[C:1]1([CH2:7][N:8]2[C:17](=O)[C:16](=O)[N:15]3[C@H:10]([CH2:11][O:12][CH2:13][CH2:14]3)[CH2:9]2)[CH:6]=[CH:5][CH:4]=[CH:3][CH:2]=1.[H-].[H-].[H-].[H-].[Li+].[Al+3].O.[OH-].[Na+]>CCOCC.CCOC(C)=O.C1(CN2C(=O)C(=O)N3[C@H](COCC3)C2)C=CC=CC=1>[C:1]1([CH2:7][N:8]2[CH2:17][CH2:16][N:15]3[C@H:10]([CH2:11][O:12][CH2:13][CH2:14]3)[CH2:9]2)[CH:2]=[CH:3][CH:4]=[CH:5][CH:6]=1
To a 0° C. mixture of two combined batches of (9aS)-8-(phenylmethyl)hexahydropyrazino[2,1-c][1,4]oxazine-6,7-dione (combined total 42.29 g, 162.5 mmol) in Et2O (406 mL) was added 1 M LiAlH4 in Et2O (406 mL, 406 mmol) via dropping funnel over 40 min. The mixture was then warmed to 35° C. and stirred for 6 days. The mixture was then cooled to 0° C., and EtOAc (100 mL) was slowly added, followed by water (20 mL), 15% aqueous NaOH (20 mL), and water (60 mL). The mixture was vigorously stirred for 1 h, and then diluted with EtOAc (500 mL). The mixture was filtered, and the filter cake was diluted with 1 N aqueous NaOH (500 mL) and extracted with Et2O (2×200 mL). The combined organic phase (filtrate and Et2O extractions) was dried over anhydrous Na2SO4, filtered, concentrated in vacuo, azeotroped with MeOH (100 mL), and dried overnight under high vacuum. The resulting colorless oil was combined with a second batch of product prepared in the same fashion from (9aS)-8-(phenylmethyl)hexahydropyrazino[2,1-c][1,4]oxazine-6,7-dione (0.3047 g, 1.1 mmol) to give crude (9aS)-8-(phenylmethyl)octahydropyrazino[2,1-c][1,4]oxazine (combined total 38.59 g, >100% crude yield). LCMS: (M+H)+: 233.1.
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