Synthesis of peracetylated d-galactopyranosylidene dihalides

Upon treatment with aluminium trichloride in absolute chloroform, β-d-galactopyranose penta-O-acetate was converted in high yield into tetra-O-acetyl-β-d-galactopyranosyl chloride. In the presence of N-bromosuccinimide in boiling carbon tetrachloride, homolytic substitution of the anomeric hydrogen atom in this β-chloride occurred selectively to afford tetra-O-acetyl-1-bromo-β-d-galactopyranosyl chloride, in admixture with minor amounts of tetra-O-acetyl-d-galactopyranosylidene chloride. Bromine substitution by fluorine took place with a low stereoselectivity in tetra-O-acetyl-1-bromo-β-d-galactopyranosyl chloride when treated with 1.25 equiv silver fluoride in acetonitrile, whereas tetra-O-acetyl-d-galactopyranosylidene fluoride could be prepared in 54 % yield with 3.3 equiv AgF. The gem-difluoride could be deacetylated quantitatively. Upon treatment with 1,4-diazabicyclo〚2.2.2〛octane, peracetylated 1-bromo-β-d-galactopyranosyl chloride underwent 1,2-elimination of hydrogen bromide to afford 2,3,4,6-tetra-O-acetyl-d-lyxo-hex-1-enopyranosyl chloride in 33 % yield.