A solution of 25 (535 mg, 1.99 mmol), 1 (335 mg, 2.09 mmol), and acetic acid (629 mg, 10.48 mmol) in methanol(2 mL)/dichloroethane(1 mL) was treated with sodium cyanoborohydride (130 mg, 2.06 mmol) at room temperature. The resulting mixture was stirred at room temperature overnight. The solvent was removed in vacuo and the residue dissolved in dichloromethane and sodium carbonate solution. The aqueous layer was extracted with two additional portions of dichloromethane and the combined organic extracts were washed with brine, dried over Na2SO4, and concentrated under reduced pressure. PCTLC (SiO2, 4 mm, 10% EtOH-90% CHCl3) afforded the title compound (26).
