Methanol → HCID 19782180, Verbindung

50 g (0.23 mol) of the compound from Example 13 are initially introduced in 780 ml of absolute CH2Cl2 at -78° C. 114 ml (1.2 mol) of boron tribromide in 360 ml of absolute CH2Cl2 are then added dropwise and the mixture is stirred at -78° C. for 30 minutes. It is then stirred at 0° C. for 2.5 h, subsequently cooled to -78° C. again and 690 ml of absolute MeOH are added cautiously. After 15 minutes, the mixture is evaporated, the residue is dissolved in 818 ml of MeOH, 272 ml of 2N NaOH solution are added and the mixture is stirred at 20° C. for 2 h. The methanol is evaporated in vacuo, the aqueous phase is washed with 200 ml of ethyl acetate and then acidified to pH 1-2 using concentrated hydrochloric acid with ice cooling. The aqueous phase is shaken twice with 100 ml of ethyl acetate each time, and the combined ethyl acetate phases are dried over Na2SO4 and evaporated. The residue is recrystallized from toluene.