Search results for "Steric effects"
showing 10 items of 354 documents
Probing a Polar Cluster in the Retinal Binding Pocket of Bacteriorhodopsin by a Chemical Design Approach
2012
Bacteriorhodopsin has a polar cluster of amino acids surrounding the retinal molecule, which is responsible for light harvesting to fuel proton pumping. From our previous studies, we have shown that threonine 90 is the pivotal amino acid in this polar cluster, both functionally and structurally. In an attempt to perform a phenotype rescue, we have chemically designed a retinal analogue molecule to compensate the drastic effects of the T90A mutation in bacteriorhodopsin. This analogue substitutes the methyl group at position C(13) of the retinal hydrocarbon chain by and ethyl group (20-methyl retinal). We have analyzed the effect of reconstituting the wild-type and the T90A mutant apoprotein…
The reactivity of 2-vinylindoles with dimethyl acetylenedicarboxylate
1984
Abstract The overall rates of reaction of 1-substltuted-1- (1-methyl-2-indolyl)ethenes with dimethyl acetylenedicarboxylate are considerably lower than those of the corresponding 2-vinyl- pyrroles. Steric interaction between the N-methyl group on the indole ring and the 1-substituent on the ethenyl group prevents the diene system adopting a coplanar cisoid configuration and, thereby, inhibits the π4 + π2 cycloaddition reaction of the system with dimethyl acetylenedicarboxylate. Under such conditions, the indolylethene preferentially undergoes a Michael addition reaction at the 3-position. The π4 + π2 cycloaddition reaction is promoted at elevated temperatures. No evidence was found for a Co…
Veresterung und Veretherung sterisch gehinderter Kohlenhydrat‐Hydroxylfunktionen mit dem System Triphenylphosphan/Azodicarbonsäureester
1979
Die 6-0-Benzoyl-3-0-Benzyl-1,2-0-isopropyliden-α-D-glucofuranose (11) reagiert unter der Einwirkung von Azodicarbonsaure-diethylester (2) (DEAD) und Triphenylphosphan (1) mit p- Nitro benzoesaure, Benzoesaure, p-Toluolsulfonsaure- und Mesitylensulfonsaure-methylester sterisch nicht einheitlich. Es bildet sich das erwartete Substitutions-(L-ido)-Produkt (unter Inversion an C-5), aber daneben auch das D-gluco-Epimere; fur dessen Bildung wird ein Acylierungsmechanismus diskutiert. – Die Reaktion der 1,2:5,6-Di-O-isopropyliden α-D-glucofuranose(7) mit dem Reagenzsystem 1/2 und Arylsulfonsaure-methylesters bleibt auf der Alkoxyphosphoniumsalze 8 stehen. Durch Umsetzung mit Alkoholat erhalt man a…
( E )‐ und ( Z )‐Enole von β‐Ketocarbonsäureamiden
1990
(E)- and (Z)-Enols of β-Ketocarboxylic Acid Amides Reaction of dimesitoyldiazomethane (1) with ammonia (3a), primary and secondary amines (3b – o and 3p – v, respectively) leads to the completely enolized amides 4a – v. Besides the chelated Z configuration the unusual E isomer exists in the most cases, too. The Z/E distribution in the equilibrium depends on the steric and electronic effects present in the amide group CONR2R3 and on the solvent.
