Search results for "synthesis"

Stereoselective synthesis of carane-based chiral β- and γ-amino acid derivatives via conjugate addition

2015

Abstract Michael addition of dibenzylamine to (−)-tert-butyl isochaminate, prepared in three steps from (−)-perillaldehyde, furnished a carane-based β-amino acid derivative in a highly stereospecific reaction. The resulting amino ester was transformed to the bicyclic amino acid, a promising building block for the synthesis of 1,3-heterocycles and peptidomimetics. The conjugate addition of nitromethane to α,β-unsaturated methyl ester likewise resulted in nitro esters in stereospecific reactions. Catalytic reduction of the nitro group yielded a γ-amino ester. Under acidic conditions, the hydrolysis of the methyl ester resulted in an unexpected aminolactone-type product through rearrangement o…

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.

Plasma membrane transporters for arginine

2004

The supply of arginine may become rate limiting for enzymatic reactions that use this semiessential amino acid as a substrate (e.g., nitric oxide, agmatine, creatine, and urea synthesis), particularly under conditions of high demand such as growth, sepsis, or wound healing. In addition, arginine acts as a signaling molecule that regulates essential cellular functions such as protein synthesis, apoptosis, and growth. In the past decade, a number of carrier proteins for amino acids have been identified on the molecular level. They belong to different gene families, exhibit overlapping but distinctive substrate specificities, and can further be distinguished by their requirement for the cotran…

Enantioselective epoxidation of olefins with molecular oxygen catalyzed by gold(III): A dual pathway for oxygen transfer

2009

Abstract A chiral gold(III) complex has been prepared that performs the epoxidation of olefins in the presence of O 2 , PhIO, or bleach. Catalytic experiments with 18 O show that O 2 is activated on the catalyst and can be directly incorporated into the epoxide through a non-radical mechanism that probably involves formation of gold, oxo, or peroxo species. In addition to this, there is a parallel radical mechanism operating that yields α , β -unsaturated ketones and alcohols as subproducts. Electrochemical and UV–Vis experiments confirmed the occurrence of a Au(III)/Au(I) redox cycle during the catalytic epoxidation in a mechanism sustained by molecular oxygen.

Synthesis of all-syn Functionalized Triphenylene Ketals

2011

The stereoselective synthesis of triphenylene ketals offers access to unique scaffolds. For a good performance in supramolecular applications an all-syn orientation of the functional groups is essential. The oxidative trimerization of catechol ketals by molybdenum pentachloride or mixtures with titanium tetrachloride leads to a template-directed formation. Several heterocyclic moieties are suitable for this transformation. A template-directed isomerization of anti,anti,syn isomers to the desired C 3 -symmetric derivative was demonstrated in two cases.

Exploring new activating groups for reactive cysteine NCAs

2016

Abstract Due to its ability to reversibly crosslink proteins, cysteine has a unique role as an amino acid in nature. For controlled, asymmetric formation of disulfides from two thiols, one thiol needs to be activated. While few activating groups for cysteine have been proposed, they are usually not stable against amines making them unsuitable for solid phase peptide synthesis or amine initiated polymerization of α-amino acid-N-carboxy-anhydrides (NCAs). In this Letter we describe a series of new thiol activated cysteines, as well as their NCAs and explore the link between electron deficiency of the leaving group and control over NCA polymerization.

Synthetic Tumor-Associated Glycopeptide Antigens from the Tandem Repeat Sequence of the Epithelial Mucin MUC4

2004

In cancer research, the development of vaccines against tumor-associated antigens is of particular interest. Epithelial cells express mucin type glycoproteins, which are extensively O-glycosylated. In case of cancer, the expression of these mucins is increased, and their carbohydrate side chains show an aberrant glycosylation pattern. A set of single and double glycosylated hexadecapeptides representing the tandem repeat sequence of the epithelial mucin MUC4 carrying different tumor-associated carbohydrate antigens was prepared by sequential solid-phase glycopeptide synthesis. The crucial glycosyl amino acid building blocks containing the T N , T, sialyl-T N and (2,6)-sialyl-T antigens were…

Asymmetric Synthesis Using Polymer-Immobilized Proline Derivatives

2011

Characterization of 1-aminocyclopropane-1-carboxylic acid oxidase from barley (Hordeum vulgare L.) seedlings and pine (Pinus sylvestris L.) needles

1999

Abstract Biochemical characteristics of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase extracted from barley ( Hordeum vulgare L.) leaves and pine ( Pinus sylvestris L.) needles were investigated. ACC oxidase from barley leaves was soluble while for complete recovery of ACC oxidase from pine needles addition of Triton X-100 to extraction medium was necessary. The enzyme required Fe 2+ , ascorbate and NaHCO 3 for maximum activity. A non-linear time course of ACC oxidase reaction indicated possible catalytic inactivation of the enzyme. The maximum activity was measured at pH 7.0–7.2 for ACC oxidase from both barley leaves and pine needles. The apparent K m for ACC was found to be 77 and …

Properties and structure of raised bog peat humic acids

2013

Abstract Humic substances form most of the organic components of soil, peat and natural waters, and their structure and properties differ very much depending on their source. The aims of this study are to characterize humic acids (HAs) from raised bog peat, to evaluate the homogeneity of peat HAs within peat profiles, and to study peat humification impact on properties of HAs. A major impact on the structure of peat HAs have lignin-free raised bog biota (dominantly represented by bryophytes of different origin). On diagenesis scale, peat HAs have an intermediate position between the living organic matter and coal organic matter, and their structure is formed in a process in which more labil…