Electron-withdrawing anomeric substituents, interaction with

Figure 3.9a may also represent the interaction of a nonbonded ( lone-pair ) orbital with an adjacent polar n or a bond [67]. If a polar n bond, one can explain stabilization of a carbanionic center by an electron-withdrawing substituent (C=0), or the special properties of the amide group. If a polar a bond, we have the origin of the anomeric effect. The interaction is accompanied by charge transfer from to A, an increase in the ionization potential, and a decreased Lewis basicity and acidity. These consequences of the two-electron, two-orbital interaction are discussed in greater detail in subsequent chapters. 

Anomeric effect - The stabilization ( x—>ocz ) of a ring conformation by interaction of a lone pair of electrons ( x)on a ring heteroatom with an antibonding c orbital (oCz ) of an adjacent electron-withdrawing substituent Z is known as the anomeric effect. This type of stabilization (i.e., o >°cz ) was first invoked to explain the preference for the axial orientation of electronegative substituents Z at the 2-position (anomeric position) of tetrahydropyrans. For examples see Sections 2.2.3.1, 2.2.4.3, and 2.4.4.4. 

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