A Comparison in the Efficiency of Six Standard 2-Amino-2-Deoxy-Glucosyl Donors for the Synthesis of (2-Deoxy-2-Phthalimido-β-D-Glucopyranosyl) (1→ 4)-β-D-Glucopyranosides

Synthesis of 2-Amino-2-deoxy-β-D-galactopyranosyl-(1→4)-2-amino-2-deoxy-β-D-galactopyranosides: Using Various 2-Deoxy-2-phthalimido-D-galactopyranosyl Donors and Acceptors

A systematic study is presented of the efficiency of the most common glycosylation methods using standard 2-deoxy-2-phthalimidogalactopyranosyl donors ethyl 4-O-acetyl-3,6-di-O- benzyl-2-deoxy-2-phthalimido-1-thio-β-D-galactopyranoside (3a), 4-O-Acetyl-3,6-di-O-benzyl- 2-deoxy-2-phthalimido-β-D-galactopyranosyl bromide (4), 4-O-acetyl-3,6-di-O-benzyl-2-deoxy-2-phthalimido-β-D-galactopyranosyl fluoride (5b), O-(4-O-acetyl-3,6-di-O-benzyl-2-deoxy-2-phthalimido-β-D-galactopyranosyl) trichloroacetimidate (7) and ethyl 3,6-di-O-benzyl-2-deoxy-2-phthalimido-1-thio-β-D-galactopyranoside (8), pent-4-enyl 3,6-di-O-benzyl- and 3-O-allyl-6-O-benzyl-2-deoxy-2-phthalimido-β-D-galactopyranoside (10a) and (10b) and pent-4-enyl 3,6-di-O-benzyl-2-deoxy-2-phthalimido-4-O-(trimethylsilyl)-β-D-galactopyranoside (11) as glycosyl acceptors in the synthesis of 2-amino-2-deoxy-β-D-galactopyranosyl-(1→4)-2-amino-2-deoxy-β-D-galactopyranosides 12, 16a and 17a. It was found that due to a low reactivity of the axial OH(4) group of glycosyl acceptors, disaccharides 16b and 17b with α(1→4) bond were also formed. The unexpected intermolecular migration of ethylsufanyl group from the reducing end of glycosyl acceptor 8 the reducing end of the activated form of glycosyl donor 4 in the glycosylation step to give ethylsulfanyl derivative 3a was proved. For preparation of the glycosyl donors and glycosyl acceptors with galacto configuration an approach based on epimerization of 4-O-mesyl derivatives of appropriate synthons with gluco configuration 2a and 2b was employed.

Synthesis of the Sda determinant and two analogous tetrasaccharides

To contribute to the possibility of studying in greater detail the biological significance of Sda-containing glycans as occur in Tamm-Horsfall glycoprotein, the following three spacer-linked tetrasaccharides have been synthesized: the Sda determinant alpha-Neu p5Ac-(2-->3)-[beta- D-GalpNAc-(1-->4)]-beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->O)(CH2)5NH 2 (1), the Gal-analogue alpha-Neup5Ac-(2-->3)-[beta-D-Galp-(1-->4)]-beta-D-Galp-(1-->4) -beta-D-GlcpNAc-(1-->O)(CH2)5NH2 (2), and the GlcNAc-analogue alpha-Neup5Ac-(2-->3)-[beta-D-GlcpNAc-(1-->4)]- beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->O)(CH2)5NH2 (3). The general trisaccharide acceptor 5-azidopentyl (methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy- D-glycero-alpha-D-galactonon-2-ulopyranosylonate)-(2-->3)-(2 ,6-di-O-benzyl- beta-D-galactopyranosyl)-(1-->4)-3,6-di-O- benzyl-2-deoxy-2-phthalimido-beta-D-glucopyranoside was prepared, using methyl (phenyl 5- acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-thio-D-glycero-D-galacto- non- 2-ulopyranosid)- onate as the sialyl donor. For the syntheses of 1, 2, and 3 the glycosyl donors 3,4,6-tri-O- acetyl-2-deoxy-2-phthalimido-alpha-D-galactopyranosyl bromide, 2,3,4,6-tetra-O-acetyl-alpha-D- galactopyranosyl bromide, and 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido -beta-D-glucopyranosyl trichloroacetimidate, respectively, proved to be the most suitable.

