Synthesis of 1-aryl-3H-[1,2,5]triazepino[5,4-a]benzimidazol-4(5H)-ones and quantum chemical investigation of the rotamers of the Boc-protected hydrazide key intermediate

3H-[1,2,5]Triazepino[5,4-a]benzimidazol-4(5H)-ones were obtained in five steps involving C-acylation of benzimidazole, its N-alkylation with ethyl bromoacetate, the ester hydrolysis, condensation with BocNHNH₂, and the acid-catalyzed heterocyclization of thus obtained 2-(2-aroyl-1H-benzimidazol-1-yl)-N′-(tert-butoxycarbonyl)acetohydrazides. The geometry of tert-butyl carbazate rotamers was estimated with quantum chemical calculations.