摘要:

Easily accessible 1,6-anhydro-2,3-O-(S)-benzylidene-β-D-mannopyranose was converted in four steps to 1,6-anhydro-3,4-di-O-benzyl-β-D-talopyranose. Glycosylation of the latter with ethyl 2,3,4-tri-O-acetyl-1-thio-α-L-rhamnopyranoside gave, after further processing, 1-O-allyl-3,4-di-O-benzyl-2-O-(2,3,4-tri-O-benzyl-α-L-rhamnopyranosyl)-L-ribitol.

ISSN:0732-8303    

DOI:10.1080/07328309108543909

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Hydrazine acetate is shown to effect selective deacetylation on peracetylated glycals like D-xylal, D-glucal, D-galactal and L-rhamnal. It is demonstrated that the observed selectivity on the -4 position cannot be explained by acetyl migration during the reaction.

ISSN:0732-8303    

DOI:10.1080/07328309108543910

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

3,6-Di-O-acetyl-4-O-(2,3,4,6-tetra-O-acetyl-β-D-galactopyranosyl)-2-deoxy-2-hydroxyimino-α- and -β-D-arabino-hexopyranosides ofN-benzyloxycarbonyl-L-serine methyl ester as well as of ethanol have been synthesised from D-lactal hexaacetate via nitrosyl chloride, followed by condensation with L-serine derivatives and ethanol, respectively. The compounds of L-serine thus obtained were modified at C-2 and C-3 to afford L-serine derivatives attached to disaccharides containing terminal α-D-gluco-, 2-acetamido-α-D-gluco-, β-D-manno, 2-acetamido-β-D-manno-pyranosyl, 3-azido-2-hydroxyimino-α-D-arabino-, and α-D-ribo-hexopyranosyl residues.

ISSN:0732-8303    

DOI:10.1080/07328309108543911

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Several L-iduronic acid derivatives were prepared by chemical synthesis including the reducing sugar, the α– and ß-methyl glycosides and L-iduronolactone. The ß-methyl glycoside, as well as two isomeric orthoesters, were the unexpected products of glycosylation of methyl (2,3,4-tri-O-acetyl-α-L-idopyranosyl bromide) uronate. EI-MS was used to distinguish between the orthoesters and the glycosides. The crystal structure of the ß-methyl glycoside was determined by direct methods and refined by full-matrix least squares to a final value of R = 0.067 for 1739 reflections. The pyran ring was found to occur in the1C4 conformation, with the three acetoxy substituents in axial orientations. In aqueous solution, the α-anomer of the reducing sodium salt is almost entirely in a2SO ring conformation and the α-methyl glycoside is an equilibrium mixture of conformations which is sensitive to pH: The ß-anomers all have spectral data consistent with predominant or only slightly distorted1C4 chairs.

ISSN:0732-8303    

DOI:10.1080/07328309108543912

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The synthesis of β-(1→3)-linked galactobiose octaacetate (14) was achieved in a 6-step reaction sequence in more than 30% overall yield, starting from the readily available laminaribiose octaacetate (2). The key steps in this synthesis involved the efficient transformation of 4,6:4′,6′-di-O-benzylidene derivative (5) into the diol (6) through regioselective reductive ring opening of the two benzylidene acetals, the preparation of the corresponding ditriflate (12) and dimesylate (8), and their subsequent SN2 displacement by benzoate and/or thiolacetate anion to give the β-(1→3)-linked Gal-Gal disacharide derivative (10) and related thio compounds 11 and 13. The big difference of the reactivities between the two mesyloxyl groups of 8 enabled their replacement with different nucleophiles.

