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Groups Increases (groups + increase)

Distribution by Scientific Domains

Chemistry	40%

Selected Abstracts

Theoretical and Experimental Study of the Regioselectivity of Michael Additions

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 2 2004
David C. Chatfield
Abstract Nucleophilic attack at an ,,,-unsaturated carbonyl moiety usually results in conjugate addition at the ,-carbon atom (1,4 or Michael addition) or, occasionally, in addition at the carbonyl carbon atom (1,2 addition). Recently, however, addition at the ,-carbon atom has been observed when strongly electron-withdrawing groups are positioned at the carbon atom , relative to the carbonyl group [e.g., methyl 3,3-bis(trifluoromethyl)propenoate (8) and ethyl 3-(2,4-dinitrophenyl)propenoate (24)]. We have performed theoretical calculations [HF/6,31+G(d) and B3LYP//HF/6,31+G(d)] for the addition of cyanide anion to model ,,,-unsaturated carbonyl compounds to determine trends in the regioselectivity with respect to properties of the substituents. The difference between the reaction barriers for ,- vs. ,-addition decreases as the strength of electron-withdrawing groups increases until, for sufficiently strong electron-withdrawing groups, ,-addition becomes favored. The calculations are in agreement with the experimental results. We show that the regioselectivity can be predicted from partial atomic charges and properties of the frontier orbitals of the reactants. We also report new experimental evidence of ,-addition to polysubstituted cinnamates and cinnamaldehydes. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

Influence of Decreasing Solvent Polarity (1,4-Dioxane/Water Mixtures) on the Acid,Base and Copper(II)-Binding Properties of Guanosine 5,-Diphosphate,

HELVETICA CHIMICA ACTA, Issue 3 2005
Emanuela
The acidity constants of twofold protonated guanosine 5,-diphosphate, H2(GDP),, and the stability constants of the [Cu(H;GDP)] and [Cu(GDP)], complexes were determined in H2O as well as in 30 or 50% (v/v) 1,4-dioxane/H2O by potentiometric pH titrations (25°; I=0.1M, NaNO3). The results showed that in H2O one of the two protons of H2(GDP), is located mainly at the N(7) site and the other one at the terminal , -phosphate group. In contrast, for 50% 1,4-dioxane/H2O solutions, a micro acidity-constant evaluation evidenced that ca. 75% of the H2(GDP), species have both protons phosphate-bound, because the basicity of pyridine-type N sites decreases with decreasing solvent polarity whereas the one of phosphate groups increases. In the [Cu(H;GDP)] complex, the proton and the metal ion are in all three solvents overwhelmingly phosphate-bound, and the release of this proton is inhibited by decreasing polarity of the solvent. Based on previously determined straight-line plots of log,Kvs. pK (where R represents a non-interacting residue in simple diphosphate monoesters ROP(O,)(O)OP(O)(O,)2, RDP3,), which were now extended to mixed solvents (based on analogies), it is concluded that, in all three solvents, the [Cu(GDP)], complex is more stable than expected based on the basicity of the diphosphate residue. This increased stability is attributed to macrochelate formation of the phosphate-coordinated Cu2+ with N(7) of the guanine residue. The formation degree of this macrochelate amounts in aqueous solution to ca. 75% (being thus higher than that of the Cu2+ complex of adenosine 5,-diphosphate) and in 50% (v/v) 1,4-dioxane/H2O to ca. 60%, i.e., the formation degree of the macrochelate is only relatively little affected by the change in solvent, though it needs to be emphasized that the overall stability of the [Cu(GDP)], complex increases with decreasing solvent polarity. By including previously studied systems in the considerations, the biological implications are shortly discussed, and it is concluded that Nature has here a tool to alter the structure of complexes by shifting them on a protein surface from a polar to an apolar region and vice versa. [source]

Polyurethanes with Pendant Hydroxyl Groups: Synthesis and Characterization

MACROMOLECULAR RAPID COMMUNICATIONS, Issue 3 2004
Luc Ubaghs
Abstract Communication: Phenoxycarbonyloxymethyl ethylene carbonate 4 was synthesized from glycerol carbonate and phenyl chloroformate. Polyurethanes with pendant hydroxyl groups were obtained from polycondensation reactions of this AA* monomer with diamines. These polymers contain primary as well as secondary hydroxyl groups. The obtained polyurethanes are amorphous materials. The glass transition temperature decreases with increasing number of methylene groups between the urethane groups. As the number of methylene groups increases between the urethane groups, the glass transition temperature of the polymer decreases. [source]

Perceived Group Status Differences and the Effects of Gender, Ethnicity, and Religion on Social Dominance Orientation

POLITICAL PSYCHOLOGY, Issue 1 2004
Shana Levin
Social dominance orientation (SDO) is a measure of general desire for group-based dominance. According to social dominance theory, the higher the status of one's group, the higher one's level of SDO should be. This study examined the extent to which between-group differences in SDO increase as the size of the perceived status gap between the groups increases. Data were collected in Israel, Northern Ireland, and the United States. In agreement with expectations, differences in SDO between arbitrary-set groups (e.g., ethnic and religious groups) were found to be greater when the status gap between the groups was perceived to be larger. In contrast, gender differences in SDO did not vary as a function of the size of the perceived status gap between men and women: Men had higher levels of SDO than women even when the gender status gap was perceived to be very small. These findings highlight the effects of perceived group status on SDO and the degree to which these effects vary depending on whether the salient group distinction is based on gender, ethnicity, or religion. [source]

Pretransplant MELD Score As a Predictor of Outcome After Liver Transplantation for Chronic Hepatitis C

AMERICAN JOURNAL OF TRANSPLANTATION, Issue 5 2003
Nicholas N. Onaca
The Model of End-Stage Liver Disease (MELD) score, an accurate predictor of mortality in patients awaiting liver transplantation (OLTX), did not predict graft or patient survival in the post-transplant setting. Our aim was to test the model in patients who underwent OLTX for chronic hepatitis C. Two hundred and eighty-seven adult patients who underwent primary OLTX for chronic hepatitis C between December 1993 and September 1999 were studied from a prospectively maintained database. The group was stratified by MELD scores of less than 15, 15,24, and greater than 24. Patient survival, graft survival, and interval liver biopsy pathology were reviewed. Both patient and graft survival at 3, 6, and 12 months were significantly lower in the higher MELD score groups, as was patient survival at 24 months (p-values, 0.01,0.05). The difference in survival between the low, medium, and high MELD score groups increases in time. The survival without bridging fibrosis in the allograft at 1 year post-transplant was significantly lower with higher MELD scores (p = 0.037). The decrease in survival seen in hepatitis C patients with MELD scores greater than 24 raises questions of transplant suitability for these patients. Therapeutic modalities to decrease post-transplant graft injury in these patients should be explored. [source]