Acetic acid anhydride (498 μl, 5.17 mmol) and formic acid (237 μl, 6.28 mmol) were stirred first 2 h at 50° C. and then overnight at rt. Subsequently, the mixture was diluted with dichloromethane (5.1 ml), and 1.16 ml of this solution were added to a solution of Example 49 (233 mg, 370 μmol) in pyridine (89 μl). After stirring at rt for 2 h, the mixture was diluted with methanol and then evaporated. The residue was purified by preparative RP-HPLC (Reprosil C18, gradient 10-95% acetonitrile/0.1% aq. formic acid) affording 77 mg (35% of th.) of title compound.
