Search results for "Selectivity"
showing 10 items of 1148 documents
A DFT study of the asymmetric (S)-5-(pyrrolidin-2-yl)-1H-tetrazole catalyzed Michael addition of carbonyl compounds to nitroalkenes
2007
Abstract The role of the ( S )-5-(pyrrolidin-2-yl)-1 H -tetrazole in the asymmetric organocatalyzed Michael addition of carbonyl compounds to nitroalkenes has been studied using DFT methods at the B3LYP/6-31G ∗∗ computational level. For the C–C bond-formation step, several reactive channels have been analyzed. The B3LYP/6-31G ∗∗ results, which are in reasonable agreement with the experiments, allow us to explain the stereoselectivity and the increase of the reactivity achieved by an intramolecular HB between the acid hydrogen of the tetrazole moiety and one oxygen atom of the nitroalkene.
Boron aldol additions with erythrulose derivatives: dependence of stereoselectivity on the type of protecting group
1999
Abstract Boron aldol additions of 1- O -silylated 3,4-di- O -benzyl- and 3,4-di- O -benzoyl- l -erythrulose and achiral aldehydes using dicyclohexylboron chloride have been investigated. The dibenzyl derivative gave syn/syn stereoisomers with high stereoselectivity, whereas the dibenzoyl derivative gave syn/anti stereoisomers. It is believed that, while the dibenzoyl erythrulose gives rise to the E enolate in the presence of dicyclohexylboron chloride, as usually observed with this reagent, only the Z enolate is formed in the case of the dibenzyl derivative.
ChemInform Abstract: Designing P*-Chirogenic Organophosphorus Compounds: From Ligands to Organocatalysts
2015
The interest of organophosphorus compounds bearing chirality on the phosphorus atom has greatly expanded in last decade, due to the development of efficient stereoselective synthetic methods, using...
Dynamic formation of hybrid peptidic capsules by chiral self-sorting and self-assembly.
2014
Owing to their versatility and biocompatibility, peptide-based self-assembled structures constitute valuable targets for complex functional designs. It is now shown that artificial capsules based on β-barrel binding motifs can be obtained by means of dynamic covalent chemistry (DCC) and self-assembly. Short peptides (up to tetrapeptides) are reversibly attached to resorcinarene scaffolds. Peptidic capsules are thus selectively formed in either a heterochiral or a homochiral way by simultaneous and spontaneous processes, involving chiral sorting, tautomerization, diastereoselective induction of inherent chirality, and chiral self-assembly. Self-assembly is shown to direct the regioselectivit…
Using theozymes for designing transition-state analogs for the intramolecular aldol reaction of δ-diketones
2001
Two theozymes for the intramolecular aldol reaction of δ-diketones have been studied using ab initio methods. The presence of both acid/base residues favors several steps of the aldol reaction. The appropriate positioning of these residues can accelerate one of two diastereromeric reaction pathways, the catalyzed aldol reaction being highly stereoselective. Analysis of the geometrical parameters, charge distribution, and the shape of molecular electrostatic potential for the corresponding acid/base catalyzed transition structure allows us to design adequate transition-state analogs to favor a reactive channel of this intramolecular aldol reaction. © 2001 John Wiley & Sons, Inc. Int J Quant …
ChemInform Abstract: Stereoselective Synthesis of Bromopiperidinones and Their Conversion to Annulated Heterocycles.
2010
N-Galactopyranosyland N-glucopyranosyl imines of aliphatic, aromatic and heteroaromatic aldehydes react with 1-methoxy-3-trimethylsilyloxy-1,3-butadiene in a domino Mannich-Michael reaction cascade to give 2-substituted 5,6-dehydro-piperidin-4-ones with high diastereoselectivity. Treatment of these cyclic enaminones with N-bromo-succinimide yields the corresponding 2-substituted 5-bromo-5,6-dehydropiperidinones which react with L-Selectride R © or methylcuprate to afford the saturated bromo-piperidinones with high diastereoselectivity. Condensation reactions of these products with thioamides afford thiazolopiperidines.
Catalytic conversion of glucose to 5-hydroxymethylfurfural over biomass-based activated carbon catalyst
2020
Abstract Selective and efficient dehydration of glucose to 5-hydroxymethylfurfural (HMF) has been widely explored research problem recently, especially from the perspective of more sustainable heterogeneous catalysts. In this study, activated carbon was first produced from a lignocellulosic waste material, birch sawdust. Novel heterogeneous catalysts were then prepared from activated carbon by adding Lewis or Bronsted acid sites on the carbon surface. Prepared catalysts were used to convert glucose to HMF in biphasic water:THF system at 160 °C. The highest HMF yield and selectivity, 51% and 78%, respectively, were obtained in 8 h with a catalytic mixture containing both Lewis and Bronsted a…
Quantitative Characterization of the Local Electrophilicity of Organic Molecules. Understanding the Regioselectivity on Diels−Alder Reactions
2002
Regional electrophilicity at the active sites of the reagents involved in polar Diels−Alder processes may be described on a quantitative basis using an extension of the global electrophilicity inde...
Stereoselective synthesis of 2-amino-1-hydroxy-3-phenylpropylphosphonic acid
1996
Abstract A highly stereoselective synthesis of 2-amino-1-hydroxy-3-phenylpropylphosphonic acid was achieved by simple addition of diethyl phosphite to enantiomeric N-blocked phenylalaninals. These compouds exhibit significant herbicidal activity.
ChemInform Abstract: Stereoselective Synthesis of 2-Amino-1-hydroxy-3- phenylpropylphosphonic Acid.
2010
Abstract A highly stereoselective synthesis of 2-amino-1-hydroxy-3-phenylpropylphosphonic acid was achieved by simple addition of diethyl phosphite to enantiomeric N-blocked phenylalaninals. These compouds exhibit significant herbicidal activity.