A solution containing about 1.26 g of Compound 39 in about 20 ml anhydrous DMF under nitrogen was cooled in an ice-salt bath. About 0.55 ml of N-methylmorpholin was added to the cooled solution, followed 5 minutes later by about 0.65 ml isobutylchloroformate. After stirring for a period of about 20 minutes, 1.20 g diethyl-L-glutamate hydrochloride was added, followed immediately by about 0.55 ml N-methylmorpholin. The reaction mixture was warmed to room temperature and stirred for 12 hours. At this time, the activation cycle was repeated using 1/2 the amounts of reagents indicated above, after which the reaction mixture was warmed to room temperature and stirred for an additional 12 hours. The solvents were removed under reduced pressure, and the residue was dissolved in about 100 ml CH2Cl2, washed with about 75 ml water, 50 ml 0.1 NHCl, and about 50 ml saturated NaCl. The organic layers were dried (MgSO4) and filtered. The filtrate was evaporated under reduced pressure and the residue was flash chromatographed on silica gel (2.4×24 cm), eluting first with CH2Cl2 and then with 1% methanol in CH2Cl2. Fractions showing a single spot corresponding to the product were pooled and evaporated under reduced pressure to yield about 1.29 g of Compound 34.
