Showing papers on "Glycal published in 1997"
TL;DR: In this paper, a linear tetrasaccharide containing exclusively β-glucosyl-(1→4) linkages was prepared in high yields, employing a novel coupling protocol involving thioethyl 2-pivaloyl glycosyl donors and glycal acceptors.
Abstract: Glycals were converted into thioethyl glycosyl donors through 1,2-anhydrosugar intermediates. Various participating groups in the C-2 position were examined for formation of β-glucosyl, β-galactosyl, and α-mannosyl linkages. A number of disaccharides was prepared employing a novel coupling protocol involving thioethyl 2-pivaloyl glycosyl donors and glycal acceptors. Using this methodology, a linear tetrasaccharide containing exclusively β-glucosyl-(1→4) linkages was prepared in high yields. Ready application to α-mannosylation and C2 branching are other hallmarks of the method.
88 citations
TL;DR: In this paper, a general and convenient synthesis of carbohydrate 2-C-analogs by addition of malonates to glycals is described, which is applicable to glyals derived from hexoses and pentoses and is characterized by easily available precursors.
Abstract: A general and convenient synthesis of carbohydrate 2-C-analogs by addition of malonates to glycals is described. The method is applicable to glycals derived from hexoses and pentoses and is characterized by easily available precursors. The reactions are mediated by manganese(III) or cerium(IV) and proceed via intermediarily generated malonyl radicals. All additions exhibit a very high degree of regioselectivity, since only 2-C-branched sugars were obtained. This result can be best rationalized by favorable orbital interactions between the SOMO of the malonyl radical and the HOMO of the double bond. Variation of the steric demand of the malonate or the glycal allows the stereoselectivities to be increased up to >98%. Highest selectivities were obtained with tri-O-acetyl-d-galactal and di-O-acetyl-d-arabinal, where the attack occurs exclusively from one face of the carbohydrate. For all cerium(IV)-mediated reactions, methyl glycosides are formed as main products in 73−89% yield, which can be isolated in ana...
82 citations
Journal Article•
66 citations
TL;DR: The synthesis of O-silyl- and O-acyl-protected furanose glycals from free thymidine was investigated and the method of glycal formation reported by Pedersen et al. was successfully expanded to include 5-ester (toluoyl) protected glycals.
Abstract: The synthesis of O-silyl- and O-acyl-protected furanose glycals from free thymidine was investigated. The method of glycal formation reported by Pedersen et al. was successfully expanded to include 5-ester (toluoyl) protected glycals as well as various combinations of 5‘-ester and 3- and 5-tert-butyldimethylsilyl and tert-butyldiphenylsilyl protection. Gram quantities of furanoid glycals can be prepared in a few days in two−four synthetic steps in overall yields ranging from 17 to 80%.
62 citations
TL;DR: The stereoselectivity of d-glucal derivatives with PhSCl and PhSeCl is dependent on the presence of an electronegative heteroatom substituent and the nature of the functionality at C(4) as discussed by the authors.
57 citations
TL;DR: In this paper, it was shown that 4-O-acetyl-6-bromo-3-O-(tert-butyldimethylsilyl)-2-deoxy-2-thiophenyl-1-trichloroacetimido-α- d -glucopyranose 11b is the most efficient and selective donor for use in the synthesis of 2deoxyβ-glycosides of the series of glycosyl donors examined.
54 citations
TL;DR: The tungsten pentacarbonyl-induced cyclization of an acyclic alkynol substrate bearing protected oxygen and nitrogen functional groups provides the cyclic Tungsten oxacarbene, which is readily converted into a pyranose glycal structurally related to the carbohydrate moieties of the pluramycin and vancomycin families of antitumor antibiotics as mentioned in this paper.
45 citations
TL;DR: The results show that the substituents positioned on the glycal ring have a pronounced influence on whether a chair-like or twist-boat transition state geometry is adopted primarily and reveal for the first time the fundamental importance of exothermicity and of substitution in these spiro glycosidation reactions.
Abstract: The susceptibility of glycal-derived carbinols to acid-catalyzed ring expansion is described. In the systems prepared from cyclopentanone, Ferrier ionization precedes the pinacol-like Wagner−Meerwein shift, thermodynamic control operates, and high stereoselectivity is seen if a C(6) substituent is present. In contrast, the adducts to cyclobutanone exhibit release of ring strain under kinetically controlled conditions and intercept the oxonium species reversibly formed via direct proton transfer. The results show that the substituents positioned on the glycal ring have a pronounced influence on whether a chair-like or twist-boat transition state geometry is adopted primarily. The composite reaction profiles reveal for the first time the fundamental importance of exothermicity and of substitution in these spiro glycosidation reactions. Since optical activity is preserved in all instances, the utility of this chemistry for the synthesis of bis-C,C-glycosides and more complex oxacyclics appears promising.
