A solution of 2′,3′-dideoxy-3′-fluoroguanosine (1.35 g, 5 mmole), 3,3-bis (N-CBZ-L-valyloxymethyl)-propionic acid (3.6 g, 6 mmole), DMAP (0.061 g, 0.5 mmole) and HOBT (0.81 g, 6 mmole) was coevaporated two times with DMF and reduced to about 120 ml. DCC (1.24 g, 6 mmole) was added and the mixture was stirred overnight at room temperature. The mixture was filtered and the solution was evaporated under reduced pressure. Ethyl acetate (200 ml) was added and the organic phase washed twice with 5% acetic acid, 5% sodium hydrogen carbonate and water. The organic phase was dried with sodium sulfate and evaporated under reduced pressure. The product was isolated by silica gel column chromatography. Yield: 2.7 g
