Terbutyldimethylsilyl chloride (80 μmol, 12 mg) is added to a solution of the compound of Example 5 (80 μmol; 27 mg) and imidazole (160 μmol; 11 mg) in DMF (0.5 ml). The solution obtained is stirred for 20 hours then the reaction mass is poured into water. The aqueous phase is extracted twice with ethyl acetate, the organic phase is washed with water then with a solution of sodium chloride. The organic phase is dried over magnesium sulphate, filtered then evaporated. The residue is eluted on silica with a mixture of isopropyl acetate and dichloromethane (20/80). 20 mg of product is obtained in the form of a white powder.
