Reversibility substitution effect

Ab initio calculations at the MP2 level of theory of a collection of substituted 1,3-dioxanes, 1,3-oxathianes, and 1,3-dithianes have been employed to study both the position of the conformational equilibria and the validity of the Perlin effect <2005T7349>. The 7h,c coupling constant proved to be a valuable tool in conformational analysis both twist conformers, in addition to the chair and alternative chair forms, could be readily identified simply by comparing experimental Vh,c coupling constants to the corresponding calculated values in the particular forms. In addition, the Perlin and reversed-Perlin effects of the C2-H fragments, c in 1,3-dioxanes,... 

In addition to the chain length, the hydrocarbon chain unsaturation also plays a very important role. This is clearly shown by the data on cationic PCs, which demonstrate drastic transfection increase with increase of the number of double bonds per lipid from 0 to 2 (Fig. 13a). Studies on double chained pyridinium compounds SAINT (Synthetic Amphiphile INTeraction) (Fig. 17, inset) have shown that, while elongation of the saturated alkyl chains from 06 0/06 0 to 08 0/08 0 resulted in a reduction by a factor of about two in the transfection efficiency, introduction of double bonds reversed this effect and resulted in very strong increase of the transfection efficiency (Fig. 17). When substituting only one of the saturated 08 0 alkyl chains for unsaturated 08 1 chain, the transfection efficiency increased by an order of magnitude, while the diunsaturated compound,... 

Another remarkable anomeric effect has been observed which has beCn named the reverse anomeric effect .37 By protonation of the imidazole-substituted D-xylo derivative the equilibrium shifts from mainly axial form to mainly equatorial form (Scheme 1.3). There are no changes in the steric requirement between the two compounds and therefore only a stereoelectronic explanation can account for this anomaly. Lemieux has... 

Some general trends have been observed 38 (i) as for sulphones, resonances of aryl and vinyl sulphonates occur at appreciably higher fields than those of alkyl sulphonates (ii) in alkyl, aryl and vinyl sulphonates, the 33S chemical shift strongly depends on the electronic properties of substituents on the molecule. In the case of 2-substituted ethanesulphonates22 and 3- and 4-substituted benzenesulphonates,3357,59 61 63 the occurrence of a reverse substituent effect has been observed electron-withdrawing substituents cause un upfield shift of the 33S resonance, whereas electron-releasing groups produce a downfield shift. 

Also in 2-substituted ethanesulphonates,35 the 33S chemical shift has a reverse substituent effect and correlates with both Taft substituent constants and the chemical shift of the carboxylic carbon in related carboxylic acids. It seems that the substituent effect does not depend on conformation, but prevailingly on intramolecular electronic effects. Density functional theory (DFT) calculations of 33S nuclear shielding constants and natural bond orbital (NBO) analysis made it possible to conclude that substituents cause a variation in the polarization of the S-C and S-O bonds and of the oxygen lone pairs of the C — S03 moiety. This affects the electron density in the surroundings of the sulphur nucleus and consequently the expansion or contraction of 3p sulphur orbitals. 

Pakula, A. A., and Sauer, R. T. Reverse hydrophobic effects relieved by amino-acid substitutions at a protein surface. Nature (London) 344, 363-364 (1990). Baker, E. N., and Hubbard, R. E. Hydrogen bonding in globular proteins. Prog. Biophys. Molec. Biol. 44, 97-179 (1984). 

Allylic sulfoxides are known to equilibrate with their isomeric sulfenates via a 2,3-sigmatropic shift, which has been developed into a useful allylic alcohol synthesis when the unstable sulfenate is trapped (Scheme 14). However, this rearrangement is reversible and so need not necessarily interfere with pyrolytic elimination of the sulfoxide, e.g. the long-chain hydroxydiene (69) was obtained on pyrolysis of the sulfoxide (68 equation 32). A study of substitution effects on the relative rates of rearrangement... 

The products are derived by breaking one of the ring single bonds the regiochemistry of this ring opening is often reversed from photochemical to catalyzed reactions. Examples are given by the reactions of 86 57 Substituted effects in reactions catalyzed by... 

Part V is devoted to the study of H transfers in organic and organometallic reactions and systems. In Ch. 18 Koch describes kinetic studies of proton abstraction from CH groups by methoxide anion, of the reverse proton transfer from methanol to hydrogen bonded carbanion intermediates, and of proton transfer associated with methoxide promoted dehydrohalogenation reactions. Substitutent effects, kinetic isotope effects and ah initio calculations are treated. Of great importance is the extent of charge delocalization in the carbanions formed which determine the kinetic and thermodynamic acidities. 

A Study on the consequences of substitution in the aglycon on the configurational equilibria of neutral or protonated iV-aryl 5-thioglucopyranosylamines 35 concluded that there is no evidence in support of a generalized reverse anomeric effect. 

Paulsen H, Gyorgydeak M, Friedmann M (1974) Conformational analysis. V. Influence of the anomeric and reverse anomeric effect on conformational equilibria on N-substituted N-pentopyranosides. Chem Ber 107 1590... 

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