To a 100 mL 3-neck flask equipped with magnetic stirrer and under argon atmosphere was added dichloromethane (7.0 mL), acetonitrile (7.0 mL) and triethylsilane (5.09 mL, 31.9 mmol) successively at room temperature. The above mixture was cooled to −20 to −25° C. and BF3.Et2O (3.03 mL, 23.9 mmol) was added in one portion. Another 100 mL flask was charged with crude (2S,3R,4S,5S,6R)-2-(4-chloro-3-(4-ethoxybenzyl)phenyl)-6-(hydroxymethyl)-2-methoxytetrahydro-2H-pyran-3,4,5-triol (3.5 g, 7.97 mmol), dichloromethane (7.0 mL) and acetonitrile (7.0 mL), and the resulting mixture was shaking for 20 min at ambient temperature until a clear solution was obtained. Under an atmosphere of nitrogen, the (2S,3R,4S,5S,6R)-2-(4-chloro-3-(4-ethoxybenzyl)phenyl)-6-(hydroxymethyl)-2-methoxytetrahydro-2H-pyran-3,4,5-triol solution in dichloromethane and acetonitrile was transferred to an addition funnel and was slowly added to the solution of BF3-Et2O and triethylsilane over a period of 1 h while keeping the internal temperature between −15 to −20° C. After the addition was completed, the mixture was stirred at a temperature between −15 to −20° C.
