Iridoids are a class of secondary metabolites found in a wide variety of plants and in some animals. They are monoterpenes biosynthesized from isoprene and they are often intermediates in the biosynthesis of alkaloids. Chemically, the iridoids usually consist of a cyclopentane ring fused to a six-membered oxygen heterocycle. The chemical structure is exemplified by iridomyrmecin, a defensive chemical produced by the Iridomyrmex genus, for which iridoids are named. Cleavage of a bond in the cyclopentane ring gives rise to a subclass known as secoiridoids, such as oleuropein and amarogentin. Iridoids are typically found in plants as glycosides, most often bound to glucose.
Iridoids are found in many medicinal plants and may be responsible for some of their pharmaceutical activities. Isolated and purified, iridoids exhibit a wide range of bioactivities including cardiovascular, antihepatotoxic, choleretic, hypoglycemic, analgesic, anti-inflammatory, antimutagenic, antispasmodic, antitumor, antiviral, immunomodulator, and purgative activities.
The iridoids are produced by plants primarily as a defense against herbivores or against infection by microorganisms. The Variable Checkerspot butterfly also uses iridoids obtained through its diet as a defense against avian predators. To humans and other mammals, iridoids are often characterized by a deterrent bitter taste.
Aucubin and catalpol are two of the most common iridoids in the plant kingdom. Iridoids are prevalent in the plant subclass Asteridae, such as Ericaceae, Loganiaceae, Gentianaceae, Rubiaceae, Verbenaceae, Lamiaceae, Oleaceae, Plantaginaceae, Scrophulariaceae, Valerianaceae, and Menyanthaceae.
The iridoid ring scaffold is synthesized, in plants, by the enzyme iridoid synthase. In contrast with other monoterpene cyclases , iridoid synthase uses 10-oxogeranial as a substrate. The enzyme uses a two-step mechanism, with an initial NADPH-dependent reduction step followed by a cyclization step that occurs through either a Diels-Alder reaction or an intramolecular Michael addition. 
- Didna, B., Debnath, S., Harigaya, Y. "Naturally Occurring Iridoids. A Review, Part 1." Chem. Pharm. Bull. 55(2) 159-222 (2007).
- Tundis R, Loizzo MR, Menichini F, Statti GA, Menichini F. Biological and pharmacological activities of iridoids: recent developments. Mini Rev Med Chem. 2008 Apr; 8(4):399-420.
- Bowers, M. D. "Unpalatability as a Defense Strategy of Western Checkerspot Butterflies (Euphydryas Scudder, Nymphalidae)." Evolution 35.2 (1981): 367-75.
- Geu-Flores, F.; Sherden, N. H.; Courdavault, V.; Burlat, V.; Glenn, W. S.; Wu, C.; Nims, E.; Cui, Y.; O'Connor, S. E. (2012). "An alternative route to cyclic terpenes by reductive cyclization in iridoid biosynthesis". Nature 492 (7427): 138–142. doi:10.1038/nature11692. PMID 23172143.