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- W2483348102 abstract "We have investigated experimentally the reverse anomeric effect (AE), the origin of the AE, and stereoelectronic control. (1) To probe the reverse AE claimed for cationic substituents, the proportions of axial anomers of N-alkylglucopyranosylamine derivatives were determined by 1H NMR in a variety of solvents, including acidic media. The change upon N-protonation is small and can be accounted for on the basis of steric effects and a small normal AE. Thus we conclude that the reverse AE does not exist. (2) To determine whether MO or electrostatic interactions are more important for the AE, the conformational equilibrium of 2-methoxy-1,3-dimethylhexahydropyrimidine was studied by 1H and 13C NMR. The proportion of axial conformer is almost the same as in 2-methoxy-l,3-dioxane, but steric corrections indicate that the AE is weaker in the former. Since nitrogen is less electronegative than oxygen, its stronger MO interactions ought to have increased the AE. We therefore conclude that in nonpolar solvents electrostatic interactions predominate. Moreover, it is shown that dipoledipole interactions can account for structural changes considered to be evidence for MO interactions. (3) To evaluate stereoelectronic control in cleavage of tetrahedral intermediates, product ratios from hydrolyses of cyclic amidinium ions and rates of methoxy exchange in 2-methoxy-4,6-dimethyl-1,3-dioxanes were determined. The results indicate a weak preference for antiperiplanar lone pairs in six-membered rings, but not in five- or seven-membered ones. The amidinium results cannot be rationalized by assuming reaction via a boat conformer." @default.
- W2483348102 created "2016-08-23" @default.
- W2483348102 creator A5044101697 @default.
- W2483348102 date "1993-11-23" @default.
- W2483348102 modified "2023-09-25" @default.
- W2483348102 title "Anomeric Effects" @default.
- W2483348102 doi "https://doi.org/10.1021/bk-1993-0539.ch005" @default.
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