To a solution of oxalyl chloride (1.05 mL, 17.22 mmol) in dry tetrahydrofuran (40 mL) cooled to -78° C., dry dimethyl sulfoxyde (1.3 mL, 18.04 mmol) was added dropwise and then stirred during 35 min at -35° C. The reaction mixture was cooled again to -78° C. and methyl 2,3,4-tri-O-benzyl-α-D-glucopyranoside (6 g, 16.4 mmol) dissolved in tetrahydrofuran (20 mL) was added and the mixture was stirred during 15 min at -35° C., then triethylamine (11.5 mL, 82.65 mmol) was added and the mixture was stirred during 1 h at -35° C. This aldehyde was used without purification and isolation in a Wittig reaction described as follows. To dried triphenylmethylphosphonium bromide (11.7 g, 32.8 mmol) suspended in tetrahydrofuran (700 mL) was added dropwise at -78° C. a 1.42 M solution of n-butyllithium in hexane (23 mL, 32.66 mmol). The reaction mixture was warmed to room temperature and stirred during 1.5 h. Then the mixture was cooled to 0° C. and potassium tertio-butylate (3.68 g, 32.8 mmol) and dry tertio-butyl alcohol (3mL, 31.8 mmol) were added. The mixture was stirred again at room temperature during 30 min. The reaction mixture was cooled to -78° C. and the tetrahydrofuran solution of the aldehyde prepared above was added dropwise. The reaction mixture was warmed to room temperature and stirred during 2 h. A saturated aqueous solution of ammonium chloride and the solvents were evaporated under reduced pressure. The residue was dissolved in ether and washed with water. The organic layer was dried over sodium sulfate, filtered and concentrated under reduced pressure to afford a brown oil. Flash chromatography on silica gel and elution with a 4:96 mixture of ethyl acetate and toluene afforded the expected olefine methyl 2,3,4-tri-O-benzyl-6,7-dideoxy-α-D-glucohept-6-enopyranoside (3.26 g, 55%) which crystallized from hexane; m.p. 46°-47° C.
