二萜 Diterepene
精油化學
占星油樂園
二萜是兩種單萜單元的組合，總共20個碳。由
於它們的高分子量，這些分子難以通過蒸汽蒸餾提取，不容易存在精油中，即使有也是以非常低的量被發現。在結構上，這些是可在常規蒸餾條件下收集的最大的揮發性芳族化合物。
精油成分通常具有低分子量，這有助於其高揮發性。如果分子量高於低於300，則不能通過蒸汽蒸餾收集。如果在精油中發現分子量大於此數量的分子，則表明萃取條件或摻假不當。它們很少見於大多數精油中。可能存在於精油中的二萜包括樟腦，咖啡醇，咖啡白脂(kahweol)和單環二萜(cambrene A)。
Diterpenes are a class of chemical compounds composed of two terpene units, often with the molecular formula C20H32. Diterpenes consist of four isoprene subunits. They are biosynthesized by plants, animals and fungi via the HMG-CoA reductase pathway, with geranylgeranyl pyrophosphate being a primary intermediate. Diterpenes form the basis for biologically important compounds such as retinol, retinal, and phytol. They are known to be antimicrobial and antiinflammatory.
As with most terpenes a huge number of potential structures exists, which may be broadly divided according to the number of rings present.
Diterpenes are derived from the addition of one IPP unit to FPP to form geranylgeranyl-pyrophosphate (GGPP). From GGPP, structural diversity is achieved mainly by two classes of enzymes: the diterpene synthases and cytocromes P450. Several diterpenes are produced by plants and cyanobacteria. GGPP is also the precursor for the synthesis of the phytane by te action of the enzyme geranylgeranyl reductase. This compound is used for the biosynthesis of tocopherols and the phytyl functional group is used in the formation of chlorophyll a, ubiquinones, plastoquinone and phylloquinone.