Synthesis of peracetylated D-galactopyranosylidene dihalides

Upon treatment with aluminium trichloride in absolute chloroform, beta -D-g alactopyranose penta-O-acetate was converted in high yield into tetra-O-ace tyl-beta -D-galactopyranosyl chloride. In the presence of N-bromosuccinimid e in boiling carbon tetrachloride, homolytic substitution of the anomeric h ydrogen atom in this beta -chloride occurred selectively to afford tetra-O- acetyl-1-bromo-beta -D-galactopyranosyl chloride, in admixture with minor a mounts of tetra-O-acetyl-D-galactopyranosylidene chloride. Bromine substitu tion by fluorine took place with a low stereoselectivity in tetra-O-acetyl- 1-bromo-beta -D-galactopyranosyl chloride when treated with 1.25 equiv silv er fluoride in acetonitrile, whereas tetra-O-acetyl-beta -galactopyranosyli dene fluoride could be prepared in 54% yield with 3.3 equiv AgF. The gem-di fluoride could be deacetylated quantitatively. Upon treatment with 1,4-diaz abicyclo[2.2.2] octane, peracetylated 1-bromo-beta -D-galactopyranosyl chlo ride underwent 1,2-elimination of hydrogen bromide to afford 2,3,4,6-tetra- 0-acetyl-D-lyxo-hex-1-enopyranosyl chloride in 33 % yield. (C) 2001 Academi e des sciences / Editions scientifiques et medicales Elsevier SAS.