[CH3:1][N:2]1[C:6]2[C:7]([CH:11]([CH2:15][CH2:16][CH3:17])[CH2:12][CH2:13][CH3:14])=[CH:8][CH:9]=[CH:10][C:5]=2[NH:4][C:3]1=O.P(Cl)(Cl)([Cl:21])=O>C(OCC)(=O)C>[Cl:21][C:3]1[N:2]([CH3:1])[C:6]2[C:7]([CH:11]([CH2:15][CH2:16][CH3:17])[CH2:12][CH2:13][CH3:14])=[CH:8][CH:9]=[CH:10][C:5]=2[N:4]=1The crude 1-methyl-7-(1-propylbutyl)-1,3-dihydrobenzimidazol-2-one (710 mg, 2.88 mmol) was dissolved in 10 mL of phosphorous oxychloride and heated at 100° C. overnight. The reaction was allowed to cool to room temperature and concentrated in vacuo. The residue thus obtained was dissolved in ethyl acetate, washed with aqueous sodium bicarbonate, dried over sodium sulfate, filtered and concentrated in vacuo to give 655 mg (86%) of the title compound, which was used in the next step without further purification.
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