Cbz-Val-Phe-OH (4.67 g, 11.7 mmol, obtained from Sigma Chemical Company, St. Louis, Mo. 63178) is combined with paraformaldehyde (5 g) and p-toluenesulfonic acid monohydrate (500 mg, 2.6 mmol) in benzene (120 mL). The reaction is heated at reflux for 23 hours with continuous removal of water with a Dean-Stark trap. The reaction then cooled to room temperature and concentrated under vacuum. The residue is dissolved in ethyl acetate (100 mL) and saturated aqueous sodium bicarbonate (60 ) is added with mixing. The layers are then separated and the aqueous layer is extracted with ethyl acetate (2×50 mL). The organic layer and organic extracts are combined and dried over anhydrous magnesium sulfate, passed through a short pad of silica gel and the filtrate is concentrated under vacuum. The residue is then purified by flash chromatography (hexane/ethyl acetate, 95:5 to 90:10 to 80:20, silica gel) to provide [S-(R*,R*)]-[2-methyl-1-[[5-oxo-4-(phenylmethyl)-3-oxazolidinyl]carbonyl]propyl]-carbamic acid, phenylmethyl ester (2.00 g, 42%) as a foam; [α]20D +113.95 (c 0.55, CHCl3); IR (CHCl3) 3314, 3032, 2934, 1804, 1714, 1659, 1437, 1233 cm-1 ; MS m/z 411 (M+H+), 367, 268, 234, 206, 178, 162, 91.
