Acetic acid (10 μL, 0.17 mmol, 5.4 eq) is added at room temperature to a stirred solution of [trans-4-(3-hydroxy-6-methoxy-3,4-dihydro-2H-1-oxa-4,5,9-triaza-phenanthren-3-yl)-cyclohexyl]-carbamic acid benzyl ester (15 mg, 0.03 mmol, 1.0 eq) in dichloromethane (3 mL) and methanol (3 mL), followed by sodium cyanoborohydride (10 mg, 0.16 mmol, 5.0 eq). After 30 minutes stirring at room temperature, the reaction mixture is extracted with ethyl acetate (3×10 mL) and a saturated ammonium chloride aqueous solution (10 mL). The combined organic layers are dried over sodium sulfate, filtered and concentrated to give a residue that is purified by column chromatography (silica gel, eluent: dichloromethane:methanol, 10:1, v/v) to afford [trans-4-(6-methoxy-3,4-dihydro-2H-1-oxa-4,5,9-triaza-phenanthren-3-yl)-cyclohexyl]-carbamic acid benzyl ester as a white solid (11 mg, 76% yield).