αα- and αβ-Zinc-meso-A2B2-tetraarylporphyrins with large optical responses to triethylamine
2012
Synthesis and separation of αα- and αβ-meso-A(2)B(2)-zinc(II) tetraarylporphyrin atropisomers with A = mesityl and B = ortho-phenylethynyl-phenyl are reported. Both isomers exhibit large optical responses upon axial NEt(3) coordination which are visible to the human eye and could therefore be beneficial for the design of smart amine sensing materials. The larger spectral changes as compared to Zn(TPP) are attributed to pronounced distortions of the porphyrin π-system due to steric interactions of the coordinating amine with the porphyrin periphery. This effect as well as the coordination site of NEt(3) at the αα-isomer have been studied by NMR experiments and were rationalized by DFT calcul…
Formation of Triple‐Stranded Dinuclear Helicates with Dicatecholimine Ligands: The Influence of Steric Hindrance at the Spacer
2005
A series of new imine-bridged dicatechol ligands 3a–f-H4 with sterically demanding groups at the spacers are used for the formation of titanium(IV) complexes M4[(3)3Ti2]. All three ligands 3a–c-H4 form triple-stranded dinuclear helicates. When the bulky ligands 3a-H4 or 3c-H4 are used with potassium as the countercation, oligomeric or polymeric side products are also observed. The imine-bridged ligand 3e-H4 quantitatively forms helicates M4[(3e)3Ti2] and not a M4L6 tetrahedron as observed with Raymond’s analogous amide-bridged dicatechol ligand 3i-H4. NMR spectroscopic investigations at variable temperature show that ligand 3f-H4, which possesses a spiro fluorenyl group at the central unit …
Bis(pentamethylcyclopentadienyl)-Substituted Phosphanes: Synthesis and Structure
1998
The bis(1,2,3,4,5-pentamethyl-1,3-cyclopentadien-1-yl)phosphanes 3−7 are formed in good yields by the reaction of the halogenophosphanes 2a or 2b with the appropriate nucleophile. Following another route, the dialkylaminobis(pentamethylcyclopentadienyl)phosphanes 11a−c have been synthesized by the treatment of dichloro(dialkylamino)phosphanes with two equivalents of (pentamethylcyclopentadienyl)lithium. The compounds 3−11 have been characterized by multinuclear NMR spectroscopy and 3, 5, 6, and 11a have also been characterized by single-crystal X-ray diffraction studies. The molecular structure of 11a is governed by steric congestion, which typically would lead to a parallel arrangement of …
1‐Oxa‐3‐cyclooctin
1990
1-Oxa-3-cyclooctyne According to an MNDO calculation, 1-oxa-cyclooctyne (13) and 1-oxa-3-cyclooctyne (14) are 8-membered heterocyclic ring systems with high steric energy (ring strain). On the basis of THF (1) and acetoacetic ester (4), a multi-step synthesis is described for 14. The triple bond is introduced in the last step by a thermal fragmentation of the 1,2,3-selenadiazole 12.
β‐Ketocarbonsäureester mit trans ‐Enolisierung
1986
Durch die Einfuhrung von Mesitylsubstituenten gelingt es, offenkettige β-Ketocarbonsaureester 10/11 herzustellen, die bevorzugt in der E-Konfiguration 2 der Enolform vorliegen. Der Energiegewinn der intramolekularen Wasserstoffbrucke in der Z-Konfiguration 1 wird durch die sterische Energie kompensiert. Sterische und elektronische Effekte bewirken, das die Ketoformen 10/11 energetisch so hoch liegen, das sie im Gleichgewicht keine Rolle spielen. Auser den Substituenteneinflussen von R1, R2 und R3 werden die Solvensabhangigkeit und die basekatalysierte Einstellung der Gleichgewichte 1⇄2 untersucht. Die hohe Energiebarriere fur die E,Z-Isomerisierung gestattet die Reindarstellung und Lagerung…
Two mixed valence diruthenium(ii,iii) isomeric complexes show different anticancer properties
2021
In this paper it is demonstrated that the nature of the ligands of two Ru2(II,III) paddlewheel complexes dramatically affects the overall anticancer properties in cells. Herein, the complex [Ru2(EB776)4Cl] was found to be more active against a glioblastoma model with respect to its isomer [Ru2(EB106)4Cl]. These different effects depend on the steric hindrance, on the allowed conformations of the complexes and on the presence of hydrophilic regions in [Ru2(EB776)4Cl], which overall lead to a lower “steric protection”.