O-[2-Oxo-1(2H)-pyridyl]-N,N,N',N'-tetramethyluronium Tetrafluoroborate

Experimental procedure: at 0 °C NMM (23.9 mL, 212 mmol) is added to 1 (43.5 g, 193 mmol) and TPTU (73.9 g, 244 mmol) in dichloromethane (720 mL), followed by 2 (28 mL, 212 mmol) in dichloromethane (80 mL). The solution is stirred for 2 h at 0 °C and warmed to room temperature overnight. Additional 2 (5 mL, 39 mmol) is added and stirring is continued for 4 h. The solution is poured on ice water, the layers are separated, and the aqueous layer is extracted with dichloromethane. The combined organic layers are washed with water, dried over MgSO₄, filtered, and evaporated. Flash chromatography on silica gel (hexane:ethyl acetate = 4:1 ~ 1:1) yields 3 (64.8 g, 94%) as a light yellow oil.