摘要:

The13C NMR spectra of a range of di-O-isopropylidene acetals of α,α-trehalose and its analogues1, 2, 4-7have been studied Attention has been focussed on the chemical shifts of the acetal carbon and methyl groups of the acetals. These parameters are characteristic of ring-size (1,3-dioxolane and 1,3-dioxane). Di-n-butylstannylene and cyclic orthoester intermediates9and12of 2,6-di-O-benzoyl-α-D-galactopyranosyl 2,6-di-O-benzoyl-α-D-galactopyranoside (8) were used to synthesize the partially protected trehalose analogue having chain extension at positions 4,4′ and 3,3′ (10and13) respectively. Acetonation of the synthetic trehalose type disaccharide yielded mainly 3,4:3′,4′-di-O-isopropylidene derivative4. The benzoylation of4followed by acid hydrolysis gave8in 85% yield, which was the key intermediate for the synthesis of10and13

ISSN:0732-8303    

DOI:10.1080/07328309608005683

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The properly protected ethyl 1-thio-abequopyranoside11and ethyl 1-thio-tyvelo-pyranoside26were prepared by a sequence of reactions, the key step of which was the regioselective hydride-mediated ring-opening of the cyclic sulfate function in compounds8and18. Iodonium ion-assisted glycosylation of allyl mannopyranoside30with the individual ethyl 3,6-dideoxy- 1-thio-D-hexopyranoside donors11and26furnished, after deprotection, the respective allyl 3-O-(α-D-abequopyranosyl)-α-D-mannopyranoside1and allyl 3-O-(α-D-tyvelopyranosyl)-α-D-mannopyranoside2.

ISSN:0732-8303    

DOI:10.1080/07328309608005684

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The asymmetric α-alkylation of enolates of chiral esters derived from 2,3:4,5-di-O-isopropylidene-β-D-fructopyranose (1) was studied. The diastereoselectivities range from 1:1 to 7:3. The absolute stereochemistry of the majorS-isomers were established by chemical correlation. The diastereoselectivities of the alkylated products were similar to those observed in the deprotonation steps. The stereochemical outcome can be interpreted by an intramolecular complexation of the lithium cation of the carbohydrate ester enolates.

ISSN:0732-8303    

DOI:10.1080/07328309608005685

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Addition reactions ofO-acetylated glycal esters of Kdo mono-, α-(2→8)- and α-(2→4)- linked Kdo disaccharide derivatives1a - cwith NIS in acetic acid afforded good yields oftrans-diaxial as well as minor amounts oftrans-diequatorial andcis-configured 2-O-acetyl-3-deoxy-3-iodo derivatives, which were efficiently reduced with Bu3SnH/AIBN to give the corresponding per-O-acetylated Kdo methyl ester derivatives. Similar reactions of1awith NBS or NCS furnished thetrans-diaxial 2-O-acetyl-3-bromo-3-deoxy- as well as 3-chloro-3-deoxy derivatives as the main products. Reaction of1awith NBS in aqueous MeCN provided the 2,3-trans-bromohydrin derivative11c, which upon treatment with DBU in MeCN gave the elimination product11and the α-2,3-anhydro derivative12as a suitable donor of glycosides with D-glycero-D-talo- or D-glycero-D-galactoconfiguration, respectively.

ISSN:0732-8303    

DOI:10.1080/07328309608005686

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Plasmalopsychosines and plasmalocerebrosides comprise a novel class of human brain glycosphingolipids consisting of long chain fatty aldehydes conjugated to the galactose moiety of psychosine or cerebroside via cyclic acetal linkages. In order to clarify questions concerning the detailed stereochemistry of the acetal linkages in the natural lipids, model compounds having a simplified aglycone were synthesized. Methyl β-D-galactopyranoside was condensed with 1,1 -dimethoxyhexadecane in the presence ofp-TsOH to give a mixture of stereoisomeric cyclic acetals. After acetylation, the mixture of acetals was separated by HPLC and the structure of each isomer was established by 1-D NOE experiments. By comparison of NMR data from the model compounds with spectra of synthetic plasmalopsychosines it was possible to determine the stereochemistry of the acetal chains in the latter, and, by extension, their stereochemistry in the natural lipids.

ISSN:0732-8303    

DOI:10.1080/07328309608005687

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

As part of our ongoing studies on the isolation and characterisation of α-(1,3)-fucosyltransferase (FucT) enzymes, an improved method for the synthesis and purification of the key biological tool GDP-hexanolamine (1a), together with detailed experimental conditions and product characterisation are reported. We demonstrate the viability of the product obtained, by its competitive inhibition of an α-(1,3)-fucosyltransferase. Subsequently, we are able to use it to derivatise a sepharose gel column which we show is highly efficient in the purification of a recombinant α-(1,3)-fucosyltransferase from a crude preparation. The GDP-hexanolamine is converted to the photoaffinity probe GDP-hexanolaminyl-4-azidosalicylate (1b), useful in the characterisation of fucosyltransferase enzymes.

ISSN:0732-8303    

DOI:10.1080/07328309608005688

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Synthetic pathways leading to 5-deoxy-5,5,5-trifluoropentofuranoses are reported. The main difficulty was to obtain a good diastereoselectivity at the C-4 carbon bearing the CF3group. For the ribose and the lyxose derivatives the problem was partially solved by reacting trifluoromethyltrimethylsilane with 1,4-lactones of suitable glyconic acids leading to hemiketalic 1-deoxy-1,1,1-trifluoro-2-ketoses. Reduction of these ketoses with NaBH4or LiA1H4allowed the desired configuration at the C-4 carbon. For the arabinose and the xylose derivatives it was found more convenient to synthesize uncyclised 1-deoxy-1,1,1-trifluoro-2-ketopentoses by Dess-Martin oxidation of the corresponding protected alcohols. Highly selective reductions of these trifluoromethylketones led toarabinoorxyloderivatives depending on the reducing agent.

ISSN:0732-8303    

DOI:10.1080/07328309608005689

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Evidence suggests that the glucuronide conjugates of retinoids may be active metabolites of the parent compounds with potential utility as drugs.1We have recently shown that theO-glucuronide metabolite1of the synthetic retinoidN-4-hydroxyphenylretinamide (2) shows reduced toxicity and a significant chemopreventive advantage relative to the parent retinoid in a carcinogen-induced rat mammary cancer model.2In efforts to determine whether these conjugates function as intact molecules or must be hydrolyzed back to2, we designed and synthesized a series of C-glycosyl and C-glucuronosyl analogues of13. Our syntheses of these compounds are lengthy (10-12 steps) and lead to the desired products in less than 4% overall yields. Limitations on the availability of these compounds led us to evaluate them using a modified protocol for the carcinogen-induced rat mammary tumor model. While many of our analogues showed cancer chemopreventive activity in this assay, C-glucuronosyl analogue3has proven to be the most effective analogue of2we have yet evaluated in this model.4Unfortunately, compound3was prepared in the poorest overall yield among these analogues. However, its potent activity and low toxicity make it an important candidate for expanded biological studies. Because additional studies of this important new compound will benefit from improvements in its synthesis, we briefly describe below recent dramatic improvements we have made in the preparation of 3.

ISSN:0732-8303    

DOI:10.1080/07328309608005690

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor