Synthesis of fluorinated thiodigalactoside analogues

Synthesis of fluoro- and seleno-containing D-lactose and D-galactose analogues

Synthetic deoxy-fluoro-carbohydrate derivatives and seleno-sugars are useful tools in protein-carbohydrate interaction studies using nuclear magnetic resonance spectroscopy because of the presence of the ¹⁹F and ⁷⁷Se reporter nuclei. Seven saccharides containing both these atoms have been synthesized, three monosaccharides, methyl 6-deoxy-6-fluoro-1-seleno-β-D-galactopyranoside (1) and methyl 2-deoxy-2-fluoro-1-seleno-α/β-D-galactopyranoside (2α and 2β), and four disaccharides, methyl 4-O-(β-D-galactopyranosyl)-2-deoxy-2-fluoro-1-seleno-β-D-glucopyranoside (3), methyl 4-Se-(β-D-galactopyranosyl)-2-deoxy-2-fluoro-4-seleno-β-D-glucopyranoside (4), and methyl 4-Se-(2-deoxy-2-fluoro-α/β-D-galactopyranosyl)-4-seleno-β-D-glucopyranoside (5α and 5β), the three latter compounds with an interglycosidic selenium atom. Selenoglycosides 1 and 3 were obtained from the corresponding bromo sugar by treatment with dimethyl selenide and a reducing agent, while compounds 2α/2β, 4, and 5α/5β were synthesized by the coupling of a D-galactosyl selenolate, obtained in situ from the corresponding isoselenouronium salt, with either methyl iodide or a 4-O-trifluoromethanesulfonyl D-galactosyl moiety. While benzyl ether protecting groups were found to be incompatible with the selenide linkage during deprotection, a change to acetyl esters afforded 4 in a 17% overall yield and over 9 steps from peracetylated D-galactosyl bromide. The synthesis of 5 was performed similarly, but the 2-fluoro substituent led to reduced stereoselectivity in the formation of the isoselenouronium salt (α/β ∼ 1 : 2.3). However, the β-anomer of the uronium salt could be obtained almost pure (∼98%) by precipitation from the reaction mixture. The following displacement reaction occurred without anomerisation, affording, after deacetylation, pure 5β.

The Synthesis and Glycoside Formation of Polyfluorinated Carbohydrates

Fluorinated carbohydrates have found many applications in the glycosciences. Typically, these contain fluorination at a single position. There are not many applications involving polyfluorinated carbohydrates, here defined as monosaccharides in which more than one carbon has at least one fluorine substituent directly attached to it, with the notable exception of their use as mechanism-based inhibitors. The increasing attention to carbohydrate physical properties, especially around lipophilicity, has resulted in a surge of interest for this class of compounds. This review covers the considerable body of work toward the synthesis of polyfluorinated hexoses, pentoses, ketosugars, and aminosugars including sialic acids and nucleosides. An overview of the current state of the art of their glycosidation is also provided.