Pericyclic Reactions: Cycloadditions and Diels-Alder Reaction
Cycloaddition products are chiral if they do not contain any elements of symmetry including axes of symmetry. Without asymmetric induction, both enantiomers are formed in equal parts.
Asymmetric induction is possible in two ways:
- Chiral auxiliaries
- Chiral catalysis
Addition of BF3 increases the reactivity of acrylate. The reaction is more selective since it can be carried out at lower temperatures. By adding the Lewis acid to the carbonyl function, a more electron-poor and, therefore, more reactive dienophil is generated:
The use of chiral catalysts, mostly chiral Lewis acids, represents an alternative and elegant method to increase the enantioselectivity of the Diels-Alder reaction.