Lamiaceae, iridoid glycosides

Figure 7.14 Distribution of iridoid glycosides in the family Lamiaceae, reconstructed from a rbcL data set. (After Wink and Kaufmann, 1996.) (See Plate 20 in colour plate section.)...

Iridoids are synthesised through the mevalonic acid pathway and are technically known as cyclopentan-[c]-pyran monoterpenoids. They occur mainly as glycosides althongh non-glycosidic iridoids also occur— these are covered in Chapter 5. The name iridoid is derived from the common Anstralian meat ant Iridomyrex detectus, from which it was first detected in 1956 (Sticher 1977). Iridoid glycosides are derived from plants belonging to many families, most notably the Rubiaceae, Lamiaceae, Scrophulariaceae and Gentianaceae. 

Naturally occurring iridoids and their glycosides are widely distributed in plants and serve as important taxonomic markers. The iridoids were named after ants of the genus Iridomirmex, from which iridodial, iridomyrmecin, and related compounds were isolated and found to be involved in the defense mechanisms of these insects. Similar structures also exist in plants for example nepetalactone from Nepeta cataria L. (Lamiaceae), or teucriumlactone C from Teucrium marum L. have marked properties (the effects of the former on cats earned it some evocative names catnip, Katzenmelisse, herbe-aux-chats). 

The bitter iridoid monoterpenes usually occur as glycosides (see Chapter 4). Non-glycosidic iridoids include the sedative valepotriates found in valerian—Valeriana spp. (Valerianaceae). They are closely related to nepetalactone, the volatile component of essential oil of catnip, Nepeta cataria (Lamiaceae), responsible for attracting cats to the plant. 

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