Organic chemistry
Alkaloids, terpenoids, flavonoids
Three large families of natural products arise from different biosynthetic building blocks and show wide structural variety and biological activity.
IntuitionIntuition: one plant chemistry, three signatures
A chemist sees similar natural products differently once the biosynthetic origin is identified. Nitrogen-rich alkaloids often interact with receptors, enzymes or ion channels; terpenoids are assembled from C₅ isoprene units; flavonoids are polyketide-phenylpropanoid pigments and signalling molecules.
SchoolSchool level: recognize the families
Definition:
A compound produced by a living organism, often with ecological, physiological or defensive roles. Many medicines, fragrances and dyes were first identified in this class.
Alkaloids such as caffeine, nicotine, morphine or quinine usually contain basic nitrogen. Terpenoids include monoterpenes like limonene and complex structures such as taxol or steroids. Flavonoids include quercetin, catechins and anthocyanins, often contributing to colour and antioxidant behaviour.
| Family | Biosynthetic source | Familiar example |
|---|---|---|
| Alkaloids | Often amino-acid-derived | Morphine, caffeine |
| Terpenoids | C₅ isoprene units | Limonene, taxol |
| Flavonoids | Phenylpropanoid + polyketide | Quercetin, catechin |
UndergraduateUniversity: biosynthetic pathways and classification
Classification can be structural, biosynthetic or functional. For terpenoids, counting C₅ building blocks gives mono-, sesqui-, di-, tri- and tetraterpenes. For alkaloids, a ring or amino-acid precursor often gives more informative categories than an arbitrary structural motif alone.
Example: Assign a natural product by biosynthesis
A plant metabolite contains a heterocyclic basic nitrogen and derives from tryptophan. Which family is more plausible: terpenoid or alkaloid?
Solution
The biosynthetic precursor points to an alkaloid. Tryptophan is an amino-acid precursor for many indole alkaloids; a terpenoid would instead reflect isoprene-unit assembly.
AdvancedAdvanced: stereochemistry and biological role
The same gross structure may have very different effects depending on stereochemistry, protonation state, glycosylation or esterification. Biosynthetic hypotheses therefore help isolate what is essential: a scaffold, a recognition element, or a functional group pattern.
References
- Medicinal Natural Products: A Biosynthetic Approach (3rd ed.) · Paul M. Dewick, 2009
- Alkaloids: Biochemistry, Ecology, and Medicinal Applications · Margaret F. Roberts, Michael Wink (eds.), 1998