Toluene (3 ml) and trimethyl orthoformate (79 μl) were added to the (2S,4S)-tert-butyl 4-(4-amino-5-((3,5-dimethoxyphenyl)ethynyl)-7H-pyrrolo[2,3-d]pyrimidin-7-yl)-2-(hydrazine carbonyl)pyrrolidine-1-carboxylate (93.6 mg) obtained in Step 2, and the resulting mixture was stirred at 110° C. overnight. Acetic acid (400 μl) was added to the reaction mixture, and stirred at 110° C. for 6 hours. Ethyl acetate and water were added to the reaction mixture to separate the organic layer. The organic layer was dried over anhydrous magnesium sulfate, and the solvent was distilled off under reduced pressure. The resulting residue was purified by silica gel column chromatography (developing solvent: chloroform/methanol) to obtain the title compound as a light-yellow, amorphous substance (50 mg). Physical properties: m/z [M+H]+ 532.2
