Anomeric effects defined

As would be anticipated for a phenomenon which is fundamentally electrostatic, this effect is attenuated in polar solvents a-glucosylimidazole in water does not change conformation on protonation [17]. There is evidence that the anomeric effect itself is likewise solvent sensitive. Table 1 gives representative data on the strength of the anomeric effect (defined as the additional stabihsation of the axial substituent in the glucose series, compared with cyclohexane). 

Praly and Lemieux109 defined a total anomeric effect (AA) that has an endo-(from 0-6) and an exo- (from 0-2) component. In the case of the equatorial... 

A parameter that has been useful for defining the anomeric stereochemistry of glycopyranosides is the chemical shift of the anomeric carbon atom, C-l. Similarly, the 13C NMR shifts of C-l in methyl septanosides also diagnostic for their anomeric configuration. In general, the Sc.i values for a-septanosides are slightly upfield (99-104 ppm in completely deprotected septanosides) relative to (S-septanosides (104-111 ppm) (Table I). This distribution of chemical shifts mirrors the trends for pyranosides namely, the C-l chemical shifts for a-pyranosides lie upfield from those of p-pyranosides. This observation suggests that the anomeric effect is operative in... 

Quantitatively, the anomeric effect is defined as the difference in conformational free energy for the process shown in Scheme 28 and the corresponding process in cyclohexane. Conformational energies for a range of substituents are available (82JA3635). 

The MM2 program [52] is by faf the most widely used force field program in the area of hydrocarbon or moderately polar molecules and numerous publications have demonstrated its usefulness. Still the anomeric center had not until recently been properly defined by the force field parameters. Now there are parameters available which include the proper treatment of acetal fragments and hence reproduce the anomeric as well as the exo anomeric effect accurately [53]. Due to the full optimization of the geometry the MM2 program can treat only a limited number of atoms. 

The bases are in syn or anti orientation. The orientation of the base relative to the sugar moiety is defined by torsion angle x which is constrained by steric interactions, and by the anomeric effect. The main conformations are referred to as syn and anti (see Fig. 17.5). In syn, j X is close to 0° (sp for torsion angle definition, see Box 13.3 Fig. 13.12), and the base is I oriented "above the ribose ring causing steric interactions which in the anti conformation with x close to 180° (ap) are avoided. The anti conformation is therefore preferred and is the j only form observed in double-helical DNA and RNA. An exception is the left-handed Z-DNA with alternating purine/pyrimidine nucleotide sequence where the purines are in the syn conformation. 

This transformation was significant from a mechanistic standpoint. The formation of the a-ace-tonitrilium ion was not expected on the basis of the reverse anomeric effect (originally defined as the tendency of positively charged substituents at C-1 of a pyranose ring to adopt the equatorial orientation [58]). The authors, however, unambiguously established the a-orientation of... 

In this lecture, the author will attempt not to mix two kinds of approach to the problem of the "Origin and Consequences of the Anomeric Effect", on one side the hard facts of experimental data and calculations conducted with clearly-defined approximations, and on the other side, semi-empirical theories which are more easily grasped and put to use with complex molecules. Discussion of these is reported to the last section. 

A negative value of AG(AE1) represents the reverse anomeric effect. It could, perhaps, also be defined in the context of Eq. 4. Here, the reference saturated hydrocarbons exhibit negative energy AE(AE3), and the reverse anomeric effect could be defined as an excess energy (in al lute value) over that for the reference molecules. 

Observed anomeric C and 5- 0 kinetic effects (1.004 and 1.015 respectively) were both less than the quilibrium effects (defined as [p]L[a]H/[p]H[ ]L) of 1.007 and 0.975, respectively, indicating little changing of bonding at these... 

Steric isotope effects are less clear cut, possibly because many small effects considered to be steric are in fact a mix of steric and stereoelectronic. Early work on the racemisation of optically active biphenyls gave a value of kne/ De of 0.85 for a 2,2 -dimethylbiphenyl also containing a 6,6 ethylene bridge (4,5-dimethyl 9,10 dihydrophenanthrene), and 1-deuteriocyclohexane prefers the deuterium-axial conformation by 25 J mol V but the preference decreases next to a heteroatom. Effects of deuterium substitution of carbon-bound protons in glucose on the anomeric equilibrium in water cannot be simply rationalised by a single effect the equilibrium isotope effect (defined as [P]H[a]D/[p]D[a]H) being 1.043 for HI, 1.027 for H2, 1.027 for H3, 1.001 for H4, 1.036 for H5 and 0.998 for H6,6. ... 

---