Asymmetric Synthesis (Chiral Synthesis)

Asymmetric Synthesis (Chiral Synthesis) is a method of creating compounds with a preferred chirality to produce optically active molecules.

Definition:

• Asymmetric synthesis is a chemical reaction that forms chiral products in a way that favors one stereoisomer (enantiomer or diastereomer) over the other.

• Essential in drug synthesis, since enantiomers can have different biological effects.

Types of Asymmetric Synthesis

• Partial Asymmetric Synthesis (Enantioselective/Diastereoselective)

  • Produces both enantiomers, but one in greater amount.
  • Measured by enantiomeric excess (ee):

$ee = \left| \frac{R – S}{R + S} \right| \times 100$

1. • Achieved by using chiral catalysts, auxiliaries, or reagents.
   • Example:
   • Hydrogenation of prochiral alkenes using chiral Rh-catalyst gives more of one enantiomer.

2. Absolute Asymmetric Synthesis

   • Produces only one stereoisomer—100
   • Achieved with strong chiral influence (e.g., chiral catalysts, enzymes, chiral ligands).
   • Example:
     • Sharpless epoxidation gives a single enantiomer of epoxide.
     • CBS reduction converts ketones to alcohols with very high enantioselectivity.
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