[C:1]([O:5][C:6](=[O:12])[C@@H:7]([CH:9]([CH3:11])[CH3:10])[NH2:8])([CH3:4])([CH3:3])[CH3:2].C(O)(=O)C.[O:17]=[C:18]1[C:26]2[C:21](=[CH:22][CH:23]=[CH:24][CH:25]=2)[C:20](=[O:27])[N:19]1[CH2:28][CH:29]=O.C([BH3-])#N.[Na+].C([O-])(O)=O.[Na+]>CO.C1COCC1>[O:17]=[C:18]1[C:26]2[C:21](=[CH:22][CH:23]=[CH:24][CH:25]=2)[C:20](=[O:27])[N:19]1[CH2:28][CH2:29][NH:8][C@H:7]([CH:9]([CH3:10])[CH3:11])[C:6]([O:5][C:1]([CH3:4])([CH3:3])[CH3:2])=[O:12](R)-Valine t-butyl ester (4.9 g; 28.28 mmol) was dissolved in methanol (100 ml). Acetic acid (1.62 ml; 28.28 mmol) and (1,3-dioxo-1,3-dihydroisoindol-2-yl)-acetaldehyde (5.40 g; 28.28 mmol) were added thereto. Then, sodium cyanoborohydride (1.95 g; 31.11 mmol) dissolved in THF (20 ml) was added. The mixture was stirred at RT for 6 h and then saturated NaHCO3 solution (200 ml) was added. Methanol was removed under reduced pressure, and the residue was extracted with EtOAc (3×; 100 ml). The combined organic phases were dried over MgSO4. Removal of the solvent under reduced pressure and purification of the residue by column chromatography (SiO′) resulted in t-butyl (R)-2-(2-(1,3-dioxo-1,3-dihydroisoindol-2-yl)ethylamino)-3-methylbutyrate (6.60 g; 19.05 mmol). MS: 347.20 (M+H); Rt: 1.02 min (method: gradient 0 min 96% H2O (0.05% TFA) 2.0 min 95% acetonitrile, 95% acetonitrile to 2.4 min, 4% acetonitrile 2.45 min; flow rate 1 ml/min; column 0.4 μL (YMC J'sphere ODS H80 20X2 1.4μ); 30° C.)
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