1,721,189 research outputs found
Formylation of carbohydrates and the evolution of synthetic routes to artificial oligosaccharides and glycoconjugates
No full textA highly effective method for the synthesis of formyl C-glycosides is described via addition of 2-lithiothiazole or 2-lithiobenzothiazole to sugar lactones, deoxygenation of the resulting ketols, and releasing of the formyl group from the heterocyclic ring. The synthetic utility of these sugar aldehydes is demonstrated by the development of synthetic methods to more elaborate C-glycosides. These include various (1 → 6)-C-disaccharides and some higher oligomers up to a C, C, C, C-pentasaccharide C-glycosyl amino acid isosteres of N-glycosyl asparagines; the methylene isostere of β-D-galactosyl ceramide; linear and cylic (2 → 1)-ketoside oligomers. An alternative synthesis of ethylene-bridged glycosyl asparagine isosteres is illustrated by the use of ethynyl C-glycosides as starting materials
Heterocycles in organic synthesis: thiazoles and triazoles as exemplar cases of synthetic auxiliaries
No full textThis Perspective article illustrates the key role of thiazole and triazole in the work carried out in the author’s laboratory over three decades and deals with the synthesis of carbohydrate-based bioactive molecules. The first part reports on the development of synthetic strategies exploiting the use of various thiazole-based reagents and the ready conversion of thiazole into the formyl group. After describing the chain elongation of monosaccharides into higher-carbon homologues, the synthesis of target natural and non-natural carbohydrates, or their ultimate precursors, is presented. These include some sphingoids, neuraminic and destomic acids, lincosamine, various 3-deoxy-2-ulosonic acids (KDO, KDN, iso-Neu4Ac), iminosugars (nojirimycin, mannojirimycin, galactostatin) and homoazasugars. Also prepared were the disaccharide subunit of bleomycin A2 and the side-chain of taxol and taxotere. The use of 1,2,3-triazole is discussed in the second part of the paper. The service of this heterocycle that is easily formed by the Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC) is considered in light of its use as a robust linker (a sort of keystone) of complex and diverse molecular architectures. Thus, the assembly of triazole-linked glycosyl amino acids, non-natural nucleotides, 1,6-oligomannosides, sialoside clusters on calixarene platfom via CuAAC is described and the biological relevance of these compounds is discussed in brief
The thiazole aldehyde synthesis
No full textAldehydes with diverse structural arrays are prepared by construction of carbon chains at C-2 of the thiazole ring and conversion of the heterocycle into the formyl group. Various methods are described that differ depending on the type of thiazole-based reagent employed in the initial carbon-carbon bond forming reaction leading to 2-substituted thiazole intermediates. The most common reagents are metalated thiazoles, phosphoranes, and carbonyl derivatives. On the other hand the unmasking of the formyl group from the thiazole ring is carried out by a general protocol involving a sequence of three reactions: N-methylation, reduction, and hydrolysis. The essential neutral conditions under which these reactions take place are tolerated by various hydroxy and amino protective groups and compatible with unprotected functional groups (azido, phosphonate). Some applications of these methods are illustrated by the synthesis of natural carbohydrates and carbon-linked analogues. Also, the viabili..
