Sulfonic esters of carbohydrates. II

Recent trends in the synthesis of O-glycosides of 2-amino-2-deoxysugars

The discovery of new methods for stereoselective glycoside synthesis and convergent oligosaccharide assembly has been critical for the area of glycosciences. At the heart of this account is the discussion of the approaches for stereoselective synthesis of glycosides of 2-amino-2-deoxysugars that have emerged during the past two decades. The introductory part provides general background information and describes the key features and challenges for the synthesis of this class of compounds. Subsequently, major approaches to the synthesis of 2-amino-2-deoxyglycosides are categorized and discussed. Each subsection elaborates on the introduction (or protection) of the amino functionality, synthesis of glycosyl donors by introduction of a suitable leaving group, and glycosidation. Wherever applicable, the deprotection of a temporary amino group substituent and the conversion onto the natural acetamido functionality is described. The conclusions part evaluates the current standing in the field and provides a perspective for future developments.

Epoxide Migration and Pseudo-Epoxide Migration of 1,6:2,3- and 1,6:3,4-Dianhydro-β-D-hexopyranoses. Synthesis of Some Deoxy Halo Derivatives of 1,6-Anhydro-β-D-hexopyranoses

Epoxide or pseudo-epoxide migration of 1,6:2,3-dianhydro- and 1,6:3,4-dianhydro-β-D-hexopyranoses was effected by treatment with aqueous sodium hydroxide or sodium iodide in acetone to give equilibrium mixtures. Various iodo derivatives of 1,6-anhydro-β-D-hexopyranoses were prepared as potential intermediates for pseudo-epoxide migration. NMR was used for following the reaction mechanism of epoxide and pseudo-epoxide migrations and analysis of reaction mixtures. Experimental data were compared with DFT calculations. Chair-boat equilibration of 1,6-anhydro-3-deoxy-3-halo-β-D-glucopyranoses was discussed.

Sucrose analogues modified at position 3: chemoenzymatic synthesis and inhibition studies of dextransucrases

Conditions for the large-scale (molar) oxidation of sucrose by Agrobacterium tumefaciens were improved, thus leading to homogeneous solutions of 3-ketosucrose in 40% yield. Treatment of this solution with hydroxylamine or methoxylamine afforded the corresponding oximes 3a and 3b (isolated as acetates) in excellent yield. Dissolving-metal reduction of these oximes gave mixtures of amino disaccharides in which the gluco epimer (3-amino-3-deoxysucrose) was predominant. A more efficient approach to this amino sucrose was provided by the highly stereoselective hydrogenation of 3-ketosucrose peracetate (7), which gave exclusively the allo isomer 8 (2,4,6-tri-O-acetyl-alpha-D-allopyranosyl 1,3,4,6-tetra-O-acetyl-beta-D-fructofuranoside). Upon reaction with lithium azide, the triflate derived from 8, compound 9, afforded 3-azido-3-deoxysucrose peracetate (10) which was converted into 3-amino-3-deoxysucrose (12). The reaction of triflate 9 with potassium ethylxanthate led to a mixture of products (the expected 3-S-ethoxythiocarbonyl-3-thiosucrose derivative and the peracetates of 3-thiosucrose and of 3-thiosucrose disulfide), which could be all converted into 3-thiosucrose (17). Sucrose analogues 12 and 17 were not substrates of dextransucrases from various strains of L. mesenteroides, nor did they participate in glycosyl transfer reactions to an acceptor (maltose). Compounds 3a and 12 were found to be strong competitive inhibitors of the dextran synthesis process (dextransucrase from strain B-1397). These results indicate that 3a and 12 compete effectively with sucrose for the sucrose binding site but are unable to participate as glycosyl donors in the polymerization or glycosyl-transfer processes.