1,720,976 research outputs found
Dosaggio di nicotina e nornicotina via GC-MS
No full textNell’ambito del progetto “Nuove varianti di tabacco con limitata presenza di precursori di sostanze tossiche nel fumo di tabacco” ETI (2003-2005) è stato messo a punto ed applicato un nuovo metodo di dosaggio, via GC-MS, di nicotina e nornicotina.
“Fatty Acid Hydroxytyrosyl Esters: Structure/Antioxidant Activity Relationship by ABTS and in Cell-Culture DCF Assays”
No full textA large series of hydroxytyrosyl esters of C2-C18 fatty acids with increasing lipophilicity was prepared by a new highly efficient method based on acylation of methylorthoformate-protected hydroxytyrosol. All products were tested for the relative antioxidant effect using ABTS assays in ethanolic medium and DCF assays in L6 cells. No linear correlation between lipophilicity and antioxidant effect was found. ABTS assays showed a growing antioxidant capacity, with respect to hydroxytyrosol, only for medium-sized ester chains (C4-C10), and a nearly constant capacity for the higher homologues. This has been rationalized by molecular dynamics experiments in terms of partial shielding of the catecholic hydroxyls by long chained esters. A similar and dose-dependent pattern was observed in DCF assays in L6 cells, but a sharp antioxidant activity drop resulted for long chain esters, probably due to membrane entrapment.
Desymmetrisation of meso-Methylcyclooctanones. Highly Enantioselective Synthesis of C8 syn-Isoprenoid and syn,syn-Deoxypropionate Subunits from a Bicyclo[3.3.1]nonane Precursor.
No full textThe methyl esters of 3R, 7S-dimethyl- and 3R, 5R, 7S-trimethyl-8-hydroxyoctanoic acids have been prepared in good yields and with ee > 98% by chemical elaboration of the known exo, exo-3,7-dimethylbicyclo[3.3.1]nonan-9-one, the key step involving the desymmetrisation of the intermediate meso-ketones cis-3,7-dimethyl- and cis,cis-3,5,7-trimethyl-cyclooctanone through the corresponding chiral enolates generated by the lithium amide of the (+) bis[(R)-(1-phenylethylamine)]. The very high enantioselectivity observed might be related to the conformational features of the eight-membered ring.
High-Yielding Synthesis of Methyl Orthoformate-Protected Hydroxytyrosol and Its Use in Preparation of Hydroxytyrosyl Acetate
No full textThe new methyl orthoformate of the powerful antioxidant hydroxytyrosol (or 2- (3,4-dihydroxyphenyl)ethanol) has been synthesized by a two-step high yielding procedure. The protection stabilizes hydroxytyrosol against fast oxidation and allows both easy chromatographic purification and long term storage. The protective group is hydrolyzed over pH = 10 and below pH = 5, thus allowing the release of the active principle under physiological conditions. The use of the methyl orthoformate-protected hydroxytyrosol allows the preparation of protected hydroxytyrosyl esters, like the acetate herein reported, by selective esterification of the alcoholic function. The subsequent quantitative deprotection under non-aqueous and mild conditions affords the hydroxytyrosyl acetate in high yields
HSAB Driven N1-Alkylation of Uracil Derivatives
No full textThe lithium salts of 4-methoxy-derivatives of pyrimidinic bases undergo chemoselective N1-methylation or ethylation when treated with the appropriate alkylsulphate in dry dioxane. Under the same conditions, 4-unprotected pyrimidinic bases afford mixtures of N1-, O2- and N1,N3-alkylation products. This is explained in terms of HSAB principle.
Il principio HSAB nell’alchilazione regioselettiva di derivati uracilici
No full textThe lithium salts of 4-methoxy-derivatives of pyrimidinic bases undergo chemoselective N1-methylation or ethylation when treated with the appropriate alkylsulphate in dry dioxane. Under the same conditions, 4-unprotected pyrimidinic bases afford mixtures of N1-, O2- and N1,N3-alkylation products. This is explained in terms of HSAB principle.
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
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