Amines anomeric effects

Finally, the simple nucleophilic amine Me2NH attacks the epoxide with inversion of configuration to give methyl mycaminoside. The conformational drawing shows that all substituents are equatorial except the MeO group, which prefers to be axial because of the anomeric effect. 

Although the cost of the correlated models has limited their application to fairly small molecules, -277 implementations of the methodology at simply the Hartree—Pock level have been used to study the basicity of methyl-amines,conformational equilibria in esters and amides,the influence of solvation on the anomeric effect, 5 2 reactions,and even structural properties of biopolymers like In addition, considerable attention... 

It would be very interesting to obtain AEl and AEd values for, for example, O—C—O, S—C—S, and Se—C—Se anomeric interactions as a function of appropriate torsion angles so as to establish the relative importance of stabilizing and destabilizing interactions as a source of the observed anomeric effects. So far, NBO analysis has been applied to the elucidation of anomeric effects involving silicon centers (145) (see ref. 144 for a PMO approach), to Se—C—Se anomeric interactions (148), and to the study of fluorinated amines (146) F AH NH2. The amine study showed that due to the two... 

Alder RW, Camiero TMG, Mowlam RW, Orpen AG, Petillo PA, Vachon DJ, Weisman GR, White JM (1999) Evidence for hydrogen-bond enhanced structural anomeric effects from the protonation of two aminals, 5-methyl-l,5,9-triazabicyclo[7.3.1]tridecane and 1,4,8,11-tetraazatricyclo [9.3.1.1(4.8)]hexadecane. J Chem Soc Perkin Trans 2 589-599... 

Randell KD, Johnston BD, Green DF, Pinto BM (2000) Is there a generalized reverse anomeric effect Substituent and solvent effects on the configurational equilibria of neutral and protonated A-arylglucopyranosyl amines and W-aryl-5-thioglucopyranosylamines. J Org Chem 65 220-226... 

Figure 6,74 Comparison of anti- and syn-anomeric effects for fluoromethyl amine.

Irwin, J. J., Ha, T.-K., Dunitz, J. D. (1990). Stereoelectronic Aspects of the Anomeric Effect in Eluoromethylamine. Helvetica Chimica Acta, 73(1), 1805-1817. See also Reed, A. E., Schleyer, P. v. R. (1988). The anomeric effect with central atoms other than carbon. 2. Strong interactions between nonbonded substituents in mono- and polyfluorinated first- and second-row amines, F AH NH. Inorganic Chemistry, 27(22), 3969-3987. 

Anomeric effects Hyperconjugative electron transfer into a C-H orbitals can lead to increased shielding and decreased chemical shift of protons antiperiplanar to unshared electron pairs. This effect in tris-amines is illustrated in Figure 12.22. In the acyclic compound, where free rotation is possible, the methyl resonance appears at 3.02 ppm. When lone pairs are forced to be syn-periplanar, the relative inefficiency of anomeric effects allows... 

In amines, dialkoxyl subsfilufion resulls in much higher barriers to inversion fhan in alky famines a fad that has also been explained in terms of an electronegativity effect increased p-character in the a bonds results in more s-character for the electron pair on nitrogen. The planar-inversion transition state is therefore destabilized since, in it, the lone pair must develop pure p-character. This transition state is also destabilized by a six-electron anti-bonding interaction between heteroatom lone pairs and, additionally, the better anomeric overlap that is possible in sp rather than sp systems may also play a role (Figure 4) . These phenomena have also been rationalized theoretically ... 

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