To 349 mg (0.75 mmol) of di-tert-butyl (4S)—N-(tert-butoxycarbonyl)-4-(4-hydroxy-benzyl)-L-glutamate (Example 1 b) in 15 mL NA-dimethylformamide were added 138 mg (1.0 mmol) of potassium carbonate and 332 mg (1.0 mmol) of 2-phenyl-1,3-dioxan-5-yl 4-methylbenzenesulfonate (Studii si Cercetari de Chimie (1960), 8, 187-99.) and the resulting suspension was heated for 2 h at 100° C. in a microwave oven. The reaction mixture was then filtered, the solvent evaporated and the residue was taken up in DMSO. The crude product obtained in this manner was chromatographed on a PrepCon® C-18 reversed phase column using a water/acetonitrile gradient (35/65 to 0/100), and the appropriate fractions were combined and concentrated.
