Synthesis of 1(11)H-2,3,4,5-tetrahydro[1,3]diazepino[1,2-a]benzimidazole starting from benzimidazole-2-sulfonic acid.Intramolecular cyclization of 2-(8-chlorobutylamino)benzimidazole

摘要

The intramolecular cyclization of 2-(5-chlorobutylamino)benzimidazole(3c)follows the unusual pathway involving the predominant attack on the exocyclic amino group rather than on the much more nucleophilic endocyclic nitrogen atom.This reaction affords 2-pyr-rohdinobenzimidazole and 1(11)H-2,3,4,5-tetrahydro[1,3]diazepino[1,2-a]benzimidazole as the major product and the by-product,respectively.The cyclization can be directed exclusively toward the annulation of the diazepine ring only after the acetylation of the amino group of compound 3c.According to the quantum chemical calculations,the unusual regioselectivity of the cyclization of chloramine 3c is associated primarily with a substantially less steric strain and the higher entropy of pyrrolidine transition states compared to diazepine transition states.