iridoids

Iridoids are a class of monoterpenoid secondary metabolites typically found in plants as glycosides. They are based on an iridoid skeleton, a cyclopent[2]pyran ring system that is highly oxygenated and often substituted with various hydroxyl, carbonyl, and glycosyl groups. Most iridoids occur as iridoid glycosides, where the aglycone is linked to one or more sugar units.

Biosynthesis and structure: Iridoids are derived from geraniol via the mevalonate or MEP pathway, followed by cyclization to the characteristic cyclopentane–pyran framework and oxidation. Many iridoids are seco-iridoids, in which one of the rings is opened. Glycosylation produces a wide array of iridoid glycosides such as aucubin and catalpol.

Distribution and sources: Iridoids are widespread in higher plants, particularly in families such as Rubiaceae and Gentianaceae, as well as several members of Plantaginaceae, Oleaceae, and Apocynaceae. Gardenia jasminoides is a well-known source of geniposide, and aucubin and catalpol are common in Plantago species.

Biological roles and uses: They commonly function as defense compounds against herbivores and pathogens. Pharmacologically, iridoids and their glycosides have been reported to possess anti-inflammatory, antioxidant, antimicrobial, hepatoprotective, and neuroprotective activities in various models. They are studied as pharmaceutical leads and chemotaxonomic markers for plant identification and quality control.

Examples: Aucubin, catalpol, geniposide, loganin, and the aglycone genipin are well-known iridoids.