摘要:

Stereocontrolled, stepwise synthesis of methyl α-L-rhamnopyranosyl-(1→2)-[α-D-glucopyranosyl-(1→3)]-α-L-rhamnopyranoside (A(E)B, 1) and methyl 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→2)-α-L-rhamnopyranosyl-(1→2)-[α-D-glucopyranosyl-(1→3)]-α-L-rhamnopyranoside (DA(E)B, 2) is described; these constitute the methyl glycosides of fragments of theO-specific polysaccharide ofShigella flexneriserotype 5a. Two routes to trisaccharide1were considered. Route 1 involved the coupling of a precursor to residueAand a disaccharideEB, whereas route 2 was based on the condensation of a precursor to residueEand a disaccharideAB. Rather surprisingly, the latter afforded the β-anomer of1, namely methyl α-L-rhamnopyranosyl-(1→2)-[β-D-glucopyranosyl-(1→3)]-α-L-rhamnopyranoside as the major product. Route 1 was preferred. Overall, several observations made during this study suggested that, for the construction of higher fragments, a suitable precursor to rhamnoseAwould require protecting groups of low bulkiness at position 3 and 4. Therefore, the 2-O-acetyl-3,4-di-O-allyl-α-L-rhamnopyranosyl trichloroacetimidate (35) was the precursor of choice to residueAin the synthesis of the tetrasaccharide2. The condensation product of35and methyl 2,3,4,6-tetra-O-benzyl-α-D-glucopyranosyl-4-O-benzyl-α-L-rhamnopyranoside was selectively deacylated and condensed to 2-trichloroacetamido-3,4,6-tri-O-acetyl-2-deoxy-α-D-glucopyranosyl trichloroacetimidate to afford the corresponding fully protected tetrasaccharide45. Controlled stepwise deprotection of the latter proceeded smoothly to afford the target2. It should be emphasised that the preparation of45was not straightforward, several donors and coupling conditions that were tested resulted only in the complete recovery of the acceptor. Distortion of several signals in the13C NMR spectra of the fully or partially protected tetrasaccharide intermediates suggested that steric hindrance, added to the known low reactivity of HO-2 of rhamnosyl acceptors, probably played a major role in the outcome of the glycosidation attempts.

ISSN:0732-8303    

DOI:10.1080/07328309908544033

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

An efficient methodology for regiospecific glycosylation was developed by using 6-O-tert-butyldiphenylsilylN-acetylglucosamine derivatives3and5which bear two free hydroxyl groups as acceptors. The regiospecificity was attributed to the presence of thetert-butyldiphenylsilyl group at theO-6 position of theN-acetylglucosamine derivatives. Glycosylation of suitably protected galactoside donors10-14with acceptors3and5gave only β (1→4) linked disaccharides15-19in good yields. Fucosylation of disaccharide18led to Lewis X (Lex) trisaccharide21in high yield.

ISSN:0732-8303    

DOI:10.1080/07328309908544034

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Some 1→5 and 1→6 α-linked furano-disaccharide derivatives were synthesized using 1,2-anhydromannofuranose as the glycosyl donor. Comparison of the glycosyl acceptors indicated that the activity of primary hydroxyl groups of glycofuranoses was much lower than that of glycopyranoses.

ISSN:0732-8303    

DOI:10.1080/07328309908544035

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Consistent force fields for carbohydrates were hitherto developed by extensiveoptimization of potential energy function parameterson experimental data and onab initioresults. A wide range of experimental data is used: internal structures obtained from gas phase electron diffraction and from x-ray and neutron diffraction, vibrational frequencies, dipole moments, unit cell dimensions and lattice energies. The range of model compounds covered so far includes alkanes, ethers, alcohols, ketones and mono- and disaccharides. Electrostatic interactions are handled by fractional charges assigned to individual atoms. Charges are modeled such that Mulliken population analyses are reproduced. Morse functions are used for all bonded interactions; experimentally derived dissociation energies are used as parameters. Van der Waals interactions are modeled with Lennard-Jones 12-6 functions. The anomeric and exo-anomeric effects are accounted forwithout addition of specific terms. The work is done in the framework of the Consistent Force Field which originated in Israel and was further developed in Denmark. The actual methods and strategies employed have been described previously. Extensive testing of the force field is reported, and ways and means of improvement are indicated. Principles of mapping of conformational space are discussed, and a discussion on which properties to preferentially reproduce in modeling is invited.

ISSN:0732-8303    

DOI:10.1080/07328309908544036

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Methods for the construction of naphthalene oligodeoxy disaccharide fragments, common structural elements of the angucycline antibiotics, are investigated. The triphenylphosphonium bromide- or scandium triflate-catalyzed addition of rhodinal to 4-OH silyl-protected olivoses affords the required 3-O-α-oligodeoxy disaccharides selectively and in high yields.

ISSN:0732-8303    

DOI:10.1080/07328309908544037

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Both α- and β-anomers of acetylated 5-thio-D-xylopyranosyl azide lead upon thermolysis to a new sugar-derived tetrahydrothiazepine. This compound arises from a ring expansion involving the preferred migration of the ring sulfur atom, which probably assists the decomposition of the azido group with departure of molecular nitrogen.

ISSN:0732-8303    

DOI:10.1080/07328309908544038

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

A novel route for the synthesis of the asparagine-linked pentasaccharide1, the core unit of all asparagine-linked sugar chains, is described. To construct this pentasaccharide derivative, we first employed a regioselective coupling of a mannosyl donor to the C-3 and C-6 positions of a galactosyl chitobiose acceptor, and then by means of a conversion involving two simultaneous inversion reactions of the stereochemistry of the C-2 and C-4 hydroxyl groups of a galactose residue, a branched pentasaccharide containing the difficult to synthesize β-mannose unit was easily obtained.

ISSN:0732-8303    

DOI:10.1080/07328309908544039

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

1,6-Anhydro-2-(dicyanomethylene)-2,3-dideoxy-4-S-ethyl-4-thio-β-D-erythro-hexopyranose (1) reacted with tosyl azide or sulfur and triethylamine to furnish the 5-aza-10, 11-dioxatricyclo[6.2.1.02,6]undeca-2(6),3-diene-3-carbonitrile2and the 10,11-dioxa-5-thiatricyclo[6.2.1.02,6]undeca-2(6),3-diene-3-carbonitrile3, respectively. The reactions of1with arylisothiocyanates furnished the 11,12-dioxa-5-thiatricyclo[7.2.1.02,7]dodeca-2(7),3-diene-3-carbonitriles4and5. 3underwent cyclization with triethyl orthoformate and ammonia or hydrazine hydrate to afford the 5,7-diaza-14,15-dioxa-9-thiatetracyclo[10.2.1.02,10.03.8]pentadecatetra(tri)enes7and8, respectively.

ISSN:0732-8303    

DOI:10.1080/07328309908544040

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor