Search results for "Reagent"
showing 10 items of 699 documents
Der Phenylacetyl-(PhAc-)Rest als enzymatisch ablösbare Schutzgruppe für Peptide und Kohlenhydrate: Selektive Schutzgruppenabspaltungen mit Penicillin…
1988
N-Phenylacetyl-(PhAc-)-aminosauren werden mit Aminosauremethyl-, -benzyl-, -allyl- und -tert-butylestern mit Hilfe des modifizierten Carbodiimidverfahrens oder unter Verwendung von EEDQ in guten Ausbeuten zu vollgeschutzten Dipeptiden verknupft. Der PhAc-Rest ist bei den Abspaltungen der C-terminalen Schutzgruppen stabil, kann jedoch aus den Peptiden unter nahezu neutralen Bedingungen selektiv mit Penicillin-Acylase (EC 3.5.1.11) abgelost werden. Das Enzym hydrolysiert den PhAc-Rest auch aus der 2-Position von acetylgeschutzter Glucose, der 3-Position von Di-O-isopropylidenglucose und der primaren OH-Gruppe von Di-O-isopropyliden-L-sorbose. Bei den enzymatischen Abspaltungsreaktionen bleibe…
N-Butylpyrrolidone (NBP) as a non-toxic substitute for NMP in iron-catalyzed C(sp2)–C(sp3) cross-coupling of aryl chlorides
2021
Although iron catalyzed cross-coupling reactions show extraordinary promise in reducing the environmental impact of more toxic and scarce transition metals, one of the main challenges is the use of reprotoxic NMP (NMP = N-methylpyrrolidone) as the key ligand to iron in the most successful protocols in this reactivity platform. Herein, we report that non-toxic and sustainable N-butylpyrrolidone (NBP) serves as a highly effective substitute for NMP in iron-catalyzed C(sp2)–C(sp3) cross-coupling of aryl chlorides with alkyl Grignard reagents. This challenging alkylation proceeds with organometallics bearing β-hydrogens with efficiency superseding or matching that of NMP with ample scope and br…
ChemInform Abstract: Catalytic Enantioselective Addition of Terminal Alkynes to Aromatic Aldehydes Using Zinc-Hydroxyamide Complexes.
2010
A mandelamide ligand, derived from (S)-mandelic acid and (S)-phenylethanamine, catalyzes the addition of aryl-, alkyl- and silyl-alkynylzinc reagents to aromatic and heteroaromatic aldehydes with good yields and good to high enantioselectivities.
Catalytic enantioselective addition of terminal alkynes to aromatic aldehydes using zinc-hydroxyamide complexes
2009
[EN] A mandelamide ligand, derived from (S)-mandelic acid and (S)-phenylethanamine, catalyzes the addition of aryl-, alkyl-and silyl-alkynylzinc reagents to aromatic and heteroaromatic aldehydes with good yields and good to high enantioselectivities.
Tailoring the ligand structure to the reagent in the mandelamide-Ti(IV) catalyzed enantioselective addition of dimethyl- and diethylzinc to aldehydes
2007
Amides derived from (S)-(+)-mandelic acid in the presence of titanium isopropoxide catalyze the enantioselective addition of dimethyl- and diethylzinc to aldehydes with good yields and ee up to 90%. Because of the modular character of the mandelamides, the structure of the ligand can be tailored to obtain the best results with each reagent. Thus, best results with dimethylzinc are obtained with N-benzyl mandelamide while N-(pyridin-2-yl) mandelamide is the best ligand for the addition of diethylzinc.
Solute retention in reversed-phase chromatography as a function of stationary phase properties: Effect of n-alkyl chain length and ligand density
1988
Two series of bonded phases were synthesized employing LiChrospher Si 100, 10 μm and n-alkyldimethylmonochlorosilanes as silanizing reagents. In series A the n-alkyl chain length, n, of the bonded phase was varied between 1 and 20 at a constant ligand density of 3.5±0.2μmol·m−2. In series B the ligand density, d, was gradually changed from 0 to 4.1μmol·m−2 on the C1, C4, C6, C8 and C18 bonded phases, respectively.
Quantitative characterization of the global electrophilicity pattern of some reagents involved in 1,3-dipolar cycloaddition reactions
2003
Abstract The global electrophilicity power, ω, of a series of dipoles and dipolarophiles commonly used in 1,3-dipolar cycloadditions may be conveniently classified within a unique relative scale. The effects of chemical substitution on the electrophilicity of molecules have been evaluated using a representative set of electron-withdrawing and electron-releasing groups for a series of dipoles including nitrone, nitrile oxide and azide derivatives. The absolute scale of electrophilicity is used to rationalize the chemical reactivity of these species as compared to the static reactivity pattern of the reagents involved in the Diels–Alder reactions.
Fluorinated Heterocyclic Compounds. An Expedient Route to 5-Perfluoroalkyl-1,2,4-triazoles via an Unusual Hydrazinolysis of 5-Perfluoroalkyl-1,2,4-ox…
2003
The hydrazinolysis reaction of 5-perfluoroalkyl-1,2,4-oxadiazoles has been investigated. Nucleophilic addition of the reagent to the C(5)-N(4) double bond of the oxadiazole ring, followed by ring-opening and then ring-closure involving the beta-nitrogen atom of the hydrazino moiety and the C(3) of the oxadiazole ring, explains the formation of 5-perfluoroalkyl-1,2,4-triazoles as final products. Useful applications in synthesis of this uncommon hydrazinolysis can be claimed.
Investigation of α-amino acid N-carboxyanhydrides by X-ray diffraction for controlled ring-opening polymerization
2019
Abstract The need for a scalable synthesis of not sequence defined polypeptides as biomaterials is met by the ring-opening polymerization of α-amino acid N-carboxyanhydrides (NCAs). Even though this polymerization technique appears straight forward, it holds pitfalls in terms of reproducibility and overall control over the polymerization conditions, which depends, beside choice of solvent or initiator, significantly on reagent purity. In addition, the synthesis of monomers can lead to the formation of racemic amino acids. Thus, in this work, we describe the benefits of highly pure monomers in order to control nucleophilic ring-opening polymerization NCAs. Hereby, monomer purity is investiga…
Conjugate addition of organolithium reagents to α,β-unsaturated carboxylic acids
1999
Abstract Conjugate addition of primary, secondary, tertiary alkyl and phenyl lithium reagents to 2-alkenoic acids affords good yields of branched saturated carboxylic acids. Methyl groups at the α- and β-carbon of the 2-alkenoic acid decrease reactivity as acceptors, and foster deprotonation, respectively. The lithium enediolate resulting from the conjugate addition can react with electrophiles. PM3 calculations are in agreement with the substituent effects.