Search results for "CONDENSATION"
showing 10 items of 468 documents
Double diastereoselection in aldol reactions mediated by dicyclohexylchloroborane between L-erythrulose derivatives and chiral aldehydes. The Felkin-…
2003
Both matched and mismatched diastereoselections have been observed in aldol reactions of the B,B-dicyclohexylboron enolate of a protected l-erythrulose derivative with a range of chiral aldehydes. The stereochemical outcome of reactions with alpha-methyl aldehydes can be adequately explained within the Felkin-Anh paradigm. In the case of alpha-oxygenated aldehydes, however, strict adherence to this model does not allow for a satisfactory account of the observed results. In such cases, the Cornforth model provides a much better explanation.
Multiple interactions between molecular and supramolecular ordering
1999
We report studies of the interplay among processes of molecular conformational changes, spinodal demixing of the solution, and molecular crosslinking involved in the physical gelation of a biopolysaccharide-water system. Multiple interactions and kinetic competition among these processes were studied under largely different absolute and relative values of their individual rates by appropriate choices of the quenching temperature at constant polymer concentration. Quenching temperature strongly affects the rate of growth but not the final value of the fractal dimension of the gel. Kinetic competition plays a central role in determining the final conformation of individual molecules and the s…
Diastereoselective Functionalization of a Spherand‐Type Calixarene
1994
Condensation of 5,5'-di-tert-butyl-2,2'-dihydroxybiphenyl (1) with formaldehyde yields a cyclic trimer 2 (and tetramer 3) having three (four) methylene groups less than a calix[6]arene (calix[8]arene). Alkylation of the (flexible) trimer with ethyl bromoacetate gives exclusively the stereoisomer 4 with C 2 symmetry, while the isomer with D 3 symmetry is not observed. Two isomers 6a and 6b (C 2 and C 1 symmetry) are obtained by treatment with tert-butyl bromoacetate which both are converted by transesterification with methanol into the same hexamethyl ester 5 having C 2 symmetry. These results are rationalized by restricted rotation around Ar-Ar bonds for larger O-alkyl groups also around Ar…
Synthesis of new soluble polybenzyls by Friedel-Crafts reactions
1999
Soluble polybenzyls were prepared by a catalytic electrophilic Friedel-Crafts type polycondensation between α,α'-dichloro-p-xylene and substituted (by an alkyl or an alkoxy side-chain) mesitylene. The influence of the length of the side-chain on the solubility of the polymer was examined. The polymers were found to be soluble in common organic solvents. Polymer characterizations were made by 13 C nuclear magnetic resonance spectroscopy (NMR), size exclusion chromatography (SEC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and elemental analysis.
An efficient synthesis of uracil derivatives from 2-alkyl-Δ2-oxazolines and nitriles
2008
An efficient and convenient synthesis of new fluorinated and non-fluorinated uracils is described herein. The condensation of nitriles with enolates generated from 2-alkyl-Δ 2 -oxazolines (I) affords fluorinated β-enamino acid derivatives, which react with triphosgene to give an isomeric mixture of oxazolopyrimidinones. These can then be easily transformed into a single C-6 pyrimidindione derivative through reaction with a suitable nucleophile.
ChemInform Abstract: Convenient High Yielding Gram Scale Solution Synthesis of Methionine-Enkephalin.
2010
A simple, large-scale synthesis of a cytokine, methionine-enkephalin, Tyr-Gly-Gly-Phe-Met, has been elaborated. Classical solution peptide chemistry methods without protection of amino acid side-chain functions and 1+(2+2) segment condensation were used. A nine-step synthesis from commercial amino acid derivatives was developed with yields ranging from 86% to 99%, averaging 92%. The purity of all intermediates was found to be 99.0-100% by HPLC. The process has been used to prepare greater than 150 g quantities of the pentapeptide as a monohydrate of 100% purity. Hydantoin formation was observed during saponification of Boc-Tyr-Gly-Gly-Phe-Met-OMe and minimised.
Mit Donorzentren versehener korbförmiger Molekülhohlraum — Darstellung, Struktur, Eigenschaften
1991
Basket-shaped Molecular Cavity-Containing Donor Centres — Synthesis, Structure, Properties The macrocyclic basket-shaped molecule 2, composed of three 4-donor-substituted pyridine units, is synthesized by cyclisation of the chloromethyl compound 7 with sulfonamide 8. The X-ray structure analysis of 2 gives an impression of the shape of the molecular basket. Furthermore it demonstrates that the toluenesulfonamide residues interlink, creating dimeric units of 2 in the crystal.
Entzündungshemmende wirkstoffe. XI. Ringschlußreaktion an 5-phenyl-2-thiobiuret
1988
Die Umsetzung von 5-Phenyl-2-thiobiuret (1) mil α-Chlorcarbonylverbindungen 2 fuhrt zu den N-Phenyl-N′-(2-thiazolyl)harnstoffen (3). Unter den neuen Verbindungen finden sich besonders die entzundungshemmende, anthelminthische und trichomonazide Wirksamkeit stark ausgepragt. The reaction of 5-phenyl-2-thiobiuret (1) with α-chlorocarbonyl compounds 2 leads to the structure type of N-phenyl-N1-(thiazolyl)urea (3). The new compounds exhibit particularly antiinflammatory, anthelminthic, and trichomonacidal activity.
A novel synthesis of 2-aryl-4-piperidones by mannich cyclization of iminoketals
1983
2-Aryl-4-piperidones have been synthesized by condensation between an aromatic aldehyde and a β-aminoketone ethylene ketal, and further cyclization of the resulting iminoketal with dry hydrogen chloride or anhydrous p-toluensulfonic acid. Alternatively, reaction of the above iminoketals with methyl fluorosulfonate followed by dry hydrogen chloride treatment and acid hydrolysis gives directly N-methyl-4-piperidones. The application of these reactions to the synthesis of some 2-aryl-3-acetylpyrrolidine systems is also described.
Effect of addition of Lewis/Brönsted acids in the asymmetric aldol condensation catalyzed by trifluoroacetate salts of proline-based dipeptides
2012
Abstract Proline-based dipeptides catalyze aldol condensations with good yield and stereoselectivity after addition of zinc or sodium acetate to the trifluoroacetate peptide. The chirality of the N-terminal proline in the catalyst determines the absolute configuration of the aldol product, but stereoselectivity depends on the configuration of both amino acids, and is higher for the enantiomeric pair R–S, S–R. Regarding the nature of the second component, optimal results in both yield and stereoselectivity are obtained when neutral unbranched primary amino acids are used.