370 mg of the product of Example 58, 307 mg of trityl chloride in the presence of 10 mg of 4-dimethylamino-pyridine, 0.25 ml of triethylamine and 4 ml of dimethylformamide were stirred at ambient temperature for 16 hours. The mixture was heated to 40° C. for 4 hours, poured into water and extraction was carried out with ether. The extracts were washed with water and dried and the solvent was eliminated under reduced pressure. The residue was chromatographed on silica (eluant: cyclohexane--ethyl acetate 75-25) to obtain 467 mg of the expected product with a Rf=0.25.
