Following the method described by Aubé and coworkers, supra, a solution of NaN3 (0.88 g, 13.7 mmol) in H2O (2.5 mL) was cooled to 0° C. and CH2Cl2 (4 mL) was added. Whilst the mixture was stirring vigorously, Tf2O (0.47 mL, 2.26 mmol) was added dropwise. The resulting solution was stirred for an additional 2 h. The mixture was then placed in a separating funnel, and the organic layer was removed. The aqueous phase was extracted further with CH2Cl2 (2×3 mL). The organic layers were combined, washed with saturated Na2CO3, dried over Na2SO4(s), and filtered. The resulting solution of TfN3 was added dropwise to solution of methyl 2-amino-6-{[(benzyloxy)carbonyl]amino}hexanoate hydrochloride salt (0.46 g, 1.40 mmol) and DMAP (0.75 g, 6.16 mmol) in CH2Cl2 (5 mL). The resulting solution was stirred overnight under Ar(g). The mixture was concentrated under reduced pressure and purified by silica gel flash chromatography, eluting with 20% EtOAc/hexanes, to give azide 2f as an oil (0.43 g, 1.34 mmol, 96% yield).