37 citations
TL;DR: In this paper, two suitably protected building blocks (11 and 33) for the preparation of amide-linked heptaglucoside mimetic 2, an analogue of the naturally occurring phytoalexin elicitor 1a, were readily accessible by glycal chemistry.
Abstract: Two suitably protected building blocks (11 and 33) for the preparation of amide-linked heptaglucoside mimetic 2, an analogue of the naturally occurring phytoalexin elicitor 1a, were readily accessible by glycal chemistry. Sequential elongation of terminal glucuronide 21 with laminaribiosyl hemiaminal 33 and anomeric amine 11 by EDC/HOBt-catalyzed condensation and two-step conversion of the C6-OTr moiety into the corresponding carboxylate function afforded homogeneous carbopeptoid 2 in high overall yield. It was found that replacement of the acetal linkages by the more rigid amide bonds destroys the phytoalexin-elicitor activity.
32 citations
16 citations
TL;DR: In this paper, a study on the mechanism of the well-documented hypervalent iodine-mediated allylic oxidation of glycals leading to 2,3-dihydro-4H-pyran-4-ones is presented.
Abstract: A study on the mechanism of the well-documented hypervalent iodine-mediated allylic oxidation of glycals leading to 2,3-dihydro-4H-pyran-4-ones is presented. Notable features are the isolation of ring-contracted by-products 6 and 7, which are produced upon oxidation of per-O-benzylated glycal 4, as well as the characterization of carbohydrate-derived tetrahydrofurfurals 12a and 13a, which are formed by the conformation-dependent oxidation of glycals 9a and 10b. In addition, the iodine(III)-mediated oxidation process has been studied by in situ NMR spectroscopy of lyxo-configured glycals 14a,b. Intermediate alkylphenyliodonium species 19b,d and 2-enoyranosides 16a and 20a have been characterized by their NMR signals. These data support a plausible mechanism that is initiated by electrophilic attack of the iodine(III) reagent on the electron-rich enol ether double bond of the glycal. This is followed by the breaking of a bond β,γ-positioned in relation to the carbohydrate-bound iodine and subsequent reductive elimination of iodobenzene. Thus, depending on the glycals employed, a number of diverse oxidation products may be formed.
Abstract: Analogs of the C-nucleoside pyrazofurin were prepared in 7–9 steps using a key Pd(0)- catalyzed coupling reaction between protected iodopyrazoles 6a and 6b and glycal 8 to form the glycosyl bond. Conditions for this reaction were improved from those previously described for related reactions in order to maximize product yields and eliminate the need for triphenylarsine. † present address: Arris Pharmaceutical, 385 Oyster Point Blvd., South San Francisco, CA 94080
TL;DR: In this paper, the synthesis of C-glycopyranosyl compounds has been studied in the context of cell recognition and differentiation processes, and the first approach has been to link a C-N synthon (HCN or CH 3 NO 2 ) to the anomeric carbon of a natural carbohydrate.
TL;DR: In this article, the utility of preparing cis-2,6-disubstituted 5,6dihydro-2H-pyrans by the methodology implied in Scheme 1 is demonstrated through the preparation of the two pyrans (1) and (2).
Abstract: The utility of preparing cis-2,6-disubstituted 5,6-dihydro-2H-pyrans by the methodology implied in Scheme 1 is demonstrated through the preparation of the two pyrans (1) and (2). The overall yields for the two nine-step sequences starting from the enals (3) and (4) were 57 and 54%, respectively, whilst the overall diastereochemical yield approached 100%. The key features of the method are (i) a highly regioselective Lewis acid-catalysed cyclocondensation reaction of a nucleophilic diene with an aldehyde to afford a 2,3-dihydro-4H-pyran-4-one, (ii) reduction of a pyranone with better than 98% regio- and stereo-chemical control to give the corresponding glycal, and (iii) an ester enolate Claisen rearrangement of a glycal acetate.
TL;DR: In this paper, a free radical chain mechanism was proposed for the reaction of acetobromoglucose with zinc dust in the presence of a N-base (1-methylimidazole, 4-methylpyridine, or triethylamine, pyridine) in ethyl acetate or benzene was efficiently inhibited by 10-30 mol % of 1,4dinitrobenzene, elemental sulfur, or carbon tetrachloride.
TL;DR: In this paper, the synthesis of 2-deoxy-α-glycosides of methyl glycyrrhetate, methyl deoxycholate, and cholesterol was performed by glycosylation of the corresponding alcohols with glycal acetates in the presence of a cation-exchange resin (H+) and LiBr.
Abstract: Stereospecific synthesis of 2-deoxy-α-glycosides of methyl glycyrrhetate, methyl deoxycholate, and cholesterol was performed by glycosylation of the corresponding alcohols with glycal acetates in the presence of a cation-exchange resin (H+) and LiBr.
