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DOI: 10.1055/sos-SD-012-00657

Grimmett, M. R.Science of Synthesis, (200212600.

Benzimidazole N-oxides and 1-hydroxybenzimidazoles are acidic compounds, but still have sufficient basicity to form salts with strong acids. The groups are readily alkylated[‌428‌,‌473‌] but acylation occurs more readily at an annular nitrogen. Acetic anhydride converts benzimidazole N-oxides into 1,3-diacetylbenzimidazolones or 1-acetyl-3-methylbenzimidazolone. Under similar conditions, 1,2-dimethyl-1H-benzimidazole 3-oxide reacts at the 2-methyl group with retention of the N-oxide group.[‌54‌] In concert with the adjacent ring carbon atom, the N-oxide group has 1,3-dipolar properties, which allow benzimidazole N-oxides to take part in 1,3-dipolar cycloadditions with a variety of dipolarophiles (e.g., phenyl isocyanate, acetylenedicarboxylic esters, and benzonitrile). The resulting tricyclic products are not particularly stable and are subject to further transformations with ring opening and oxygen transfer, leading to the formation of 2-substituted benzimidazoles.[‌54‌,‌474‌]