Anomeric effect, donor-acceptor molecules

We have already seen in Section 2.2.3.3 how conformation can be affected by anomeric interactions, which can lead electronegative substituents to be axial at the 2-position in tetrahydropyranyl rings, and sometimes cause a chain of atoms to adopt a seemingly more hindered gauche conformation 2.81-2.83, 2.85 and 2.86 rather than the more usual zigzag arrangement. Similar hyperconjugative interactions in neutral molecules between two a bonds, one a a donor and the other a a acceptor, can lead them to adopt conformations in which the stereoelectronic effect overrides the purely steric effect. 

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