Sterically accessible

It was found that utilization of QAS with increased steric accessibility of exchange center, containing nitrogen atom with three methyl substituents and one super lipophilic substituent, - tris(2,3,4-dodecyloxy)benzyl, - leads to dramatic, up to 7 orders, increase of ISE s selectivity towards sulfate in the presence of single-charged anions. Incorporation of hexyl 4-trifluoro-acetylbenzoate into membrane leads to additional substantial increase - up to 4 orders - in sulfate selectivity. 

EFFECT OF QUATERNARY AMMONIUM SAET EXCHANGE CENTER STERIC ACCESSIBILITY ON EXTRACTION OF CADMIUM BROMIDE COMPLEXES... 

Thus, the QASes of the asymmetrical stmcture with improved steric accessibility of the exchange centre show the high selectivity to double-charged cadmium bromide complex ions relative to single-charged ions. 

Some advantages of this reaction are high yield if the tosylate is in a sterically accessible position excellent isotopic purity of the product (usually higher than-95%) and perhaps most important, access to stereospecifically labeled methylene derivatives. For example, deuteride displacement of 3j -tosylates (183) yields the corresponding Sa-d derivative (185) in 96-98% isotopic purity. Application of this method to the labeled sulfonate (184), obtained. by lithium aluminum deuteride reduction of a 3-ketone precursor (see section HI-A) followed by tosylation, provides an excellent synthesis of 3,3-d2 labeled steroids (186) without isotopic scrambling at the adjacent positions. The only other method which provides products of comparable isotopic purity at this position is the reduction of the tosyl-hydrazone derivative of 3-keto steroids (section IV-B). 

A solution of bismuth trioxide in hot glacial acetic acid provides a specific method for the oxidation of acyloins. " The reaction rate is dependent on the steric accessibility of the ketol system. A 2,3-ketol requires less than one hour for completion but an 11,12-ketol is not yet fully oxidized in thirty hours." The reaction is highly selective as a-keto acids, hydrazines and phenols are not oxidized. In a direct comparison with cupric acetate, this procedure is somewhat superior for the preparation of a 2,3-diketone from a 2-keto-3-hydroxy steroid. ... 

It has been proposed that oxygen adds to the excited keto group [- (112)]. The rearrangement of the resulting hydroxyhydroperoxy diradical (112) could then proceed by intramolecular hydrogen abstraction involving a six-membered cyclic transition state, followed by fission of the former C —CO bond to form the unsaturated peracid (113) as the precursor of the final product. Such a reaction sequence demands a hydrogen atom in the J -position sterically accessible to the intermediate hydroperoxy radical. 

The Gabriel-Colman reaction can be used to prepare isoquinoline-1,4-diols regioselectively by the use of unsymmetrically substituted phthalimides. Reaction of phthalimide 32 with sodium ethoxide in ethanol provides a 1 7 mixture of 33 34. It was rationalized that attack at carbon b is preferred because of its greater steric accessibility and diminished electron density compared to carbon a. In spite of the reasonable regioselectivity observed m this reaction, the Gabriel-Colman reaction has not been substantially investigated in the preparation of non-symmetrically substituted isoquinolines. 

Sterically accessible less hindered and more easily solvated. 

Bsubunit interacts with pectic polymers and/or other cell wall components and therby sterically access of PG2 from those regions the polymer (as indicated... 

The two reactions shown below are examples of the use of the Wacker reaction in multistep synthesis. In the first case, selectivity is achieved between two terminal alkene units on the basis of a difference in steric accessibility. Both reactions use a reduced amount of Cu(I) salt. In the second reaction this helps to minimize hydrolysis of the acid-sensitive dioxolane ring. 

Dendrimers are taking their space in the tool box of the modern synthetic chemist. Dendritization might offer solutions to problems yet unsolved. Dendritic wedges, i. e. dendryl substituents of well-chosen size and generation allow us to tune molecular properties like solubility, steric accessability of reactive sites, redox behaviour, and other features. Easy-to-make dendrimers and dendrons will thus become extremely helpful for any chemist in the covalent as well as in the supramolecular world . 

Lavery, R., S. Corbin, and B. Pullman. 1982. The Molecular Electrostatic Potential and Steric Accessibility of C-DNA. Theor. Chim. Acta 60, 513. 

The creation of hydrazide probes often is based on the derivatization of a detectable molecule with a fezs-hydrazide compound. Although hydrazine itself (in the form of hydrazine hydrate) can be used in a methanolic solution to modify activated carboxylate molecules forming hydrazides, the availability of the bifunctional hydrazides provides a built-in spacer to accommodate greater steric accessibility. 

R. Lavery and B. Pullman, The molecular electrostatic potential steric accessibility and hydration of Dickerson s B-DNA dodecamer d(CpGpCpGpApApTpTpCpGpCpG), Nuc. Acid Res. 9 3765 (1981). 

The reactivity between a tertiary C-H site and a sterically accessible, secondary C-H site is relatively even in the reactions catalyzed by TpBf3Cu. This can be seen in the reaction with 2-methylpentane (Equation (7)),38,49,56 which gave rise to a mixture of only two products. No insertion into the methyl or the sterically crowded methylene C-H bonds was seen. The C-H insertion has the possibility of selectively functionalizing relatively complex alkanes. An impressive example is the C-H insertion to 1 (Equation (8)).56 A mixture of two alkylation products derived from insertion at the tertiary C-H bonds was obtained. This transformation has been extended to the selective functionalization of hydrocarbon polymers.75... 

Quantum mechanical approaches have been successfully used to predict hydrogen abstraction potentials and likely sites of metabolism of drug molecules [78-81]. AMI, Fukui functions, and density functional theory calculations could identify potential sites of metabolism. Activation energies for hydrogen abstraction were calculated by Olsen et al. [81] to be below 80 kj/mol, suggesting most CH groups can be metabolized which particular one depends on steric accessibility and intrinsic reactivities. 

The coordination of styrene is expected to be strongly influenced by substituents that are neglected in the minimal QM model A. Thus, for sake of clarity, we do not present the styrene coordination results using model A. Depicted in Figure 8 are the three most stable styrene coordinated isomers, 8a-c. The coordination energies, which are also shown in Figure 8 in kcal/mol, reveal that the initial formation of the tt-complex is slow and reversible. In fact, only for isomer 8a is the styrene coordination exothermic and here it is only exothermic by 0.5 kcal/mol. Isomers 8a-c all have the olefinic bond of the styrene lying parallel to the plane defined by the P-Pd-Si atoms. No other sterically accessible isomers could be located where this bond lies parallel to this plane. Due to steric reasons, complexes with the olefinic bond perpendicular to this plane were found to be at least 8 kcal/mol less stable. 

The substrate can change the reactivity in the interaction with the reagent by means of its electrophihcity, nucleo-philicity, and hardness [4, 141]. These properties can be accentuated by electroauxiliaries [35] that facihtate electron transfer and direct follow-up reactions. A powerful control is the potential of the elec-trophore that is determined by electronic factors, steric accessibility, and intramolecular electron transfer. This potential can... 

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