1,721,018 research outputs found
Chiral mono- and dicarbamates derived from ethyl (S)-lactate: convenient chiral solvating agents for the direct and efficient enantiodiscrimination of amino acid derivatives by (1)H NMR spectroscopy
No full textNew chiral solvating agents (CSAs) for NMR spectroscopy have been obtained from ethyl (S)-lactate, a very cheap commercially available product. By a sequence of simple chemical modifications of its functional groups, monocarbamoylated and dicarbamoylated derivatives were obtained, the potentialities of which as CSAs for NMR spectroscopy have been explored. Their ability to differentiate the resonances of enantiomeric mixtures of amino acids bearing a 3,5-dinitrobenzoyl moiety at the amino group and with the carboxyl function derivatized as methyl ester or amide has been probed. Almost every CSA was able to originate enantiodiscrimination in the (1)H NMR spectra, with (2S)-1-(3,5-dimethylphenylcarbamoyloxy)-2-(3,5-dinitrophenylcarbamoyloxy)propane (4) standing out for efficiency and versatility
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variabilità della sequenza genica del citocromo b: un confronto tra pecore sarde antiche e pecore e mufloni attuali
No full textOsteons of domestic animals: structure, interspecies differences, biomechanical implications and utility in archaeozoological studies
No full textNMR spectroscopy: a powerful tool for detecting the conformational features of symmetrical persubstituted mixed cyclomaltoheptaoses (b-cyclodextrins)
No full textImpiego di b-Ciclodestrina Permetilata quale Agente Solvatante Chirale per la Determinazione via NMR della Composizione Enantiomerica di Idrocarburi Aromatici
No full textThiourea Derivative of 2-[(1 R)-1-Aminoethyl]phenol: A Flexible Pocket-like Chiral Solvating Agent (CSA) for the Enantiodifferentiation of Amino Acid Derivatives by NMR Spectroscopy
Full text linkThiourea derivatives of 2-[(1R)-1-aminoethyl]phenol, (1S,2R)-1-amino-2,3-dihydro-1H-inden-2-ol, (1R,2R)-(1S,2R)-1-amino-2,3-dihydro-1H-inden-2-ol, and (R)-1-phenylethanamine have been compared as chiral solvating agents (CSAs) for the enantiodiscrimination of derivatized amino acids using nuclear magnetic resonance (NMR) spectroscopy. Thiourea derivative, prepared by reacting 2-[(1R)-1-aminoethyl]phenol with benzoyl isothiocyanate, constitutes an effective CSA for the enantiodiscrimination of N-3,5-dinitrobenzoyl (DNB) derivatives of amino acids with free or derivatized carboxyl functions. A base additive 1,4-diazabicyclo[2.2.2]octane(DABCO)/N,N-dimethylpyridin-4-amine (DMAP)/NBu4OH) is required both to solubilize amino acid derivatives with free carboxyl groups in CDCl3 and to mediate their interaction with the chiral auxiliary to attain efficient differentiation of the NMR signals of enantiomeric substrates. For ternary systems CSA/substrate/DABCO, the chiral discrimination mechanism has been ascertained through the NMR determination of complexation stoichiometry, association constants, and stereochemical features of the diastereomeric solvates
Hydrolysis and enantiodiscrimination of (R)-and (s)-oxazepam hemisuccinate by methylated β-cyclodextrins: An nmr investigation
Full text linkPartially and exhaustively methylated β-cyclodextrins [(2-methyl)-β-CD (MCD), heptakis-(2,6-di-O-methyl)-β-CD (DIMEB), and heptakis-(2,3,6-tri-O-methyl)-β-CD (TRIMEB)] have been compared in the hydrolysis and enantiodiscrimination of benzodiazepine derivative (R)-or (S)-oxazepam hemisuccinate (OXEMIS), using nuclear magnetic resonance (NMR) spectroscopy as an investigation tool. After 6 h, MCD induced an 11% hydrolysis of OXEMIS, remarkably lower in comparison with underivatized β-CD (48%), whereas no hydrolysis was detected in the presence of DIMEB or TRIMEB after 24 h. DIMEB showed greater ability to differentiate OXEMIS enantiomers in comparison to TRIMEB, by contrast MCD did not produce any splitting of racemic OXEMIS resonances. Both enantiomers of OXEMIS underwent deep inclusion of their phenyl pendant into cyclodextrins cavities from their wider rims, but tighter complexes were formed by DIMEB with respect to TRIMEB
- …
