Effect, stereochemical

Moss and coworkers provided an early example of the way in which micellization can control the stereochemical course of a reaction. Deamination of chiral primary aliphatic amines in water proceeds with net inversion and extensive racemization, and the extent of racemization depends upon the lifetime of the carbocation-like intermediate. The situation changes dramatically if the salts of the primary amine can self-micellize, because now the nucleophile, typically water, is directed in from the front-side so that there is extensive retention of configuration (Moss et al., 1973). 

Micellar control of stereochemistry has also been realized in 8, 1 hydrolyses of chiral sulfonic esters (Okamoto et al., 1975 Sukenik and Bergman, 1976). 

The changes of stereochemistry in these systems are due to changes in the preferred direction of nucleophilic attack upon a carbocationic intermediate or a related ion pair. These results therefore indicate that the substrates or reaction intermediates have preferred orientations at the micellar surface. 

Stereoselectivity is small, or non-existent when rates of reaction of enantiomers are compared in micelles of chiral, but chemically inert surfactants (Moss and Sunshine, 1974). However, reactions of enantiomeric substrates in chiral functionalized micelles are stereoselective as are the corresponding reactions with chiral reagents in chemically inert micelles. 

Generally speaking imidazole is the nucleophilic functional group, and many of the chiral surfactants are histidine derivatives (Brown and Bunton, 1974 Brown et al., 1981). In other cases mixed systems have been used, e.g. an inert cationic surfactant plus a chiral amphiphilic histidine or hydroxa- 

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After selection of descriptors/NN training, the best networks were applied to the prediction of 259 chemical shifts from 31 molecules (prediction set), which were not used for training. The mean absolute error obtained for the whole prediction set was 0.25 ppm, and for 90% of the cases the mean absolute error was 0.19 ppm. Some stereochemical effects could be correctly predicted. In terms of speed, the neural network method is very fast - the whole process to predict the NMR shifts of 30 protons in a molecule with 56 atoms, starting from an MDL Molfile, took less than 2 s on a common workstation. 

Mandelbaum A. Stereochemical Effects in the Retro-Diels-Alder Fragmentation... 

D. Restricted Rotation.—A study on solvent and stereochemical effects on the restricted rotation of the stabilized ylides (35) has shown that although the cisoid (Z) conformation (35a) is generally predominant there is an increase in the amount of the transoid ( ) conformation (35b) as the size... 

Steele, T.D. Nichols, D.E. and Yim, G.K.W. Stereochemical effects of 3,4-methylenedioxymethamphetamine (MDMA) and related amphetamine derivatives on inhibition of uptake of [ H]-monoamines into synaptosomes from different regions of rat brain. Biochem Pharmacol 36 2297-2303, 1987. 

Huhnerfuss, H., B. Pfaffenberger, B. Gehrcke, L. Karbe, W.A. Konig, and O. Landgraf. 1995. Stereochemical effects of PCBs in the marine environment seasonal variation of coplanar and atropisomeric PCBs in blue mussels (Mytilus edulis L.) of the German Bight. Mar. Pollut. Bull. 30 332-340. 

Identify stereochemical effects in drug activity and/or potency... 

For more information on diethylzinc addition reactions, see Ito et al.,120 Wirth,129 and others.138 For a detailed discussion of the nonlinear stereochemical effects in diethylzinc addition, see Chapter 8. 

Through the years from his laboratory studies under Thorpe until his return to London in 1944, and still afterward, the heart of Ingold s theoretical chemistry was a recognition of the complementary roles in chemical processes of three-dimensional structure and physicochemical forces. His early career is marked by an effort to sort out the directing role in reacting molecules played by electrical and polar effects, in combination with stereochemical effects studied by chemists imbued with nineteenth-century concepts of structure and valence. 

Dixon, J.S. Midgley, I. Djerassi, C. Mass Spectrometry in Stractural and Stereochemical Problems. 248. Stereochemical Effects in Electron Impact Induced Retro-Diels-Alder Fragmentations. J. Am. Chem. Soc. 1977,99,3432-3441. 

Hewlett AC, Champion TM, Wilken GH, Mechoulam R, Stereochemical effects of 11-OH-A -tetrahydrocannabinol-dimethylheptyl to inhibit adenylate cyclase and bind to the cannabinoid receptor. Neuropharmacology 29 161—165, 1990. 

A tandem Beckmann rearrangement/allylsilane was used to produce the optically active cephalotaxine framework 482 (equation 208). Asymmetry could be induced as a result of stereochemical effects. 

The design equations previously described are only valid when there are no factors which modify the kinetics of the immobilized biocatalyst (partition effects, heat and mass transfer effects and decay of biological activity) and the hydrodynamic characteristics of the reactor (back-mixing). Thus, the kinetic constants and used in those equations are intrinsic values obtained in the absence of those factors, being only dependent on the conformational and stereochemical effects inherent in the immobilization procedure used. 

The two methyl groups in the olefin-copper(I) complex 26 are crucial for asymmetric induction. The 150° dihedral angle between the a- and /3-protons of the magnesium enolate 27 provides valuable information to determine the stereochemical effects on the a center. The two magnesium enolates 27 and 28 are reversibly temperature-dependent. Enolate 27 is the major component at 253 K, while enolate 28 becomes the major component at 293 K. Therefore, temperature lower than ca 256 K is required to obtain high stereoselectivity. 

The introduction of -alkyl substituents to the secondary amine donors of the macrocycle results in anodic shifts in both oxidation and reduction potentials of the complexes relative to the parent ligand systems (Table II). The extent of anodic shifts depends on the number of alkyl groups introduced to the ligand (47,55a). That is, -alkylation makes the attainment of the Ni(I) state easier and the Ni(III) state more difficult. The stabilization of Ni(I) species by -alkylation is ascribed to solvation and stereochemical effects (55b, 60). -Ethyl groups have greater inductive effects than -methyl groups and yield less anodic shift in both oxidation and reduction potentials (47). This anodic shift of redox potentials may be attributed to weaker Ni-N interactions in the -alkylated complexes. The weaker Ni-N interaction for the tertiary amine results in the stabilization of antibonding o--orbitals of the Ni(II) complex, which makes it more favorable to add an electron, but less favorable to remove an electron. 

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