Subject gauche effect

The main subject of this paper is the gauche effect (relating to X-C-C-Y interaction). Referring to Scheme 3 we may consider the gauche effect to be attractive when the gauche conformation... 

In summary, we expect that 2,3-substitution will not greatly alter the conformational preference of the diene system, L e. conformational isomerism of 2,3-difluoro-butadiene will be subject to the same electronic factors as that of the unsubstituted diene and these factors operate in the same direction in both cases. It is expected that, by analogy to 1,3-butadiene, the order of conformer stability of 2,3-difluoro-butadiene will be dictated by steric effects, L e. it will be tram > gauche > cis. 

Consider the two systems CH2F—SH and CH2F—OH. According to our approach both are predicted to exist in a preferred gauche conformation. However, the extent to which the nx-o F interaction obtains in the two molecules may be subject to matrix element control simply because ns is a better donor than no but yields a smaller interaction matrix element with a F- The variation of these two effects may conceivably be comparable and subject to matrix element control due to the fact that the n—o orbital interaction involves well separated energy levels. Hence, one... 

The results suggest that the five-membered peptide chelate ring, in which the atoms are nearly coplanar, may be more subject to steric effects than is the case for the gauche conformation of chelate rings of ethylenediamine and its C-methyl derivatives. However, the corresponding nucleophilic displacement reactions have not been studied and it is too early to draw... 

What is the reason for this apparent discrepancy It is a solvent effect. In aqueous solution (Figure 9.6), the OH group at the anomeric C atom of the glucose becomes so voluminous due to hydration that it strives for the position in which the steric interactions are as weak as possible. Thus, it moves into the equatorial position—with a AG° value of approximately -1.6 kcal/mol—to avoid a gauche interaction with the six-membered ring skeleton. (Remember that axially oriented substituents on the chair conformer of cyclohexane are subject to two gauche interactions with the two next-to-nearest C. —C bonds. They therefore have a... 

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