To a solution of 1-(2-methoxyethyl)-2-methyl-8-phenyl-1,6,7,8-tetrahydrochromeno[7,8-d]imidazole-5-carboxylic acid (200 mg, 0.55 mmol, Step 12), triethylamine (0.30 mL, 2.18 mmol), and O-benzotriazol-1-yl-N,N,N′,N′-tetramethyluronium hexafluorophosphate (228 mg, 0.60 mmol) in N,N-dimethylformamide (5 mL) was added dimethylamine hydrochloride (49 mg, 0.60 mmol) at 0° C. After stirring at room temperature for 12 h, the mixture was poured onto water, and the aqueous layer was extracted with dichloromethane. The combined organic layer was dried over magnesium sulfate and concentrated in vacuo. The residue was purified by column chromatography on silica gel eluting with dichloromethane/methanol (20:1) to afford the title compound as a white amorphous (215 mg, quant.).
