[OH-].[Na+].[CH3:3][S:4][CH2:5][C:6]1[O:11][CH2:10][N:9]([C:12]([CH3:19])([CH3:18])[C:13]([O:15]CC)=[O:14])[C:8](=[O:20])[C:7]=1[C:21]1[CH:26]=[CH:25][CH:24]=[CH:23][CH:22]=1>C(O)C.O1CCOCC1.O>[CH3:3][S:4][CH2:5][C:6]1[O:11][CH2:10][N:9]([C:12]([CH3:19])([CH3:18])[C:13]([OH:15])=[O:14])[C:8](=[O:20])[C:7]=1[C:21]1[CH:22]=[CH:23][CH:24]=[CH:25][CH:26]=1A solution of sodium hydroxide (16.43 g) in 10 water was added to a stirred solution of ethyl 2-(2,3-dihydro-6-methylthiomethyl-4-oxo-5-phenyl-4H-1,3-oxazin-3-yl)-2-methylpropanoate (68 g) in a mixture of ethanol and 1,4-dioxan. The solution was heated at 60° C. for 6 hours, allowed to cool and the solvent was removed under reduced pressure to give an orange oil which was dissolved in water. The aqueous solution was washed with dichloromethane, acidified to pH 1 with concentrated hydrochloric acid, extracted with dichloromethane, the extract dried (magnesium sulphate) and evaporated under reduced pressure. The residue was purified by chromatography on silica gel, eluting with dichloromethane/ethyl acetate to give 2-(2,3-dihydro-6-methylthiomethyl-4-oxo-5-phenyl-4H-1,3-oxazin-3-yl)-2-methylpropanoic acid (24.2 g) as a yellow solid, m.p. 136-137° C.
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