ABSTRACT
Sugar halides are important glycosyl donors. They are usually activated by means of heavy metal salts in glycosylations referred to as the Koenigs-Knorr reaction.1 The rst sugar anomeric dihalide reported was 2,3,4,6-tetra-O-acetyl-1-bromod-glucopyranosyl bromide isolated in a 17% yield by Blattner and Ferrier from products of photobromination of β-d-glucopyranose pentacetate with bromine in the absence of base.2 This method is synthetically useful, as a C-5 brominated
Experimental .......................................................................................................... 247 General Methods ............................................................................................... 247 2,3,4,6-Tetra-O-acetyl-1-bromo-β-d-glucopyranosyl Chloride (2) .................. 247 2,3,4,6-Tetra-O-acetyl-1-chloro-β-d-glucopyranosyl Fluoride (3) ...................248 2,3,4,6-Tetra-O-acetyl-1-uoro-d-glucopyranosyl Fluoride (4) .......................249
References .............................................................................................................. 252
compound can be isolated in 82-90% yield when the reaction is carried out under non acidic conditions. Investigations with tetrahydropyranyl ethers and sugar derivatives have demonstrated that axial C-H bonds next to an oxygen atom [as in ethers, acetals, including the C-1, the C-4 (furanose) and the C-5 (pyranose) positions in cyclic sugars] are more reactive toward homolysis, as compared to equatorial C-H bonds.3 This accounts for the higher reactivity of β-congured d-glycopyranosyl derivatives, as generally observed for reactions which involve homolysis of anomeric C-H bond as the key step. Similarly, axial C-H bonds next to the oxygen atom in cyclic sugar derivatives at the C-4 or C-5 positions are prone to homolysis. Bromination selectivity depends on steric and electronic factors and is markedly inuenced by the structural elements (atoms, substituents) at or near C-1 and C-4/C5: esteried d-glycopyranosyluronic derivatives readily undergo C-5 bromination due to the additional activating effect of the nearby ester group.4 Under radicalbased bromination conditions, 5-thio-sugar derivatives show a high reactivity with formation of various products that are brominated at C-1 and/or C-5.5 Protected glycopyranosyl cyanides6 are also valuable substrates that undergo bromination at C-1. Radical-mediated brominations at ring positions of carbohydrates have been reviewed by Somsák and Ferrier.7
