Search results for "steric"
showing 10 items of 476 documents
Synthesis of N-methyl alkylaminomethane-1,1-diphosphonic acids and evaluation of their complex-formation abilities towards copper(II)
2015
Abstract A series of N -methyl alkylaminomethane-1,1-diphosphonic acids ( 3a – g ) with a common tertiary nitrogen atom (CH 3 –N–R) bearing linear or branched alkyl, cycloheptyl or phenylalkyl R substituents was synthesized in the reaction of diethylphosphonate with triethyl orthoformate and secondary amine followed by hydrolysis, and by the Eschweiler–Clarke methylation of the alkylaminomethane-1,1-diphosphonic acids with formic acid and formaldehyde. Complex-formation abilities of 3a – g towards copper(II) in solution were studied by means of pH-potentiometry, ESI-MS spectrometry, UV–Vis and EPR methods. Evaluation of stability constants for the Cu(II) complexes of 3a – g has revealed the…
ChemInform Abstract: Asymmetric Synthesis of Bicyclic Amino Acid Derivatives by Aza-Diels- Alder Reactions in Aqueous Solution.
2010
Derivatives 7–10 and 13–16 of methyl and ethyl 2-azabicyclo[2.2.1]heptane-3-carboxylates are synthesized by Aza-Diels-Alder reactions of chiral iminium ions, formed in situ from glyoxylic acid and chiral amines, with cyclopentadiene. Whereas the heterodienophiles derived from phenylglycinol and esters of sterically more demanding amino acids fail to undergo asymmetric cycloadditions, with alanine methyl ester and (R)-1-phenylethylamine hydrochoride the cycloadducts are formed in yields of 15 and 52%, respectively, reaching de values of up to 90:10 for the exo isomers.
Stereoselectivity in the Addition of Organotitanium Reagents to Carbonyl Compounds
1986
Although great strides have been made in the area of stereoselective C—C bond forming reactions [1], many problems persist. Early studies concerning chemo- selective reactions of organotitanium reagents (Chapter 3) suggested that they might also behave stereoselectively in relevant cases. In fact, the first such observation goes back to 1979 when it was noted that dichlorodi- methyltitanium (2) adds to the ketone 1 in CH2C12 to produce a mixture of tertiary alcohols 4/5 instead of the desired (±) cuparene (3) [2, 3]. The striking aspect of this transformation is the high disastereoselectivity in favor of 4. Methyllithium shows the opposite stereoselectivity (4:5 = 34:66), in line with the s…
Naphthalenophane durch doppelte Photocyclodimerisierung von Distyrylnaphthalinen
1989
Bei der Belichtung von Distyrylnaphthalinen entstehen in regiospezifischen Kopf-Kopf-Cycloadditionen Naphthalenophane. Je nach Stellung der Styryl-Gruppen beobachtet man vollkommen stereospezifische Prozesse (1d 2, 1g 3, 1h 4) und Dimerisierungen mit partiellen Stereoselektivitaten (1b 5–8, 1i 9, 10). Masgeblich sind die Rotamerenpopulation im Ausgangsprodukt 1 und die elektronischen und sterischen Effekte in den Excimeren. Prinzipiell sind Excimere mit paralleler Anordnung moglichst vieler π-Zentren bevorzugt — ein experimenteller Befund, der durch HMO-Rechnungen untermauert wird. Naphthalenophanes by Twofold Photocyclodimerizatio Reactions of Distyrylanaphthalenes Regiospecific head-to-he…
Influence of negative allosteric cooperativity in cation transport.
1992
Abstract The bis-macrocyclic ether5 seems to have a negative allosteric cooperativity and is able to transport double the amount of Na+ and K+ cations as monocyclic systems. This compound could be used as a simple model of the plasma membrane Na+−K+ ATPase which actively pumps Na+ out and K+ into the cell, respectively.
Kinetics of alkaline phosphatase from pig kidney. Influence of complexing agents on stability and activity
1976
Metal ion-complexing agents, like KCN, EDTA etc., inactivate alkaline phosphatase of pig kidney. This inactivation is reversible at low concentrations of the complexing agents and irreversible at high concentrations. The reversible inhibition is probably due to removal of Zn2+ ions from the active site, where they are necessary for catalytic action, whereas the irreversible inhibition results from the removal of Zn2+ ions necessary for preservation of the structure. The inactivation is pseudo-first order. It depends on the concentration, size and charge of the complexing agents. β-Glycerophosphate and Mg2+ ions protect the enzyme from inactivation by complexing agents. Quantitative examinat…
Acetylcholine receptors (muscarinic) (version 2019.4) in the IUPHAR/BPS Guide to Pharmacology Database
2019
Muscarinic acetylcholine receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on Muscarinic Acetylcholine Receptors [45]) are GPCRs of the Class A, rhodopsin-like family where the endogenous agonist is acetylcholine. In addition to the agents listed in the table, AC-42, its structural analogues AC-260584 and 77-LH-28-1, N-desmethylclozapine, TBPB and LuAE51090 have been described as functionally selective agonists of the M1 receptor subtype via binding in a mode distinct from that utilized by non-selective agonists [243, 242, 253, 155, 154, 181, 137, 11, 230]. There are two pharmacologically characterised allosteric sites on muscarinic receptors, one defined by it binding gallami…
Frontispiece: Positive Allosteric Control of Guests Encapsulation by Metal Binding to Covalent Porphyrin Cages
2019
Contact formation in random networks of elongated objects
2014
The effect of steric hindrance is an important aspect of granular packings as it gives rise to, e.g., limitations on the densities of ordered and disordered packings, both of which are essentially defined by the geometry of the constituents. Here we focus on random packing of rods via deposition and their distributions of contact number and segment length. Such statistical properties are relevant for mechanical properties of the structures, but the (quite large) steric effects on them have not been addressed in previous studies. We therefore develop a theory that describes the statistical properties of rod packings, while taking into account that the deposited rods cannot overlap and thus i…