Stereochemistry & Chirality

What is Chirality?

A molecule is chiral if it is non-superimposable on its mirror image — like your left and right hands. The key structural feature is a chiral centre (stereocentre): a carbon atom bonded to four different substituents. Chirality is not merely a chemical curiosity; it has profound clinical consequences.

R/S Nomenclature

The Cahn-Ingold-Prelog (CIP) system assigns R or S configuration to each stereocentre:

1. Assign priority 1–4 to the four substituents based on atomic number (higher atomic number = higher priority).
2. Orient the molecule with the lowest-priority group (4) pointing away from you.
3. If the sequence 1→2→3 is clockwise: R (rectus, Latin for right).
4. If anticlockwise: S (sinister, Latin for left).

A molecule with one stereocentre has two enantiomers (R and S). With n stereocentres, there are up to 2ⁿ stereoisomers. Pairs of diastereomers have different physical properties; enantiomers share identical physical properties (melting point, solubility) but rotate plane-polarised light in opposite directions.

Enantiomers and Biological Activity

Biological macromolecules (enzymes, receptors, transporters) are themselves chiral. They interact differently with each enantiomer of a drug. Consequences include:

• Different potency: one enantiomer may be 100× more active than the other.
• Different targets: one enantiomer may cause side effects while the other provides benefit.
• Different metabolism: CYP enzymes often metabolise one enantiomer preferentially.

Thalidomide: The Canonical Cautionary Tale

Thalidomide was prescribed in the late 1950s as a racemate (50:50 mixture of R and S enantiomers) for morning sickness. The (R)-enantiomer is sedative and antiemetic. The (S)-enantiomer is teratogenic — it inhibits angiogenesis and disrupts limb development. Tragically, even administering the pure (R)-enantiomer is insufficient: in vivo racemisation occurs rapidly under physiological conditions, regenerating the (S)-form.

Thalidomide led directly to modern regulatory requirements for stereochemical characterisation of new drugs.

Chiral Switches in New Zealand Medicines

A chiral switch is the development of a single-enantiomer drug from a previously marketed racemate. Benefits include improved efficacy-to-side-effect ratios and extended patent life.

New Zealand examples:

• Esomeprazole (S-omeprazole): the active enantiomer of omeprazole (Losec → Nexium), with better CYP2C19 metabolism profile.
• Escitalopram (S-citalopram): the pharmacologically active SSRI enantiomer; the R-form inhibits the S-form's effect at the allosteric site.
• Levocetirizine (R-cetirizine): antihistamine with reduced sedation compared to the racemate cetirizine.

Analytical Methods

Chiral drugs are separated and analysed using chiral HPLC columns, chiral GC, or NMR with chiral shift reagents. Optical rotation ([α]D) confirms enantiomeric identity but not purity (use chiral HPLC for enantiomeric excess).