Page - (000031) - in Biomedical Chemistry: Current Trends and Developments

It should be noted that in this intramolecular addition, the electrophile is generated via diphosphate ion (PPO-) elimination. The diphosphate ion leaves easily because it is itself a stable anion and the generated carbocation is also stable, as it is an allylic carbocation stabilized by resonance. The delocalized positive charge turns the terminal carbon into a strong electrophile capable of adding to the double bond; the deficiency of electrons in the electrophile does not always coincide with the atom that will be attacked. Biological examples of electrophilic reactions occur frequently in metabolic processes, such as in the β- oxidation pathway of the fatty acid metabolism (Chapter 1.1.5). 1.1.4.3 Aromatic Substitutions In simple terms, aromaticity may be described as a chemical property that arises from delocalization of electrons in a ring. These electrons may be provided by conjugated unsaturation, lone pair electrons or even orbitals; the system becomes aromatic when agreeing with the Huckel rule, which are state that the number of the total conjugated electrons must be 4n+2 electrons. It is a particularly strong form of resonance stabilization making aromatic compounds more stable than expected otherwise. The orbital alignment required for aromatic stabilization turns the aromatic (aryl) moieties planar (Figs. 1.1.4 and 1.1.5). Because of the intermediate character of its bonds (Fig. 1.1.4) the reactivity of aromatic compounds is not identical to the reactivity of other unsaturated compounds.