To a stirred solution of the compound of Formula XII(a), benzyl 2-(tert-butoxycarbonylamino)-3-hydroxypropanoate (1 g, 3.38 mmol) in acetonitrile (10 mL) was added copper(I) iodide (64.4 mg, 0.338 mmol) and sodium sulfate (48.1 mg, 0.338 mmol). The resulting mixture was stirred at 60° C. and treated with 2,2-difluoro-2-fluorosulfonylacetic acid (524 μL, 5.08 mmol), dropwise, as a solution in acetonitrile (2 mL) over a period of 1.5 h. Upon completion of the addition, the mixture was stirred for a further 30 min then cooled to room temperature. The mixture was diluted with diethyl ether and washed with brine (2×), water (3×) and brine (1×). The organic phase was dried, filtered and concentrated in vacuo then chromatographed in 0-30% ethyl acetate in hexanes, to provide the compound of Formula XIII(a), (S)-2-tert-butoxycarbonylamino-3-difluoromethoxy-propionic acid benzyl ester (458 mg, 40%) as a colorless sticky oil. 1H NMR (300 MHz, CDCl3): δ (ppm) 7.32-7.38 (m, 5H), 6.18 (wt, 1H), 5.28 (dd, 1H), 5.19 (dd, 2H), 4.55 (dt, 1H) 4.22 (td, 1H), 4.15 (m, 2H), 1.38 (s, 9H).
