Recyclization on acylation of 1-methyl-2-phenacyl-1H-benzimidazole phenylhydrazone

Abstract

We showed previously that the acylation of alkyl-, aryl-, and aroylhydrazones of 2-phenacyl-and 2-acetonyl-1H-benzimidazoles with aroyl chlorides and anhydrides of carboxylic acids readily initiates a recyclization process with the formation of pyrazole derivatives. In particular, by the action on the phenylhydrazone of 2-phenacyl-1H-benzimidazole (1a) of the acid chloride of p-nitrophenylbenzoic acid (2a) or trifluoroacetic acid anhydride (2b), the 5-(2-acylaminoanilino)pyrazoles 3a,b were obtained [1-3]. It was proposed that recyclization begins with the formation of either acylbenzimidazolium salts of type A [3], or N-acylbenzimidazoles of type B [1], and then proceeds through spiranes C (R=H). In the present work, to make more precise the initial stage of the mechanism of the reaction being considered we studied the behavior under acylating conditions of the phenylhydrazone of 1-methyl-2-phenacyl-1H-benzimidazole (1b), since a substituent is already present at the nitrogen atom of the heterocycle and the formation of intermediates of type B is impossible for it.We found that compound 1b under the action of acylating agents 2a,b is readily recyclized with the formation of 5-(2-acylamino-N-methylanilino)pyrazoles 3c,d. The structure of compound 3d was confirmed by hydrazinolysis by means of which the trifluoroacetyl group is removed and amino compound 4 is formed.According to new data obtained by us the initiation of recyclization of compounds of type 1 by acylating agents is a more general reaction than we supposed earlier, and is caused, extremely probably, by the regioselective formation of N-acylbenzimidazolium salts A, in which the carbon atom in position 2 of the heterocycle possesses enhanced electrophilicity and readily undergoes intramolecular attack by the amino group of the hydrazone fragment. Spiranes C formed in this way are converted into the final products by opening of

Synthesis and tautomerism of 2-phenacyl-1H-benzimidazoles and their hydrogen bromide salts

“…In order to study the tautomerism of compounds 1a-i, we used 1-methyl-2-phenacyl-1-benzimidazole (2) synthesized from 1,2-dimethylbenzimidazole in 88% yield by our scheme for the preparation of compound 1 [15]. We should note that compound 2 was previously prepared by Huang and Wang [14] in 41% yield by another procedure.…”