Synthesis of Condensed 1,2,4-Triazolo-Heterocycles

Dehydrogenative cyclization of hydrazones derived from aromatic aldehydes and 2-hydrazino-1,3-benzothiazole, 2-hydrazinoquinoline, 2-hydrazinolepidine and 2-hydrazinopyridine using different dehydrogenating or oxidizing agents gave the corresponding 3-aryl-1,2,4-triazolo [3,4-b] 1,3-benzothiazoles, 3-aryl-1,2,4-triazolo [4,3-a] quinolines, 3-aryl-1,2,4-triazolo [4,3-a] lepidines and 3-aryl-1,2,4-triazolo [4,3-a] pyridines respectively. 1-Aroyl-2-(1,3-benzothiazol-2-yl)-, 1- aroyl-2-(quinol-2-yl)- and 1-aroyl-2-(lepid-2-yl) hydrazines were also prepared and their dehydrative cyclization using acetyl chloride was attempted. Under the used conditions 1-aroyl-2-(quinol-2-yl) hydrazines directly cyclized to 1,2,4-triazolo [4,3-a] quinolines while 1-aroyl-2-(1,3-benzothiazol-2-yl) hydrazines and 1-aroyl-2- (lepid-2-yl) hydrazines gave 1-acety-2-aroyl-1-1-(1,3-benzothiazol-2-yl) hydrazines and 1-acetyl-2-aroyl-1-(lepid-2-yl) hydrazines respectively. The latter were successfully cyclized through the elimination of an acetic acid molecule to the corresponding 3-aryl-1,2,4-triazolo-heterocycle by heating in boiling phenol.