Gauche effects

Our model of the gauche effect is straightforward and can be understood by examining Structure 3. 

The X-ray crystal structures of N-nucleosides show the shortening of the 04 — Cl bond relative to C4 — 04 bond by about 0.03 A which has been considered as a manifestation of the anomeric effect. 

The preference of the S -CHaOH group to occupy the pseudoequatorial orientation is manifested in the positive value for the pseudorotational equilibrium 111, 112 (Fig. 2.44). From the determination of the energetics of the two-state pseudorotational equilibrium in 36 nucleosides, it was found that the combined stereoelectronic and steric contributions in the anomeric effect of the nucleobases increase in the following order adenine guanine thymine uracil cytosine. One reason for the stronger anomeric effect in p3Timidine than in purine nucleosides could be that the delocalization is more effective in the 

The strength of the anomeric effect was enhanced upon protonation (evident by the increase in N-type sugar population) relative to the neutral state. These results 

We will discuss later in more detail the anomeric effect in nucleosides, nucleotides, and nucleic acids and its influence on the conformation of the furanose ring and the chemical consequences due to its presence. 

the gauche effect will be attractive when the gauche conformatitMi is the favored one of two equilibrium conformations, determined by the calculations of known steric and polar interactions. However, if the anti-conformation is favored more than the calculations suggest, the gauche effect is repulsive. 

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In subsequent reports [87], the principle of asymmetric glycolate alkylation/ RCM sequence was applied to the first total synthesis of isolaurallene (172), that contains a densely functionalized A5-oxonene core (Scheme 32). Anticipating that the gearing effect created by two synergistic gauche effects at C6-C7 and... 

While the comparison of the OMTS and the (CH2)12 spectra helped to learn something about the kind of information solid state chemical shifts can provide, we can obtain much more detailed data about the correlation of chemical shifts and the rotational isomeric states from the spectra of larger cycloalkanes. Usually conformational shift variations are discussed by (i) the so called y-gauche effect and (ii) the vicinal gauche effect, Vg 15) ... 

No Vg effect has to be considered for the CH3 carbon atoms. So it is reasonable to explain the splitting solely by the y-gauche effect. As can be seen from Fig. 14 the steric arrangement of the two CH3 carbon atoms within the segment differs by one y-gauche position of the carbons opposite of the CH—CH bond. With respect to the other carbons the steric situation is equivalent. Hence, the chemical shift... 

Conformational shift effects could be discussed in terms of discrete rotational isomeric states. Mainly two effects could be derived empirically to explain the shift differences due to conformational isomerism they-gauche and the Vg effect. However the spectra also indicate that the y-gauche effect is not a quantity with a universal numerical value. Furthermore the spectra of the cycloalkanes show that the conformational effects do not obey simple rules of additivity. With concern to our present knowledge great care has to be taken for the interpretation of NMR-spectra on the base of conformational shift increments which were not determined for the specific molecular structures. 

The gauche effect, anomeric effect, and related phenomena... 

Figure 3.63 illustrates the gauche effect for vicinal lone pairs and polar C—F bonds with the examples of (a) hydrazine and (b) 1,2-difluoroethane, respectively. As seen in Fig. 3.63(a), the

lone pairs are anti to one another (thus squandering their powerful donor strength on vicinal moieties with no acceptor capacity) is disfavored by 3.2 kcal mol-1 relative to the preferred = 93.9° conformer in which each nN hyperconjugates effectively with... 

Figure 3.63 Rotation-barrier profiles illustrating the gauche effect for (a) lone pairs in hydrazine and (b) polar bonds in 1,2-difluoroethane. (The angle 4> = 0°, as

The relative importance of the gauche effects associated with polar bonds and lone electron pairs is polar bond-polar bond > polar bond-lone pair > lone pair-lone pair. 

Wolfe and collaborators attempted to understand the origin of the gauche effect by using an intuitive interpretation of good quality ab initio calculations. 

Other authors have successfully correlated changes in torsional angles with y-gauche effects (80,150). 

For more examples concerning various molecular systems in staggered conformations, the reader is referred to the compilations of diamagnetic y-gauche effects in refs. 7 and 9. The disappearance of a noticeable effect when the observed carbon atom is quaternary, that is, when no polarizable C-H bond is present, was also considered to support the above explanation (110,175). 

Up to this point our discussion of y-gauche effects has demonstrated that the transmission mechanisms are not yet well understood and still open to speculation. The original concept of a steric interaction is highly controversial, and as long as there is no convincing explanation, the use of the deep-rooted term steric compression shift must be discouraged. 

Finally, it should be noted that y-gauche effects are not confined to carbon atoms. Analogous shifts have been reported for l9F, 31P (191), and 15N nuclei (206). Distinct shieldings of I70 nuclei caused by y-gauchc-oriented methyl or methylene groups were found, too (207). 

R3 transition state gauche effects (cf. Scheme 6), which appear to be greater in transition state Cj than in C. ... 

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