Reaction Steps
(3S,5R)-4-N-acetyl-3-methyl-5-phenyl-3,4,5,6,-tetrahydro-2H-1,4-oxazin-2-one (68) (220 mg, 0.94 mmol) and L-alanine tert-butyl ester (66) (205 mg, 1.42 mmol, 1.5 equiv.) were dissolved in anhydrous dichloromethane (14 mL) in a doubly sealed PTFE cylinder and subjected to ultra-high pressure of 19 kbar for 48 hours. After releasing the pressure, the solution was filtered through a short pad of CELITE® diatomaceous earth and removal of solvent from the filtrate in vacuo yielded the crude product which was purified by flash column chromatography on silica eluting with diethyl ether and dichloromethane (4:1) to furnish the title product as fine colourless needles (120 mg, 34%); m.p. 38-40° C.; v(max)(KBr) 3411 (O—H), 2980 (C—H), 1735 (C═O, lactone), 1646 (C═O, amide) cm−1; δH (250 MHz, DMSO-d) 8.06 (0.5H, d, /6.95 Hz, NH×0.5), 7.55-7.24 (5H, m, Ph), 6.54 (0.5H, d, J 6.38 Hz, NH×0.5), 5.82 (0.5H, t, /4.88 Hz, OH×0.5), 5.14-5.06 (1H, m, PhCH), 4.83 (0.5H, t, J 6.56 Hz, OH×0.5), 4.35 (0.5H, q, J 6.99 Hz, CH3CH×0.5), 4.08-4.03 (2H, m, CH2OH), 3.98-3.76 (1.5H, m, CHCH3×0.5 and CHCH3×1), 2.23 (1.5H, s, COCH3), 2.14 (1.5H, s, COCH3), 1.46-1.39 (12H, m, CHCH3×3 and C(CH3)3×9), 1.19 (1.5H, d, /7.15 Hz, CHCH3×1.5), 0.97 (1.5H, d, J 7.23 Hz, CHCH3×1.5); δc (62.5 MHz, DMSO-d) 171.8, 171.7, 171.0, 170.8, 170.2, 140.0, 138.2, 129.1, 128.2, 128.0, 126.8, 80.7, 62.7, 62.5, 60.8, 56.3, 53.4, 48.8, 48.7, 27.9, 17.5, 16.9, 16.7, 15.2; m/z (CL, NH3), 379 (MH+, 70%), 361 (18%), 305 (20%), 249 (38%), 234 (100%), and 219 (40%); HRMS for C20H31N2O5 requires 379.2225 found 379.2240; [α]D20 −37.0 (c 1.06 CHCl3).
