STERICALLY CROWDED HETEROCYCLES .3. A GENERAL-APPROACH TO IMIDAZO[1,2-A]PYRIDINES BY FERRICYANIDE OXIDATION OF QUATERNARY PYRIDINIUM SALTS

Substituted 1-(pyridin-2-yl)-2,4,6-triphenylpyridinium perchlorates Ib -le were converted with potassium ferricyanide and potassium hydroxide to sterically crowded henyl-3-[(Z)-1,3-diphenyl-3-oxopropenyl]imidazo [1, 2-a]pyridines 2b-2e accompanied by minor isomeric 2-benzoyl-3,5-di phenyl-1-(pyridin-2-yl)pyrroles 3c-3e. 4-Phenyl-2,6-di(4-substituted p henyl)-1-(pyridin-2-yl)pyridinium salts 4a, 4b gave exclusively corres ponding imidazo[1,2-a]pyridines 5a, 5b while the ferricyanide oxidatio n of 1-(5-iodo- and 5-cyanopyridin-2-yl)-2,4,6-triphenylpyridinium per chlorates 6a, 6b led to mixtures of major imidazo[1,2a]pyridines 7a-7c and minor pyrroles 8a-8c. Some mechanistic aspects of the oxidation p rocedure are discussed in connection with a resistance of 2,6-diphenyl -1,3,5-trimethylpyridinium perchlorate (9c) towards the oxidizing agen ts.