iridoid

Iridoids are a class of monoterpenoid natural products characterized by a rigid bicyclic skeleton consisting of a cyclopentane ring fused to a six-membered oxygen-containing ring, typically a dihydropyran. They are usually found in plants as aglycones or, more commonly, as iridoid glycosides—iridoids bound to one or more sugar units.

Biosynthesis proceeds from the universal isoprenoid pathway to geraniol and 8-oxogeranial, followed by cyclization to form the iridoid skeleton. The core iridoid skeleton can be modified by oxidation, glycosylation, and further rearrangements; a prominent subclass is secoiridoids, in which opening of the cyclopentane ring gives linear derivatives such as oleuropein or related compounds. Secologanin is a key iridoid glycoside that serves as a precursor to monoterpene indole alkaloids.

Occurrence and function: Iridoids are widespread in angiosperms and are particularly common in the Rubiaceae (coffee family) and Plantaginaceae, among others. They often accumulate as glycosides in vacuoles and can be hydrolyzed to release reactive aglycones; many iridoids act as feeding deterrents or antimicrobial agents, contributing to plant defense.

Examples and uses: Notable iridoids include aucubin, catalpol, loganic acid, and geniposide. Harpagoside, an iridoid glycoside from Devil's Claw (Harpagophytum procumbens), is studied for anti-inflammatory effects. Some iridoids, including geniposide and catalpol, are used in traditional medicines and are investigated for various pharmacological activities such as anti-inflammatory, neuroprotective, and antioxidant effects.