TL;DR: In this article, a one-pot, three-step procedure was developed for the synthesis of peracetylated glycal derivatives from various reducing sugars including D-glucose, D-galactose, L-rhamnose and L-arabinose.
Abstract: A convenient, highly efficient, one-pot, three-step procedure has been developed for the synthesis of peracetylated glycal derivatives from various reducing sugars including D-glucose, D-galactose, L-rhamnose, L-arabinose, D-maltose, D-lactose, and maltotriose. This procedure involves peracetylation of the reducing sugars with acetic anhydride and HBr/acetic acid followed by the transformation of the anomeric acetates to the corresponding bromides with additional HBr/acetic acid and finally reductive elimination of the 1-bromo and 2-acetoxy groups with Zn/CuSO4·5H2O in acetic acid/water containing sodium acetate. The overall yields of purified peracetylated glycals from the corresponding sugars range from 50 - 98%. Fax/Tel #: 1-313-764-7371; E-mail: koreeda@Chem.LSA.UMich.Edu
TL;DR: In this paper, 2-Deoxy-α-glycosides of oleonane type triterpene alcohols and deoxycholic acid were synthesized by glycosylation with glycal acetates in the presence of iodonium dicollidine perchlorate followed by deiodination and deprotection.
Abstract: 2-Deoxy-α-glycosides of oleonane type triterpene alcohols and deoxycholic acid were synthesized by glycosylation with glycal acetates in the presence of iodonium dicollidine perchlorate followed by deiodination and deprotection.
TL;DR: In this article, 2-Deoxy-α-glycosides of oleonane type triterpene alcohols and deoxycholic acid were synthesized by glycosylation with glycal acetates in the presence of iodonium dicollidine perchlorate followed by deiodination and deprotection.
Abstract: 2-Deoxy-α-glycosides of oleonane type triterpene alcohols and deoxycholic acid were synthesized by glycosylation with glycal acetates in the presence of iodonium dicollidine perchlorate followed by deiodination and deprotection.
TL;DR: In this paper, the susceptibility of glycal-derived carbinols to acid-catalyzed ring expansion is described, and the composite reaction profiles reveal for the first time the fundamental importance of exothermicity and substitution in these spiro glycosidation reactions.
Abstract: The susceptibility of glycal-derived carbinols to acid-catalyzed ring expansion is described. In the systems prepared from cyclopentanone, Ferrier ionization precedes the pinacol-like Wagner−Meerwein shift, thermodynamic control operates, and high stereoselectivity is seen if a C(6) substituent is present. In contrast, the adducts to cyclobutanone exhibit release of ring strain under kinetically controlled conditions and intercept the oxonium species reversibly formed via direct proton transfer. The results show that the substituents positioned on the glycal ring have a pronounced influence on whether a chair-like or twist-boat transition state geometry is adopted primarily. The composite reaction profiles reveal for the first time the fundamental importance of exothermicity and of substitution in these spiro glycosidation reactions. Since optical activity is preserved in all instances, the utility of this chemistry for the synthesis of bis-C,C-glycosides and more complex oxacyclics appears promising.
TL;DR: The tungsten pentacarbonyl-induced cyclization of an acyclic alkynol substrate bearing protected oxygen and nitrogen functional groups provides the cyclic Tungsten oxacarbene, which is readily converted into a pyranose glycal structurally related to the carbohydrate moieties of the pluramycin and vancomycin families of antitumor antibiotics as mentioned in this paper.
Abstract: The tungsten pentacarbonyl-induced cyclization of an acyclic alkynol substrate bearing protected oxygen and nitrogen functional groups provides the cyclic tungsten oxacarbene, which is readily converted into a pyranose glycal structurally related to the carbohydrate moieties of the pluramycin and vancomycin families of antitumor antibiotics. In addition a molybdenum-catalyzed cycloisomerization procedure provides an alternative route to this pyranose glycal.
TL;DR: In this paper, the utility of preparing cis-2,6-disubstituted 5,6dihydro-2H-pyrans by the methodology implied in Scheme 1 is demonstrated through the preparation of the two pyrans (1) and (2).
Abstract: The utility of preparing cis-2,6-disubstituted 5,6-dihydro-2H-pyrans by the methodology implied in Scheme 1 is demonstrated through the preparation of the two pyrans (1) and (2). The overall yields for the two nine-step sequences starting from the enals (3) and (4) were 57 and 54%, respectively, whilst the overall diastereochemical yield approached 100%. The key features of the method are (i) a highly regioselective Lewis acid-catalysed cyclocondensation reaction of a nucleophilic diene with an aldehyde to afford a 2,3-dihydro-4H-pyran-4-one, (ii) reduction of a pyranone with better than 98% regio- and stereo-chemical control to give the corresponding glycal, and (iii) an ester enolate Claisen rearrangement of a glycal acetate.