Stereoelectronic control, theory

From the principle of microscopic reversibility, the stereoelectronic control theory predicts that the developing Ibne-pair orbitals on the heteroatoms must be antiperiplanar to the new carbon-oxygen (from Ser-OH and amide carbonyl) or carbon-nitrogen bond. The important point to be considered here is that the nonbonded pair of electrons on the nitrogen atom points toward the solvent and the N—H bond toward the inside of the enzyme active site. To facilitate the spacial view, the relevant atoms are superimposed on a tr ns-decalin frame (shaded area). 

Deslongchamps kindly provided us with his own view of the mechanistic path by which a-chymotrypsin and other serine proteases can hydrolyze secondary amides by stereoelectronic control (Fig. 4.7). Petkov et al. (119) also arrived at a similar proposal by studying the influence of the leaving group on the reactivity of specific anilides in a-chymotrypsin-catalyzed hydrolysis. Furthermore, the stereoelectronic control theory has been applied to the mode of action of ribonuclease A, staphylococcal nuclease and lysozyme (120). 

Nuclear motion, the principle of least, and the theory of stereoelectronic control, 24, 113 Nucleophiles, partitioning of carbocations between addition and deprotonation. 35, 67 Nucleophilic aromatic photosubstitution, 11,225 Nucleophilic catalysis of ester hydrolysis and related reactions, 5,237 Nucleophilic displacement reactions, gas-phase, 21, 197... 

Stereoelectronic control, the principle of least nuclear motion and the theory of, 24,113... 

The first indication that this relationship can be a very simple one arose as part of an investigation of stereoelectronic effects in acetal hydrolysis. According to Deslongchamps (1983) theory of stereoelectronic control (see also Sinnott, 1988), the orientation of lone-pair electrons can control reactivity in appropriate systems. In its original form the theory suggested... 

The theory was tested in simple tetrahydropyranyl acetal systems, which exist in the axial conformation [92], but can be constrained to adopt the equatorial conformation [93] by building in appropriate structural features. It was found, for example, that the equatorial isomers of the oxadecalin acetals [94] are actually more reactive than the axial compounds [95], although only the latter have a lone pair on the donor ring oxygen antiperiplanar to the C-OAr bond (Kirby, 1984, 1987). This and other similar observations do not disprove the theory of stereoelectronic control because any stereoelectronic barrier can be got round if there is sufficient conformational flexibility. In other words, these are Curtin-Hammett systems, with free energy barriers between conformations much smaller than... 

Stability and reactivity of crown-ether complexes, 17, 279 Stereochemistry, static and dynamic, of alkyl and analogous groups, 25, 1 Stereoelectronic control, the principle of least nuclear motion and the theory of, 24, 113 Stereoselection in elementary steps of organic reactions, 6, 185 Steric isotope effects, experiments on the nature of, 10, 1... 

On the other hand, Bennet and Sinnott provided evidence that an antiperiplanar lone electron pair is not needed in acid-catalyzed cleavage of glycosides via a carbocationic intermediate.53 54 Theories of stereoelectronic control must be applied to enzymes with caution ... 

The roles of nucleophilic assistance and stereoelectronic control in determining endo-versus exo-cyclic cleavage of pyranoside acetals have been investigated for a series of a- and j8-anomers.15 Exocyclic cleavage of a-anomers, via a cyclic oxocarbenium ion, is predicted by the theory of stereoelectronic control, and was found exclusively for the cases studied. The endocyclic route, with an acyclic ion, is predicted for the /1-structures, and a measurable amount was found in all cases, but its extent was dependent on temperature, solvent, and the nature of the aglycone group. 

Nuclear motion, the principle of least, and the theory of stereoelectronic control,... 

According to the present theory, these reactions can be explained in the following manner. Since dioxolenium ions are essentially planar (8, 9), the chair form of ring B of salt 98 must be distorted towards a half-chair. Hydration of 98 with stereoelectronic control must take place from the a face to give the half-chair hemi-orthoester 102 (Fig. 8), because the steric hindrance between the incoming water molecule and ring B must inhibit... 

It is appropriate to point out here that the development of the theory of stereoelectronic control has had its origin in the study of the ozonolysis of the acetal function (67, 68). It was first demonstrated that there... 

In previous studies, i,e. concurrent carbonyl-oxygen exchange in the hydrolysis of esters, acid hydrolysis of orthoesters and oxidation of acetals by ozone, the configuration of the tetrahedral intermediate was determined by the application of the principle of stereoelectronic control. There could be some ambiguity in these experiments as the theory of stereoelectronic control is used to predict both the stereochemistry of the tetrahedral intermediate as well as its breakdown. The oxidation cleavage of vinyl orthoesters can therefore be considered a more powerful experimental technique in that respect because the configuration of the hemi-orthoester... 

The Principle of Least Nuclear Motion and the Theory of Stereoelectronic Control... 

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