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Conformations, anomeric effect definition

By definition, a generalized anomeric effect is observed at carbon of an XCY system when a molecule preferentially adopts a conformation that optimizes a secondary, stabilizing electronic interaction involving overlap between the lone pair on one heteroatom with the a orbital of the bond between the central carbon atom and the second heteroatom . Figure 5a illustrates that in XNY systems, as with anomeric carbon centres, two anomeric interactions are possible and involve either an ny-CT x ° nx-o NY overlap where nx and ny represent the p-type lone pairs on X and Y and NX and NY represent the N—X and N—Y a orbitals. In either case, the result is a net stabilization of the lone pair of electrons (Figure 5b). Except where the nitrogen is symmetrically substituted, one of these interactions will be strongest. [Pg.844]

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. [Pg.273]

Definitions based on Eqs. 1,3, and 4 should, in principle, also apply for the exo-anomeric and reverse anomeric effects. There are, however, some problems with the practical application of Eq. 1 in the case of the exo-anomeric effect, because the AG values are largely not available. For the exo-anomeric effect, the conformational equiUbrium is specified by two dihedral angles, 6 and d>, and the value of AG° is needed for all six individual con-formers shown in Fig. 3. Because rotation around the exocychc bond by angle O is much less restricted in comparison with rotation by angle 6, a mixture of conformers was experimentally observed, with a difficult resolution of AG2 into individual components. If the exo-anomeric effect is treated by Eq. 3, the extra term AE(AE2) should be redefined for the whole range of values of the torsional angle O. Due to the lack of experimental data on AG or AE , for each conformer in Fig. 3, the energy values calculated correctly , for example by some molecular orbital method, are used, instead of... [Pg.58]

This approach, though simple, is somewhat misleading because it does not correspond to the original definition of the anomeric effect, that is, increased preference for the sc conformation. It does not take into account... [Pg.166]

It should be mentioned that the inverse (reverse ) anomeric effect has been postulated (183) recently for propanedinitrile CHjfCN) (95) and its derivatives, based on a synergistic destabilization by two geminal cyano substituents. The authors assumed that the anomeric effect always implies a stabilizing interaction between two substituents at an anomeric center (this assumption is not justified, see Section II.H) and a destabilizing interaction must therefore correspond to an inverse effect. However, such an assumption does not correspond to the definition of the anomeric (and reverse anomeric) effects, which is based on the relation between energies of conformers, not on the energy of interaction between substituents at an anomeric center. [Pg.225]

The definition of the anomeric effect is sometimes expanded to systems where the general shift in the conformational equilibrium is observed towards the axial conformer, even when such systems lack lone pairs in the ring. [Pg.143]


See other pages where Conformations, anomeric effect definition is mentioned: [Pg.12]    [Pg.4]    [Pg.33]    [Pg.30]    [Pg.36]    [Pg.94]    [Pg.169]    [Pg.192]    [Pg.208]    [Pg.185]    [Pg.208]    [Pg.37]    [Pg.15]    [Pg.4]    [Pg.218]   
See also in sourсe #XX -- [ Pg.195 ]




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