Edward-Lemieux effect

S. Wolfe, Z. Shi, Isr. J. Chem. 40, 343 (2000). The SCO (O-C-S) Edward-Lemieux Effect is Controlled by p-Orbital on Oxygen. Evidence from Electron Momentum Spectroscopy, Photoelectron Spectroscopy, X-Ray Crystallography, and Density Functional Theory. 

The anomeric effect or Edward-Lemieux effect is a stereoelectronic effect that describes the tendency of heteroatomic substituents adjacent to a heteroatom within a six-membered ring to prefer the axial orientation instead of the less-hindered equatorial orientation that would be expected from steric considerations. For sugars, substituents, such as halide, at the anomeric center of pyranosides prefer to adopt... 

Since the form of F as a function of 0 was not distinguishable from that of the other repulsive terms, namely V and T, they concluded that the physical origin of the Edward-Lemieux effect cannot be ascribed in any straightforward way to coulombic interactions. Consequently, analyses of... 

The concept of the generalized anomeric ejfect results from an extension of the original Edward-Lemieux effect to account for the gauche over anti... 

Wolfe S, Rank A, Tel LM, Csizmadia IG (1971) Theoretical study of the edward-lemieux effect (the anomeric effect). Stereochemical requirements of adjacent electron pairs and polar bonds. J Chem Soc Sect B Phys Organ (1) 136-145... 

Wolfe, S., A. Rauk, L. M. Tel, and T. G. Csizmadia A Theoretical Study of the Edward-Lemieux Effect (The Anomeric Effect). The Stereochemical Requirements of Adjacent Electron Pairs and Polar Bonds. J. Chem. Soc. B 1971, 136. 

The stereochemical requirements of adjacent lone electron pairs and polar bonds have been discussed in an important paper which analysed the considerable volume of available data, including those pertaining to 1,4-dithians and similar systems. The Edwards-Lemieux effect in substituted 1,4-dithians was accounted for satisfactorily by calculations using fluoromethanol as a model compound. The temperature-dependence of the F n.m.r. spectrum of perfluoro-1,4-dithian indicated a barrier to ring-inversion of 10.05 0.10 kcalmol. The fluorine chemical-shift differences at — 90 °C appeared to be consistent with a preferred chair conformation, possibly strongly puckered as in 1,4-dithian. The i.r. and Raman spectra of 1,4-dithian and 1,4-diselenan have received detailed attention, and the crystal and molecular structures of... 

The conformational anomeric effects design the contrasteric effects observed in acetals which render the more sterically encumbered gauche/ gauche conformers more stable than their anti/gauche and anti/anti conformers. Such effects were first evidenced by Jungins in 1905 and rediscovered by Edward in 1955 and by Lemieux and Chiu in 1958. They observed the higher stability of alkyl a-D-glucopyranosides in comparison with their (3-anomers (Fig. 5).8... 

Carbohydrates have been included in the wide range of molecules used in the parameterization of MM2 and of MM3. Alcohol and ether parameters have usually been determined from simple alcohols and ethers themselves. However, carbohydrates contain some unusual features in the acetal linkages, and in the many vicinal hydrogen-bonded hydroxyl groups. The "anomeric effect", first discovered by Edward (15) and popularized by Lemieux (16.), is best known in carbohydrates, although, of course, it occurs in other classes of compounds as well. One apparent result of this effect is that an axial alkoxy substituent is often more stable than the corresponding equatorial substituent when attached at the Cl position of a tetrahydropyranyl ring. This effect can be... 

According to Stoddart (1), the unexpected preference for an electronegative substituent on C-l of a pyranoid ring to assume the axial orientation, first discussed by Edward (2) and later termed "the anomeric effect" by Lemieux (3), is now a generally recognized phenomenon in the conformational analysis of heterocyclic compounds. The preference for axial orientation is related to destabilization of the equatorial conformer in which a polar bond lies between two electron pairs on a vicinal oxygen atom. 

The anomeric effect, still an object of active research (Kirby 1983 Juaristi and Cuevas 1992 Thatcher 1993), was first observed as a property of pyranose sugars (Edward 1955 Lemieux 1964). However, the anomeric effect emerges in a pure state on a very simple molecule, methyl choromethyl ether (CH3OCH2CI), represented as projected along the O-CH2CI bond of formula 2.22. 

---