摘要:

The title compound, previously unreported in either enantioform, and its 2,6-di-O-benzyl derivative have been synthesized through a stereocontrolled epimerization at C-2 of 6-O-protected methyl 3,4-O-isopropylidene-5-C-methoxy-β-D-galactopyranosides. The epimerization, performed through a high yielding sequence of oxidation-reduction owing to the cooperative role of the equatorial C-1 aglycon and the steric hindrance of the isopropylidene group, turned out to be completely diastereoselective. Whereas the unprotected L-ribo-hexos-5-ulose exists, as proved by NMR in D2O, in five main tautomeric forms in a ratio of about 4:2:2:1:1, only two anomeric 1,4-furanosic forms are present at equilibrium in its 2,6-di-O-benzyl derivative, in ratios ranging from 10:1 to 7:3, depending on the prevalence of D2O or CD3CN in the solvent mixture.

ISSN:0732-8303    

DOI:10.1080/07328309908544053

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The electroreductive conversion of carbohydrates in aprotic media leads to radical anions which stabilize to anions. Subsequent ex-situ reaction with alkylating or acylating reagents results in the formation of ether or ester derivatives at different C-positions. For instance, the reaction of D-glucal anions with benzyl bromide gives a clear preference of the substitution at OH-4 over OH-3 and OH-6. This is explained with the help of the semiempirical methods MNDO, AM1 and PM3. It turned out that the electrochemically initiated substitution pattern corresponds to the order of stability of the intermediates (different D-glucal anions) as given by the Gibbs free energies. The calculations were extended to other carbohydrates, e.g., D-galactal, D-xylal and D-arabinal. The discussion of the performance of the three methods leads to the result that AM1 is suited best for use with carbohydrates.

ISSN:0732-8303    

DOI:10.1080/07328309908544054

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

An efficient synthesis of analogues of DAH (3-deoxy-D-arabino-hept-2-ulosonic acid) and DRH (3-deoxy-D-ribo-hept-2-ulosonic acid) is described. The route exploits a previously published highly double-stereoselective hetero Diels-Alder reaction catalyzed by a chiral salenCo(II) complex. Asymmetric dihydroxylation followed by selective reduction leads to stereoselective introduction of hydroxy groups at C-4 and C-5. Oxidative cleavage of the C-6 side-chain,in situreduction of the resulting aldehyde and deprotection afford the desired targets, which may be useful precursors to the simple analogues of the anti-influenza agent GG167.

ISSN:0732-8303    

DOI:10.1080/07328309908544055

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Benzyl protected phenyl 1-thio-galactopyranoside donors which were tethered by a succinoyl linker at their positions 2 and 6, respectively, to position 3 of a blocked benzyl glucopyranoside acceptor with a 4-OH group solely afforded the corresponding α-(1→4)-linked disaccharides upon intramolecular glycosylation. 4,6-Siloxane protected mannosides react with rearrangement of the siloxane group under similar conditions.

ISSN:0732-8303    

DOI:10.1080/07328309908544056

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The isostericC-glycosyl analogues of β-L-fucopyranosyl phosphate1and β-L-rhamnopyranosyl phosphate2have been stereoselectively synthesized from the corresponding glycono-1,5-lactones3and10via methylphosphonylation, dehydration and catalytic hydrogenation, followed by deprotection.

ISSN:0732-8303    

DOI:10.1080/07328309908544057

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Six 2'-O-, 3'-O-glycosylated diosgenyl β-D-glucopyranosides (4-9), which have a typical structural pattern of diosgenyl saponins, were synthesized; their synthetic routes are discussed.

ISSN:0732-8303    

DOI:10.1080/07328309908544058

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Synthesis of 1,6-anhydro-2,3,5-tri-O-benzoyl-β-D-galactofuranose (3) has been achieved in good yield by stannic chloride catalysed ring closure of methyl 2,3,4-tri-O-benzoyl-6-O-benzyl-β-D-galactofuranoside (1). The anhydro compound 3 was converted to the furanoside donors6and7with an easily removableO-6 acetyl group. The donors6and7were utilised for the synthesis of a di- and a trisaccharide containing β-D-galactofuranosides.

ISSN:0732-8303    

DOI:10.1080/07328309908544059

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

2,6-Anhydro-3-deoxy-D-manno-non-2-en-4-ulosonic acid (4-oxo-Kdn2en,2) was synthesized from methyl 2,6-anhydro-3-deoxy-D-glycero-D-galacto-non-2-enonate (Kdn-2en methyl ester,7) by the oxidation with manganese dioxide in dry acetone in good yield. 4-Hydroxyimino-Kdn2en derivatives (3), and 4-carbethoxymethylene-Kdn2en derivatives (4) were synthesized from the peracetylated methyl ester of 4-oxo-Kdn2en by oximation and Wittig reaction in good yield, respectively.

ISSN:0732-8303    

DOI:10.1080/07328309908544060

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor