3-Hydroxy-2,2-dimethyl-propionic acid tert-butyl ester (137 mg) and sodium hydride (60%; 32 mg) were added to a solution of the compound obtained in Example 9 (1) (300 mg) dissolved in tetrahydrofuran (5 ml) and stirred at 50° C. for 2 hours. The reaction mixture was cooled to room temperature, a saturated sodium hydrogen carbonate aq. solution and ethyl acetate were added and the organic layer was separated, washed with a saturated brine, dried over magnesium sulfate and concentrated in vacuo. The resulting residue was purified by column chromatography (silica gel; hexane:ethyl acetate=7:3→2:3) to give the titled compound (227 mg). MS (m/z):607 [M+H]+.
