The b (1 eq, 20 mmol) in DCM (20 mL) was added PCC (1.5 eq) and silica gel, then was stirred for 12 h at r. t. After reacted completely, the reaction was filtrated by celite and residue was washed by DCM (2 x 20 mL). The combined filtrate was dried over anhydrous Na2SO4. After removal of the solvent, the crude residue was purified by flash column chromatography on silica gel with hexanes/EtOAc (30: 1) as the eluent to afford c as colorless oil (62% yield).