ISSN:0732-8303    

DOI:10.1080/07328309108543913

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Methyl 6-azido-4-O-benzyl-6-deoxy- (5) and methyl 6-azido-4-O- (p-bromobenzyl)-6-deoxy- α-D -mannopyranoside (6) were obtained by reaction of methyl 4-O-benzyl- (1) and methyl 4-O-(p-bromobenzyl)- 2,3-O-isopropylidene-6-O-toluene-sulfonyl-α-D-mannopyranoside (2) with sodium azide, followed by acid hydrolysis. Selective benzylation andp-bromobenzylation of 5 and 6 under phase transfer conditions afforded methyl 6-azido-2,4-di-O-benzyl-6-deoxy- (7) and methyl 6-azido-2,4-di-O-(p-bromobenzyl)-6-deoxy-α-D-mannopyranoside (8) respectively, which were acetylated and subsequently chlorinated to give 3-O-acetyl-6-azido-2,4-di-O-benzyl-6-deoxy- (11) and 3-O-acetyl-6-azido-2,4-di-O-(p-bromobenzyl)-6-deoxy-α-D-mannopyranosyl chloride (12). Ring closure of 11 and 12 was conducted in the presence of potassiumtert-butoxide to give the title anhydro sugar ethers in almost quantitative yield. The full assignments of1H and13C NMR spectra of the anhydro sugar ethers were achieved by single frequency decoupling and1H-13C two-dimensional heteronuclear correlated NMR spectroscopy. Vicinal and long-range proton-proton coupling constants suggested that the conformations of the 1,3-anhydro sugar ethers are essentially B2,5(D) for the pyranose rings and chairs for the 1.3-dioxane rings.

ISSN:0732-8303    

DOI:10.1080/07328309108543914

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A new, reversible derivatization procedure for reducing oligosaccharides was developed. Reaction of the model compound lacto-N-tetraose (β-D-Gal-(1→3)-β-D-GlcNAc-(1→3)-β-D-Gal-(1→4)-D-Glc) with aqueous ammonium bicarbonate gave the corresponding β-glycosylamine, which was reacted with fluorenylmethoxycarbonyl chloride to give anN-fluorenyl-methoxycarbonyl β-glycosylamine (FMOC derivative). The free oligosaccharide could be recovered in high yield from the FMOC derivative by brief treatment with 15% aqueous ammonia. Complex oligosaccharide mixtures from human milk were derivatized with the new procedure, and excellent separations of the derivatives on straight-phase silica HPLC columns were achieved.

ISSN:0732-8303    

DOI:10.1080/07328309108543915

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Configurational assignments for a series ofN-acyl-D-ribofuranosylamines have been established unequivocally using a set of NMR criteria. The conformation of theexo-cyclic substituents is determined.

ISSN:0732-8303    

DOI:10.1080/07328309108543916

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Three disaccharides (13, 16, and 20), compounds to be utilized in the preparation of analogs of the anticancer agent mithramycin (1), have been synthesized from appropriately protected 2,6-dideoxy sugars. In these syntheses triflate rearrangement was used to invert configuration at specific chiral centers. Silver-silicate controlled reaction generated the desired β-linkages between saccharide units. Silver-silicate also catalyzed β-glycoside formation when the glycosyl bromides derived from 13 and 20 each were coupled witho-methylphenol (a model aglycon).

ISSN:0732-8303    

DOI:10.1080/07328309108543917

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Heating methyl 4-O-benzoyl-2,6-dideoxy-3-O-triflyl-β-D-lyxo-hexopyranoside (1) at 100 °C in aqueous pyridine opens the pyranoid ring to form 3-O-benzoyl-2-O-formoyl-4-penten-2,3-diol (2) as the primary product. [Under the reaction conditions 2 is partially hydrolyzed to 3-O-benzoyl-4-penten-2,3-diol (3).] Mechanistic study of this reaction indicates that the carbocation 6 and the hemiacetal 7 are both intermediates in the reaction process. The importance of stereochemistry at C1and C3(the two chiral centers involved in the ring-opening reaction) was demonstrated by studying two additional triflates, methyl 4-O-benzoyl-2,6-dideoxy-3-O-triflyl-β-D-ribo-hexopyranoside (11) and methyl 4-O-benzoyl-2,6-dideoxy-3-O-triflyl-α-D-lyxo-hexopyranoside (14). Compound 11 experienced no ring-opening but rather exclusive triflate displacement by pyridine. For 14 the major process was E2 elimination to give methyl 4-O-benzoyl-2,3,6-trideoxy-α-D-glycero-hex-3-enopyranoside (15), while ring-opening was only a minor reaction pathway.

ISSN:0732-8303    

DOI:10.1080/07328309108543918

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor