Enantioselective synthesis

Lore & Background

Enantioselective synthesis is achieved by using a chiral feature that favors the formation of one enantiomer over another through interactions at the transition state, a biasing known as asymmetric induction. This can involve chiral features in the substrate, reagent, catalyst, or environment, and works by making the activation energy required to form one enantiomer lower than that of the opposing enantiomer. The enantioselectivity is determined by the relative rates of an enantiodifferentiating step, with the rate difference being greater at lower temperatures, so even small energy-barrier differences can lead to a noticeable effect. Approaches include enantioselective catalysis using chiral catalysts (often chiral coordination complexes), chiral auxiliaries (organic compounds coupled to the starting material to enable diastereoselective reactions), biocatalysis using biological compounds like isolated enzymes or living cells, enantioselective organocatalysis using organic compounds such as proline, and chiral pool synthesis using readily available chiral starting materials. Each method has its own advantages and limitations, such as catalyst efficiency, need for stoichiometric amounts, or specificity. Separation and analysis of enantiomers is challenging because enantiomers possess many of the same physical properties, making it difficult to determine if a process has produced a single enantiomer or to separate enantiomers from a reaction that was not 100% enantioselective. However, enantiomers behave differently in chiral environments, enabling analysis.

Reader's Guide

Enantioselective synthesis is of great importance because many biological building blocks like sugars and amino acids are produced exclusively as one enantiomer, and living systems react differently with various enantiomers. Examples include the sweetener aspartame (L-aspartame tastes sweet, D-aspartame is tasteless), the odor of carvone (R-(–)-carvone smells like spearmint, S-(+)-carvone smells like caraway), drug effectiveness (only the (S)-(+) enantiomer of Citalopram is responsible for its beneficial effects), and drug safety (D-penicillamine is used in therapy, while L-penicillamine is toxic). Enantioselective synthesis is difficult because enantiomers possess identical enthalpies and entropies, so an undirected process produces a racemic mixture. The field's legacy lies in enabling the production of single enantiomers for pharmaceuticals and other applications, using methods such as asymmetric hydrogenation, chiral auxiliaries, biocatalysis, organocatalysis, and chiral pool synthesis. The design of new catalysts is dominated by the development of new classes of ligands, with certain 'privileged ligands' like BINOL, Salen, and BOX being effective in a wide range of reactions.

Did You Know?

• Enantioselective synthesis is defined by IUPAC as a reaction producing stereoisomeric products in unequal amounts.
• The artificial sweetener aspartame has two enantiomers: L-aspartame tastes sweet, D-aspartame is tasteless.
• The antidepressant drug Citalopram is sold as a racemic mixture, but only the (S)-(+) enantiomer is responsible for its beneficial effects.
• D-penicillamine is used in chelation therapy, while L-penicillamine is toxic as it inhibits the action of pyridoxine.