Seite - (000030) - in Biomedical Chemistry: Current Trends and Developments

Scheme 1.1.9: a) Phosphorane intermediate ‒ analogue of the activated complex ‒ in red are the axial positions (collinear) and in blue are the equatorial positions (coplanar); b) metaphosphate (planar, stabilised by resonance) ‒ analogue of a carbocation. 1.1.4.2 Electrophilic Addition Reactions Electrophilic reagents can react with compounds that are electron rich in certain exposed regions, from which alkenes are a typical example. In these systems, the π bond results from overlapping of p orbitals and provides regions of increased electron density above and below the plane of the molecule. π electrons are more loosely bound than those of a σ-bond so they can interact more easily with a positively charged electrophilic species, forming a new σ-bond and a carbocation (Scheme 1.1.10). This in turn rapidly reacts with a nucleophile to form another σ- bond. In this case, the nucleophile is either an anion or a neutral moiety with free pairs of electron that will become neutral again eliminating a group or an atom. Scheme 1.1.10: Electrophilic addition reaction mechanism. 1.1.4.2.1 Synthesis of α-Terpineol ‒ Intramolecular Addition In the following example we can see the simplified (without enzyme interactions) biosynthetic mechanism of α-terpineol from linalyl diphosphate, which occurs through an electrophilic addition (Scheme 1.1.11). Scheme 1.1.11: Synthesis of α-terpineol from linalyl diphosphate.