GENERAL STEPS: Two 100 ml bottles of methanol were taken and cooled to 0°C under nitrogen protection and acetyl chloride (2.8 ml, 30 mmol) was added slowly. After keeping the reaction at 0°C for 30 min, a DMF solution (15 mL) of (2S,3S,4S,5R,6S)-6-((5,6-dihydroxy-2-(4-hydroxyphenyl)-4-oxo-4H-benzopyran-7-yl)oxy)-3,4,5-trihydroxytetrahydro-2H-pyran-2-carboxylic acid (Compound 1, 4.62 g, 10 mmol) was added and stirred for 15 min. Slowly return to room temperature and gradually a white solid precipitated. The reaction mixture was heated to 40°C to continue the reaction for 4 hours, followed by cooling to -10°C for 1 hour. The reaction mixture was filtered and the filter cake was washed with water and dried under vacuum. A yellow solid (2S,3S,4S,5R,6S)-6-((5,6-dihydroxy-2-(4-hydroxyphenyl)-4-oxo-4H-benzopyran-7-yl)oxy)-3,4,5-trihydroxytetrahydro-2H-pyran-2-carboxylic acid methyl ester was obtained (target product, 3.65 g) in 77% yield.