Synthesis of [60]fulleropyrrolidine glycoconjugates using 1,3-dipolar cycloaddition with C-glycosyl azomethine ylides
No full textHeating a mixture of [60]fullerene, N-methylglycine (sarcosine), and a sugar aldehyde in refluxing toluene resulted in the formation of a complex mixture of products from which the fulleropyrrolidine monocycloadduct was isolated in 14, 10, and 12% yield for formyl C-galactopyranoside, formyl C-glucopyranoside, and formyl C-mannofuranoside, respectively. © 2002 Elsevier Science Ltd. All rights reserved
Thiazolylketoses: a new class of versatile intermediates for glycoside synthesis
No full textAn account is provided on the very recent work carried out in the author's laboratory dealing with the preparation of thiazolylketose acetates and their use as effective glycosyl donors in reactions with oxygen, carbon, nitrogen and phosphorus nucleophiles. These reagents are formed by addition of 2-lithiothiazole to sugar lactones followed by acetylation of the resulting thiazolylketoses. Coupling reactions, promoted by TMSOTf in CH2Cl2, are described with primary and secondary sugar alcohols, trimethylsilyl azide, triethyl phosphite and various C-nucleophiles. Suitable transformations of the resulting glycosides are carried out owing to the ready conversion of the thiazole ring into the formyl group followed by reduction to alcohol and/or oxidation to carboxylic acid. Thus, anomeric glycosyl amino acids, ketosyl and ulosonyl disaccharides and phosphonates have been prepared. Special applications include the synthesis of O-glycosyl calix[4]arene derivatives (calix-sugars) and a cyclic ketotrisaccharide. The triethylsilane reduction of thiazolylketose acetates leads to thiazolyl C-glycosides that once subjected to the thiazole-to-formyl conversion afford formyl C-glycosides. These anomeric sugar aldehydes are convenient starting reagents for the synthesis of various C-glycosides and C-glycoconjugates, such as (1→6)-disaccharides and oligosaccharides, glycosyl aminoacids (glycine, serine, asparagine), and a galactocerebroside. Formyl C-glycosides participate in the azomethine ylide 1,3-dipolar cycloaddition reaction to C60 fullerene to give sugar fullerenes. The cover illustration shows some products of these reactions and a view of Palazzo dei Diamanti (Palace of Diamonds), a beautiful and unique building in the architectural context of Ferrara, constructed in the first half of 1500. The facade of the Palace is decorated with more than eight thousand five-hundred diamond shaped small blocks of marble
Thiazole as leaving group. Thermal elimination from thiazolylketoses
No full textHeating thiazolylketofuranoses and -ketopyranoses in refluxing toluene results in the elimination of thiazole and formation of the corresponding sugar lactones in nearly quantitative yield. The same reaction does not occur with 1-O-acetyl and 1-O-trimethylsilyl derivatives. Also model furyl- and thienylketofuranoses and various thiazolyl alcohols proved to be stable under the above thermolysis conditions. A possible mechanism of the observed thermolysis of thiazolylketoses involves the thiazolium 2-ylide as the actual leaving group
Multicomponent Syntheses of glycoside-decorated N-heterocyclic pharmacophores
No full textWe report in this communication on the synthesis of carbohydrate decorated N-heterocycles, namely dihydropyrimidin-2-ones, dihydropyridines, and azetidin-2-ones, via multicomponent Biginelli, Hantzsch, and Staudinger reactions respectively
Transformation of linear oligoketosides into macrocyclic neoglycoconjugates
No full textThe macrocyclization of linear D-galacto-2-heptulopyranose-containing oligoketosides has been carried out by intramolecular glycosidation and ring-closing metathesis. The aglycon fragment of the cyclic neglycoconjugates thus formed was an alkylidene or a polyether chain. One of the oligoketoside–crown ethers showed a moderate asymmetric induction in the Cram model phenyl acetate–acrylate addition
Chemical synthesis of 3-deoxy-D-glycero-D-galacto-2-nonulosonic acid (KDN) from D-mannose. Testing a Wittig-Michael type route to 3-deoxy-2-ulosonic acids
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Asymmetric organocatalysis: From infancy to adolescence
No full textAfter an initial period of validating asymmetric organocatalysis by using a wide range of important model reactions that constitute the essential tools of organic synthesis, the time has now been reached when organocatalysis can be used to address specific issues and solve pending problems of stereochemical relevance. This Review deals with selected studies reported in 2006 and the first half of 2007, and is intended to highlight four main aspects that may be taken as testimony of the present status and prospective of organocatalysis: a) chemical efficiency; b) discovery of new substrate combinations to give new asymmetric syntheses; c) development of new catalysts for specific purposes by using mechanistic findings; and d) applications of organocatalytic reactions in the asymmetric total synthesis of target natural products and known compounds of biological and pharmaceutical relevance. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA
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