Cyclization of [(4- or 5-Substituted-2-benzimidazolyl) thio] acetic Acids. Isolation and Identification of Two Possible Isomers of Substituted Thiazolo [3, 2-a] benzimidazol-3 (2H)-one

Abstract

Cyclizations of [(5-substituted-2-benzimidazolyl) thio] acetic acid (2a-e) to the corresponding thiazolo [3, 2-a] benzimidazol-3 (2H)-one (5 and 6) were successfully carried out by heating 2a-e in Dowtherm A or Ac₂O/pyridine, and gave the two possible isomers (5a-e and 6a-e) in a ratio of nearly 1 : 1 in all cases. Separation of 5 and 6 was successfully carried out, and their structures (namely, the direction of cyclization) were suggested on the basis of the NMR spectra. Dowtherm A was more effective than Ac₂O/pyridine, particularly for the cyclization of [(5-nitro-2-benzimidazolyl) thio] acetic acid (2a), which had been reported not to be cyclized with other reagents. Similar cyclization of [(4-methyl-2-benzimidazolyl) thio] acetic acid (2f) preferentially gave 8-methylthiazolo [3, 2-a]-benzimidazol-3 (2H)-one (6f).