To a stirred solution of oxalyl chloride (12.8 mL, 147.7 mmol) in DCM (150 mL) was added drop wise a solution of DMSO (13.1 mL) in DCM (90 mL) over 10 min at −78° C. and then the reaction mixture was stirred at same temperature for 40 min. Then a solution of (2S,3R,4R,5R)-2,3,4,6-tetrakis(benzyloxy)hexane-1,5-diol 2 in DCM (75 mL) was added drop wise to the reaction mixture over a period of 15 min keeping the reaction mixture below −70° C. After stifling the reaction mixture for 2 h at below −70° C., Et3N (62.15 mL, 443 mmol) was added drop wise over a period of 10 min and then the reaction mixture was allowed to warm to −5° C. over a period of 2 h. The reaction mixture was concentrated under reduced keeping the temperature below 25° C. to afford crude (2R,3R,4S)-2,3,4,6-tetrakis(benzyloxy)-5-oxohexanal 3 (18 g, 90%) as sticky yellowish oil which was used for the next step without any purification.
