and 3-pyridylnitrenes, generated by the thermolysis (or by the flash vacuum thermolysis, FVT) of triazolo [4,5-b]-and triazolo [4,5-c]pyridines, 2-azidopyridine, tetrazolo [1,5-a]pyridines and [1,2,4]oxadiazolo [2,3-a]pyridin-2-one rearrange into 2-and 3-cyanopyrroles. 2-Cyanopyrrole formation is a result of consecutive 2-pyridylnitrene ring expansion and contraction.…”

Synthesis of Substituted Quinazolin-4(3H)-ones and Quinazolines via Directed Lithiation

“…25 In some cases addition is followed by electrophilic addition at position 5 of pyridine to give 2,5-disubstituted 2,5-dihydropyridines, 26,27 which are known to be unstable. Such attack on pyridine or quinoline compounds sometimes causes ring-opening, 28 giving substituted butadienes, 29 which can cyclise again. 30 Butyllithium also undergoes exclusive addition to some fluoroquinolines, but in the case of 2-fluoro-and 7-fluoroquinolines competitive lithiation takes place at positions 3 and 8, respectively.…”

Ring opening and ring expansion of 8-cyano-tetrazolo[1,5-a]pyridine with secondary amines. Reactions of azides, tetrazoles and nitrenes with nucleophiles, Part 2

“…2 The formation of substituted 2-pyridylformamidines PyNCHNR 2 and 2-pyridylhydrazines PyNHNR 2 in dark reactions of valence tautomeric tetrazolo [1,5-a]pyridines/2-azidopyridines and amines was described in Part 1. 1 Tetrazolo[1,5-a]pyridines/2-azidopyridines 1T/1A are also convenient sources of 1H-and 5H-1,3-diazepines (e.g.…”