CLARIFICATION OF THE TRANSFER-REACTION IN THE SYNTHESIS OF MANNO-OLIGOSACCHARIDE

The cationic ring-opening reaction of ,6-anhydro-2,3,4-tri-0-benzyl-be ta-D-mannopyranose was applied to the synthesis of alpha-(1-->6)-linke d manno-oligosaccharide which is a part of natural, high-mannosetype o ligosaccharides. Dimethoxymethane was added into the reaction system a s a transfer reagent in order to diminish the molecular weight of the products. It was confirmed by gel permeation chromatography measuremen ts that additional dimethoxymethane distinctly lowered the molecular w eight of the oligomerization products. The presence of the -OCH3 group , which was transferred from dimethoxymethane, at the reducing termina l of the obtained oligosaccharide derivatives was confirmed by C-13 NM R measurements. In addition, methyl -0-benzyl-6-O-methoxymethyl-alpha- D-manopyranoside was synthesized as a model compound. From the compari son of the C-13 NMR spectrum of the reaction product with that of the model compound, -O-CH2-O-CH3 group binding to the nonreducing end was found to be present. These results showed the mechanism of the transfe r reaction. Since another product was detected by using C-13 NMR spect roscopy, isolation of the products was carried out by silica gel colum n chromatography. It was shown by H-1 and C-13 NMR spectroscopies, COS Y, HMQC, and HMBC that a 4,6-methylidene structure was also formed at the nonreducing end of the oligosaccharide derivatives.