A Curious By-Product in the Tipson-Cohen Reaction of a Carbohydrate Ditosylate

Abstract

The treatment of methyl 4,6-O-benzylidene-2,3-di-O-p-toluenesulfony-β- D- galactoside , methyl 4,6-O-benzylidene-3-O-benzenesulfonyl-2-O-p- toluenesufonyl-β-D-galactoside and methyl 4,6-O-benzylidene-2-O- benzenesulfonyl-3-O-p-touenesulfonyl-β-D-galactoside under Tipson-Cohen conditions (Zn/ Nal/dimethylformamide ) gives mainly the alkene, methyl 4,6-O-benzylidene-2,3-dideoxy-β-D-threo-hex-2-enoside. However, the first two glycosides give methyl 4,6-O-benzylidene-3-deoxy-3-C-(m-methylphenyl )-β-D- galactoside, and the last, methyl 4,6-O-benzylidene- 3-deoxy-3-C-phenyl-β-D-gaactoside, as minor by-products resulting from a curious reaction of the supposed iodo sulfonate intermediate. The structure of methyl 4,6-O-benzylidene-3-deoxy-3-C-(m- methylphenyl )-β-D- galactoside was confirmed by a single-crystal X-ray structure determination of the derived p-bromobenzoate.