(3). A mixture of 1 (4.10 g, 11.1 mmol) and tetramethylguanidine (2.39 g, 22.3 mmol) in 30 mL of anhydrous dimethylformamide was stirred in an ice bath. T,, the colti reaction mixture was added dropwise and simultaneously tert-butyloxycarbonyl azide6 (2.90 g, 22.3 mmol) and tetramethylguanidine (2.39 g, 22.3 mmol). The mixture was stirred