SYNTHESIS OF THE METHYL ALPHA-GLYCOSIDES OF A DISACCHARIDE, TRISACCHARIDE, AND A TETRASACCHARIDE FRAGMENT MIMICKING THE TERMINUS OF THE O-POLYSACCHARIDE OF VIBRIO-CHOLERAE O 1, SEROTYPE OGAWA/

Methyl do)-4,6-dideoxy-2-O-methyl-alpha-D-mannopyranoside was acetylat ed, and the fully protected methyl glycoside was treated with dichloro methyl methyl ether-ZnCl2 (DCMME-ZnCl2) reagent to give ido)-4,6-dideo xy-2-O-methyl-alpha-D-mannopyranosyl chloride (3). Condensation of 3 w ith methyl o-tetronamido)-4,6-dideoxy-alpha-D-mannopyranoside (4) gave the fully acetylated disaccharide 5, which was deacetylated yielding the methyl cr-glycoside of title disaccharide, The disaccharide glycos yl donor required for the blockwise synthesis of the title tri- and th e tetra-saccharide, o)-4,6-dideoxy-2-O-methyl-alpha-D-mannopyranosyl-( 1 --> ro-tetronamido)-4,6-dideoxy-alpha-D-mannopyranosyl chloride (12 ), was obtained by condensation of 3 with the 1-O-acetyl analog of 4, followed by treatment of the disaccharide formed with DCMME-ZnCl2. The synthesis of the methyl alpha-glycoside of the title trisaccharide in volved a condensation of 12 with 4, followed by deacetylation. Similar ly, the condensation of 12 with 15, the latter being the analog of 5 h aving a free HO-2, followed by deacetylation, gave the methyl alpha-gl ycoside of the title tetrasaccharide. All glycosylation reactions were mediated by silver trifluoromethanesulfonate in the presence of 2,4,6 -trimethylpyridine. -4,6-dideoxy-2-O-methyl-alpha,beta-D-mannopyranose was prepared for the first time, It was characterized by NMR spectros copy, and via its crystalline per-O-acetyl derivative. It is the sacch aride whose alpha-form constitutes the terminal, non-reducing end-grou p of the O-PS of V. cholerea O:1, serotype Ogawa.