Hydrolysis conformational effects

Tetrahydronaphthalene epoxide hydrolysis conformational effects on stereochemistry of diol formation 267... 

TETRAHYDRONAPHTHALENE EPOXIDE HYDROLYSIS CONFORMATIONAL EFFECTS ON STEREOCHEMISTRY OF DIOL FORMATION... 

AcOH, H2O, 89% yield. ° A factor of 400 in the relative rate of hydrolysis is attributed to a conformational effect where the lone pair on oxygen in the silyl ketals does not oveilap with the incipient cation during hydrolysis. 

Polymer properties, influence of ions, 258 Polymer surface reactions, kinetics, 322-323 Polymer transformation reactions configurational effect, 38 conformational effects, 38 hydrolysis of polyfmethyl methacrylate), 38 neighboring groups, 37-38 quaternization of poly(4-vinyl pyridine), 37-38 Polymerization, siloxanes, 239... 

As one might expect the rate of p-nitrophenyl heptanoate hydrolysis increased at low ethanol concentrations as a result of apolar binding. The rate of p-nitrophenyl acetate hydrolysis also increased markedly at low ethanol concentration. This finding was explained by a conformational effect on the polymer, that is, lower ethanol concentration brings about a shrinkage of the polymer, which increases concerted interactions of the imidazole residues. The hydrolysis of 3-nitro-4-dodecanoyloxybenzoate was found to be 1700 times faster in the presence of poly[4(5)-vinylimidazole] compared to free imidazole (77). A double-displacement mechanism was demonstrated for this system (75). 

The title of this paper asks if theories fit the facts. So far it has been argued that for cyclic esters, conformational effects make it exceptionally difficult to control the variables to test any theory but by showing that ligand reorganisation processes in reaction intermediates (as opposed to stable phos-phoranes) are limited then at least some simplification of mechanisms is possible. It is pertinent now to examine the one assumption that was so valuable in explaining the differences in hydrolysis patterns of 5-membered phosphates and phosphonates, viz, the C-P bond provides a barrier to pseudorotation. 

Abstract This chapter emphasises on the important aspects of steric and stereo-electronic effects and their control on the conformational and reactivity profiles. The conformational effects in ethane, butane, cyclohexane, variously substituted cyclohexanes, and cis- and tra/ ,v-decalin systems allow a thorough understanding. Application of these effects to E2 and ElcB reactions followed by anomeric effect and mutarotation is discussed. The conformational effects in acetal-forming processes and their reactivity profile, carbonyl oxygen exchange in esters, and hydrolysis of orthoesters have been discussed. The application of anomeric effect in 1,4-elimination reactions, including the preservation of the geometry of the newly created double bond, is elaborated. Finally, a brief discussion on the conformational profile of thioacetals and azaacetals is presented. 

Let us consider the reverse of acetal formation, i.e., acid hydrolysis of an acetal within the ambit of stereoelectronic effects and explore the underlying features. We begin by understanding the conformational profile and the associated conformational effects by representing the acetal in such a way that it appears to be part of a cyclohexane chair. In doing so, we understand the geometrical relationship of various bonds on this ring system much better. 

The hydrolysis of acetals and ortho esters is governed hy the stereoelectronic control factor previously discussed (see A and B on p. 427) though the effect can generally be seen only in systems where conformational mobility is limited, especially in cyclic systems. There is evidence for synplanar stereoselection in the... 

The preparation of ketones and ester from (3-dicarbonyl enolates has largely been supplanted by procedures based on selective enolate formation. These procedures permit direct alkylation of ketone and ester enolates and avoid the hydrolysis and decarboxylation of keto ester intermediates. The development of conditions for stoichiometric formation of both kinetically and thermodynamically controlled enolates has permitted the extensive use of enolate alkylation reactions in multistep synthesis of complex molecules. One aspect of the alkylation reaction that is crucial in many cases is the stereoselectivity. The alkylation has a stereoelectronic preference for approach of the electrophile perpendicular to the plane of the enolate, because the tt electrons are involved in bond formation. A major factor in determining the stereoselectivity of ketone enolate alkylations is the difference in steric hindrance on the two faces of the enolate. The electrophile approaches from the less hindered of the two faces and the degree of stereoselectivity depends on the steric differentiation. Numerous examples of such effects have been observed.51 In ketone and ester enolates that are exocyclic to a conformationally biased cyclohexane ring there is a small preference for... 

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