Rearrangements in the biosynthesis of valine and isoleucine

Figure 5.7 Examples of (a) elimination, (b) isomerization (aldose/ketose) and (c) a complex rearrangement of the pinacol-pinacolone type found in the biosynthesis of valine and isoleucine.

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

---