The intra-molecular addition reaction of Grignard reagent with quaternary C=N bond of benzimidazole methiodide salt is reported. and a novel method for the synthesis of cycloundecanone is provided. The intra-molecular addition reaction of Grignard reagent with quaternary C = N bond of benzimidazole methiodide salt is reported. And a novel method for the synthesis of cycloundecanone is provided.