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Catalpol is an iridoid glucoside. This natural product falls in the class of iridoid glycosides, which are simply monoterpenes with a glucose molecule attached.
Natural occurrence
First isolated in 1962, catalpol was named for plants in the genus Catalpa in which it was discovered. Later in 1969, catalpol was found to be present in larger quantities in several plants in genus Rehmannia (Orobanchaceae). It is also found in plants belonging to several families, including, but not limited to, Scrophulariaceae, Lamiaceae (including scullcap), Plantaginaceae (Bignoniaceae), all of which being in the order Lamiales.
Because they feed on these plants, variable checkerspot butterflies (Euphydryas chalcedona) contain high amounts of catalpol, which makes them unpalatable to predators and thus serves as a defense mechanism.