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.

Syntheses of disaccharides with (1----4)-beta glycosidic linkages related to the 4- and 6-sulfates and the 4,6-disulfates of chondroitin

The solution salts of the 6'-sulfate 12, the 4'-sulfate 15 and the 4',6'-disulfate 17 of benzyl 4-O-(beta-D-galactopyranosyl)-beta-D-glucopyranosiduronate 10 have been synthesized. Methyl [benzyl 2,3-di-O-benzoyl-4-O-(2,3-di-O-benzoyl-beta-D-galactopyranosyl)-beta-D-+ ++glucopyranosid]uronate (9) has been prepared as a key intermediate from benzyl 4',6'-O-benzylidene-beta-D-lactopyranoside (2). Protection of 2 at C-6 with the tert-butyldimethylsilyl group, followed by O-perbenzoylation and disilylation, gave benzyl 2,3-di-O-benzoyl-4-O-(2,3-di-O-benzoyl-4,6-O-benzylidene-beta-D-galac top yranosyl)-beta-D-glucopyranoside (7). Oxidation of the 6-position of 7 proved to be difficult. However, 7 could be converted into the tert-butyl glucuronate 8 using chromium trioxide-pyridine and tert-butanol. Simultaneous hydrolysis of the benzylidene acetal and the tert-butyl ester groups, followed by esterification of the resulting free acid with diazomethane, yielded 9. Compound was directly sulfated with sulfur trioxide-trimethylamine within 12 h to give the 6'-sulfate 11. The 4',6-disulfate 16 was accessible by running the reaction under the same conditions for 14 days. The 4'-sulfate 14 was obtained after protecting the 6'-OH group of 9 with benzoyl cyanide to give the 6'-benzoate 13 followed by sulfation under more vigorous reaction conditions. Deesterification of 9, 11, 14, and 16 was achieved by treatment with aqueous sodium hydroxide in tetrahydrofuran to give 10, 12, 15, and 17, respectively.

Synthesis of beta-D-GlcA-(1----3)-beta-D-Gal disaccharides with 4- and 6-sulfate groups and 4,6-disulfate groups

The sodium salts of the 6-sulfate 7, the 4-sulfate 10, and the 4,6-disulfate 12 of benzyl 3-O-(beta-D-glucopyranosyl uronate)-beta-D-galactopyranoside (5) have been synthesized. Methyl (2,3,4-tri-O-acetyl-1-bromo-1-deoxy-alpha-d-glucopyran)uronate (1) was coupled with benzyl 2-O-benzoyl-4,6-O-benzylidene-beta-D-galactopyranoside (2) to yield 3. The benzylidene acetal of 3 was hydrolyzed to give benzyl 2-O-benzoyl-3-O-[methyl (2,3,4-tri-O-acetyl-beta-D-glucopyranosyl)uronate]-beta-D-galactopyra noside (4). Compound 4 was utilized as a key intermediate to prepare the sulfated disaccharides 7,10, and 12. Direct sulfation of 4 with sulfur trioxide-trimethylamine for 2 days yielded the 6-sulfate 6. The 4,6-disulfate 11 was accessible by running the reaction under the same conditions for 14 days. The 4-sulfate 9 was obtained after protecting the 6-OH group of 4 by reaction with benzoyl imidazole to give the 6-benzoate 8, followed by sulfation under vigorous conditions. Treatment of the protected compounds 4, 6, 9, and 11 with aqueous sodium hydroxide in tetrahydrofuran gave the unprotected 5, 7, 10, and 12, respectively.