It is to be noted that except for inorganic salts and a few low molecular organic substances, the majority of molecules in living systems—both in plants and animals are chiral. Although these molecules can exist as a number of stereoisomers, almost invariably only one stereoisomer is found in nature. Thus nature is inherently chiral with the building blocks of life namely amino acids, nucleotides and sugars being chiral and they exist in nature in enantiomerically pure forms.

Because the interactions between molecules in living systems take place in a chiral environment, a molecule and its enantiomer will elicit a different physiological response when compared to the same molecule’s diastereomer. Thus while α-glucose is metabolically active in living systems its isomer β-glucose is non-assimilable; (+)-epinephrine is a less active adrenergic than (–)-epinephrine.

Today it is known that S-thalidomide is a sedative while its R isomer is teratogenic. The thalidomide catastrophe of the 1960s could have been prevented by marketing its ‘s’ form instead of the racemic drug. When a chemical substance containing a centre of symmetry such as thalidomide is synthesized in the lab, this generally yields a racemate that is a mixture of both enantiomers in equal amounts. Thus commercially available synthetic drugs are racemic and today we know now that this has clinical implications as enantiomers can show important differences in pharmacodynamic and pharmacokinetic behaviour.

It has been shown in a review of 1,522 pharmaceutical substances discovered in the 1960s and 1970s that out of 1,096 synthetics, 422 had an asymmetry centre and as many as 370 of these were not used as enantiomers. In contrast, 424 of the 426 natural and semi-synthetic drugs had an optically active carbon atom and only 2 were not used as enantiomers. This illustrates the selectivity and stereospecificity of natural compounds which is a clear advantage over synthetics.

• Plant SMs often have complex stereostructures with many chiral centres, which may be essential for biological activity. Hence many of these complex biomolecules cannot be synthesized economically on a commercial basis in enantiomerically pure forms. Vincristine, vinblastine and azadirachtin are a few examples.
• Thus SMs are generally recognized to afford a source of small and complex organic molecules of outstanding chemical diversity, which are highly relevant to the contemporary drug discovery process.
• Plant kingdom can thus provide us with unique chemical structures that are unlikely to be synthesized de novo on a commercial scale. Anti-cancer drug paclitaxel containing 11 chiral centres with 248 possible diastereomeric forms is a classic example.
• Several SMs often serve additionally as chemical models or templates for the design and total synthesis of new drug entities—e.g., d-tubocurarine for atracurium besylate, visnadin for sodium cromoglycate, podophyllotoxin for etoposide and artemisinin for artemether.