Example 1, Alternative Step 9aii: Aminoester 1 (63 g, 0.16M, 1 eq.; the product of reductive deprotection of a known compound—(See e.g. R. J. Cherney, WO 2004/098516 and G. V. Delucca & S. S. Ko, WO 2004/110993) was placed in a round bottom flask and MeCN (500 mL) was added. EDAC (33.1 g, 0.17M, 1.1 eq), HOBt.H2O (21.2 g, 0.16M, 1.0 eq) and N-Cbz-L-methionine (46.7 g, 0.17M, 1.05 eq) were then added followed by TEA (48.0 mL, 0.35M, 2.2 eq). An exotherm to 38° C. was observed. The reaction mass was left to stir at RT. After 30 mins, HPLC indicated complete conversion. The reaction mass was diluted with EtOAc (2.5 L) and washed with KHCO3 (4×500 mL, 20 wt % aq. solution) and brine (500 mL). The organic phase was separated, dried over MgSO4 and concentrated. The residue was dissolved in TBME and reconcentrated to give ethyl (7R,8S)-8-{(2S)-2-benzyloxycarbonylamino-4-methylsulfanyl-butyr-yl-amino}-1,4-dioxa-spiro[4.5]decane-7-carboxylate 2 as a sticky semi-solid (76.2 g, 98% yield, 93 AP purity). 1H-NMR: (300 MHz, CDCl3) δ 7.36-7.30 (m, 5H), 7.03 (d, J 9.0 Hz, 1H), 5.66 (d, J 8.0 Hz, 1H), 5.10 (s, 2H), 4.35-4.25 (m, 2H), 4.19-4.04 (m, 2H,), 3.98-3.86 (m, 4H), 2.87-2.80 (m, 1H), 2.55-2.45 (m, 2H), 2.18 (dd, J 14.0 Hz, 7.0 Hz, 1H), 2.08 (s, 3H), 2.05-1.67 (m, 6H), 1.26 (t, J 7.0 Hz, 3H). HPLC: YMC-Pack Pro C18 5 μm 4.6×150 mm, 0.05% TFA (20% MeOH, 80% H2O), to 0.05% TFA (20% MeOH, 80% MeCN), 0-100% 10 min gradient. 10.01 min (Compound 2, 93.1 AP). HRMS: m/z 495.2166 [Calc: C24H35N2O7S 495.2165].
