ethyl oxo((phenylmethyl){[(3S)-4-(phenylmethyl)-3-morpholinyl]methyl}amino)acetate → (S)-8-Benzylhexahydropyrazino[2,1-C][1,4]oxazine-6,7-dione

1-Phenyl-N-{[(3S)-4-(phenylmethyl)-3-morpholinyl]methyl}methanamine (4.35 g, 14.7 mmol) and diisopropylethylamine (3.30 mL, 19.1 mmol) in THF (50 mL) cooled to 0° C. Ethylchlorooxo-acetate (1.80 mL, 16.2 mmol) was added dropwise via syringe. The ice bath was removed and the reaction was stirred at rt for 2 h. The reaction mixture was concentrated, taken up in EtOAc and washed with half saturated NaHCO3. The aqueous layer was back extracted with EtOAc, dried over MgSO4 and azeotroped in EtOH to provide ethyl oxo((phenylmethyl){[(3S)-4-(phenylmethyl)-3-morpholinyl]methyl}amino)acetate (5.83 g, 14.7 mmol, used without further purification). Ethyl oxo((phenylmethyl){[(3S)-4-(phenylmethyl)-3-morpholinyl]methyl}amino)acetate (5.83 g, 14.7 mmol) was hydrogenated (1 atm.) with Pd on carbon (1.18 g, 20% w/w with starting morpholine) for 13 days. The reaction was filtered through Celite® and washed thoroughly with MeOH. The solution was concentrated and subsequently triturated with EtOAc providing the title compound of step E (2.06 g, 7.9 mmol, 54% over two steps). 1H NMR (400 MHz, CDCl3) δ ppm 3.00 (ddd, J=13.9, 12.3, 4.2 Hz, 1H), 3.14-3.23 (m, 3H), 3.49 (td, J=12.1, 2.9 Hz, 1H), 3.72-3.82 (m, J=10.0, 10.0, 6.3, 4.0 Hz, 1H), 3.87 (dd, J=11.4, 3.7 Hz, 1H), 4.02 (dd, J=11.7, 4.4 Hz, 1H), 4.28 (dd, J=13.9, 2.2 Hz, 1H), 4.61-4.67 (m, 1H), 4.70-4.75 (m, 1H), 7.26-7.38 (m, 5H). Chiral HPLC analysis (Chiralpak AS-H, 4.6×150 mm, methanol (0.1% isopropyl amine), 1.0 mL/min, UV 254 nm, 25° C.) ee=99%, retention time, S=2.99 min. [α]D20° C.=−9.0:I=100 mm, c=0.975 in CHCl3.