The terpenoids, sometimes called isoprenoids, are a large and diverse class of naturally occurring organic chemicals derived from the 5-carbon compound isoprene, and the isoprene polymers called terpenes. Most are multicyclic structures with oxygen-containing functional groups. About 60% of known natural products are terpenoids. Although sometimes used interchangeably with "terpenes", terpenoids contain additional functional groups, usually O-containing. Terpenes are hydrocarbons.
Plant terpenoids are used for their aromatic qualities and play a role in traditional herbal remedies. Terpenoids contribute to the scent of eucalyptus, the flavors of cinnamon, cloves, and ginger, the yellow color in sunflowers, and the red color in tomatoes. Well-known terpenoids include citral, menthol, camphor, salvinorin A in the plant Salvia divinorum, the cannabinoids found in cannabis, ginkgolide and bilobalide found in Ginkgo biloba, and the curcuminoids found in turmeric and mustard seed. The provitamin beta carotene is a terpene derivative called a carotenoid.
The steroids and sterols in animals are biologically produced from terpenoid precursors. Sometimes terpenoids are added to proteins, e.g., to enhance their attachment to the cell membrane; this is known as isoprenylation.
Structure and classification
Terpenoids are modified terpenes, wherein methyl groups have been moved or removed, or oxygen atoms added. Some authors use the term "terpene" more broadly, to include the terpenoids. Just like terpenes, the terpenoids can be classified according to the number of isoprene units that comprise the parent terpene:
|Terpenoids||Analogue terpenes||Number of isoprene units||Number of carbon atoms||General formula||Examples|
|Hemiterpenoids||Isoprene||1||5||C5H8||DMAPP, isopentenyl pyrophosphate, isoprenol, isovaleramide, isovaleric acid, HMBPP, prenol|
|Monoterpenoids||Monoterpenes||2||10||C10H16||Bornyl acetate, camphor, carvone, citral, citronellal, citronellol, geraniol, eucalyptol, hinokitiol, iridoids, linalool, menthol, thymol|
|Sesquiterpenoids||Sesquiterpenes||3||15||C15H24||Farnesol, geosmin, humulone|
|Diterpenoids||Diterpenes||4||20||C20H32||Abietic acid, ginkgolides, paclitaxel, retinol, salvinorin A, sclareol, steviol|
|Sesterterpenoids||Sesterterpenes||5||25||C25H40||Andrastin A, manoalide|
|Triterpenoids||Triterpenes||6||30||C30H48||Amyrin, betulinic acid, limonoids, oleanolic acid, sterols, squalene, ursolic acid|
|Polyterpenoid||Polyterpenes||>8||>40||(C5H8)n||Gutta-percha, natural rubber|
Terpenoids can also be classified according to the type and number of cyclic structures they contain: linear, acyclic, monocyclic, bicyclic, tricyclic, tetracyclic, pentacyclic, or macrocyclic. The Salkowski test can be used to identify the presence of terpenoids.
Paclitaxel is a diterpenoid anticancer drug.
Terpineols are monoterpenoids.
Humulones are classified as sesquiterpenoids.
Retinol is a diterpenoid.
Terpenoids, at least those containing an alcohol functional group, often arise by hydrolysis of carbocationic intermediates produced from geranyl pyrophosphate. Analogously hydrolysis of intermediates from farnesyl pyrophosphate gives sesquiterpenoids, and hydrolysis of intermediates from geranylgeranyl pyrophosphate gives diterpenoids, etc.
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