1,1,1-Trimethoxybutane → ethyl 4-(5,6-dimethyl-8-propyl-7H-imidazo[4,5-c]tetraazolo[1,5-a]pyridin-7-yl)butanoate

For Examples 40 and 41, pyridine hydrochloride (1.34 g, 11.6 mmol) and trimethyl orthobutyrate (5.42 mL, 34.1 mmol) were sequentially added with stirring to a solution of ethyl 4-[(8-amino-5,6-dimethyltetraazolo[1,5-a]pyridin-7-yl)amino]butanoate (9.1 g, 31 mmol) in toluene (310 mL) under a nitrogen atmosphere. The reaction was heated at reflux for 1.5 hours, allowed to cool to ambient temperature overnight, and concentrated under reduced pressure. The residue was partitioned between chloroform (300 mL) and saturated aqueous sodium bicarbonate (75 mL). The aqueous layer was extracted with chloroform (2×100 mL), and the combined organic fractions were washed with saturated aqueous sodium bicarbonate (2×75 mL), dried over magnesium sulfate, filtered, and concentrated under reduced pressure. The resulting solid was triturated with ethyl acetate and isolated by filtration to provide 6.0 g of ethyl 4-(5,6-dimethyl-8-propyl-7H-imidazo[4,5-c]tetraazolo[1,5-a]pyridin-7-yl)butanoate as a white solid.