Glycosylation reactions--present status future directions

A short review of the present status of glycosylation reactions is presented. The reactivity of both proven and newer glycosylation methods are briefly discussed. Emphasis is placed on the control of stereochemistry and regiochemistry. As well, the identification and avoidance of side reactions is covered. Polymer-supported synthesis of oligosaccharides is noted as a promising direction for eliminating some of the problems associated with purification. It is suggested that a better understanding of the mechanism of glycosylation reactions is necessary for future improvements to stereoselectivity and regioselectivity. A key advance would be methods for enhancing the reactivity of weakly nucleophilic hydroxyls.

Chemical synthesis and NMR spectra of a protected branched-tetrasaccharide thioglycoside, a useful intermediate for the synthesis of branched oligosaccharides

Acid-catalyzed thiophenolysis of per-O-acetylated 1,6-anhydromaltose (3) gave phenyl 2,3-di-O-acetyl-4-O-(2,3,4,6-tetra-O-acetyl-alpha-D- glucopyranosyl)-1-thio-beta-D-glucopyranoside (4) in quantitative yield. Phenyl 4-O-alpha-D-glucopyranosyl-1-thio-beta-D-glucopyranoside (5) was obtained by acid-catalyzed thiophenolysis of maltose octaacetate (2), using trimethylsilyl triflate as catalyst, and subsequent deacetylation. Standard benzylation of 5 gave phenyl 2,3-di-O-benzyl-4-O- (2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl)-1-thio-beta-D-glucopy ran oside (6) which upon treatment with N-bromosuccinimide in aqueous acetone gave 2,3,6-tri-O-benzyl-4-O-(2,3,4,6-tetra-O-benzyl-alpha-D- glucopyranosyl)-D-glucopyranose (8). Compound 8 was treated with trichloroacetonitrile in the presence of anhydrous potassium carbonate to give 2,3,6-tri-O-benzyl-4-O-(2,3,4,6-tetra-O-benzyl- alpha-D-glucopyranosyl) -alpha,beta-D-glucopyranosyl trichloroacetimidate (9), which was effectively used as the glycosyl donor in the condensation reaction with compound 4, using trimethylsilyl triflate as catalyst, to obtain the branched tetrasaccharides phenyl O-[2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl)- (1-->4)]-O-(2,3,6-tri-O-benzyl-alpha-D-glucopyranosyl)-(1-->6)-O-(2,3,4, 6- tetra-O-acetyl-alpha-D-glucopyranosyl)-(1-->4)-2,3-di-O-acetyl-1-thio-be ta-D- glucopyranoside (10) and phenyl O-[(2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl)-(1-->4)]- O-(2,3,4-tri-O-benzyl-beta-D-glucopyranosyl)-(1-->6)-O-(2,3,4,6-tetra-O- acetyl- alpha-D-glucopyranosyl)-(1-->4)-2,3-di-O-acetyl-1-thio-beta-D-glucopy ran oside (11) in 67 and 21% yield, respectively. A complete NMR interpretation of 10 is presented. Alternative methodologies for the synthesis of the branched tetrasaccharides were investigated. Chemical synthesis of the phenyl thioglycoside 5 was achieved by deacetylation of 4. Reaction of 6 with diethylaminosulfur trifluoride in the presence of N-bromosuccinimide gave 2,3,6-tri-O-benzyl-4-O-(2,3,4,6-tetra-O-benzyl-alpha-D- glucopyranosyl)-alpha,beta-D-glucopyranosyl fluoride (7) in 78% yield. Subsequent condensation of 7 and 4, using the combination silver perchlorate-stannous chloride as catalyst, gave the corresponding branched tetrasaccharides 10 and 11 in 55 and 10% yield, respectively.