PREFACE he term "anomeric effect" was introduced by Lemieux in 1958 as a result of a detailed study of the anomerization of acetylated pentoand hexo-pyranoses. The effect is well known to carbohydrate chemists, and refers to the tendency of an electronegative substituent at C - l of a pyranoid ring to assume the axial rather than equatorial orientation, in contrast to predictions based solely on steric grounds. However, the phenomenon is not restricted to carbohydrate systems, but is displayed in many types of heterocyclic compounds. Thus, the investigation of the anomeric effect has been of considerable interest to a variety of chemists, namely, theoreticians, structural chemists, physical organic chemists, and synthetic chemists. Chemists from all of these areas participated in the Symposium on The Origin and Consequences of the Anomeric Effect. This symposium was the first symposium devoted exclusively to a discussion of the anomeric effect, and it provided a mechanism for interactions between the diverse types of chemists. The chapters in this volume are not merely the texts as presented at the symposium, but they also incorporate some new interpretations by the authors resulting from these interactions.
Downloaded by HERIOT-WATT UNIV on September 24, 2016 | http://pubs.acs.org Publication Date: January 25, 1979 | doi: 10.1021/bk-1979-0087.pr001
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Several explanations have been proposed to account for the physical origin of the anomeric effect, and, in recent years, this aspect has been the subject of much debate and controversy. The first rationalization attributed the phenomenon to an unfavorable dipole-dipole interaction between the carbon-oxygen bonds on the ring and the bond from C - l to the equatorial, electronegative substituent. Another interpretation is based on the suggestion that interaction of the ring-oxygen lone pairs with an antibonding o-orbital of the ligand bond stabilizes the axial orientation of the ligand. The interaction between the oxygen p-type lone pair and adjacent a-bonds in pyranoid derivatives is discussed by David; the results of ab initio STO-3G molecular orbital calculations on 2-chlorotetrahydropyran are described, and experimental evidence for such an interaction is presented. For several years S. Wolfe and his co-workers have been concerned with the stereochemical consequences of adjacent electron pairs and polar bonds. In the symposium, Wolfe, Whangbo, and Mitchell reported the results of a theoretical study of the magnitudes and origins of the "anomeric effects'* associated with X and Y in X C H Y H molecules. This paper could not be incorporated into this volume, but it appears in Carbohydrate Research (1979) 69, 1. Ab initio molecular orbital calculations were performed at the STO-3G level 2
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Szarek and Horton; Anomeric Effect ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
on the series of molecules X C H Y H in which X = N H , C H , O H , F, and CI, and Y = O and S. The "anomeric effects" were found to be CI > F > O H as a function of X, and O > S as a function of Y. Molecules in which X — C H and N H exhibit "reverse anomeric effects." The results were analyzed by a quantitative perturbational molecular orbital (PMO) treatment which calculates orbital interactions between X C H and Y H in the case of the "anomeric effect," and between X and C H Y H in the case of the "reverse anomeric effect," using fragments and fragment orbitals generated from the ab initio wave function. The stabilizing interactions between the lone pair of Y and antibonding orbitals of X C H , and between the highest lying orbital of X and antibonding orbitals of C H Y H , were especially examined. In all instances, the trends in these stabilizing orbital interactions parallel the trends in the "anomeric" and "reverse anomeric effects," suggesting that, within the framework of the PMO model, such interactions can be regarded as the "origin" of these effects. Although both
