Diterpenes phytanes

Approximately 5 000 naturally abundant acyclic and cyclic diterpenes derived from the parent hydrocarbon phytane are known The (3i ,7i ,ll )-enantiomer of phy-tane has been found in meteorites, oil slate, other sediments and, last but not least, in human liver. Oil slate additionally eontains (-)-(3if,7if,llif)-phytanoic acid which has also been isolated from butter. 1,3(20)-Phytadiene is one among many constituents of tobacco Nicotiana tabacum (Solanaceae) -1,3-phytadiene and its (%)-isomer are found in zooplankton. Chlorophyll in the chloroplasts of plant cells exemplifies an ester of +)-(lE,lR, l/ )-2-phyten-l-ol usually referred to as phytol. 2,6,10,14-Phytatetraene-l,19-diol, better known as plaunotol, is the ehief constituent of the leaves of flie Thai medicinal plant Croton sublyratus (Euphorbiaceae) used as "plau noi" or "kelnac" as an antiulcerative. 

Mono- and Bicyclic Diterpenes derived from Phytane... 

A further group of branched-chain acids differ from those already discussed in that they are produced from isoprenoid precursors and not by the normal de novo pathway or some simple modification of it. These acids are based on the diterpene phytol derived from chlorophyll and the more common acids are phytanic (C20), pristanic (C19) and a Cie homologue. These molecules contain chiral centres and the natural forms are not always the same enantiomer. These acids occur widely, but at low levels, in the lipids of land and aquatic animals and have been identified in ancient sediments. In animals they occur in tissue, milk and blood lipids and are present as trace components of butter fat. The tissue lipids of humans suffering from the rare neurological disorder Refsum s disease contain up to 50% of phytanic acid because of an inability to metabolize this unusual branched-chain acid (Section 12.6). 

An important representative of diterpenoid alcohols is phytol (2 ,3,7ff,llff,15)-3,7,ll,15-tetramethylhexadec-2-en-l-ol, a constituent of chlorophylls, tocopherols and K group vitamins. Free phytol arises during chlorophyll hydrolysis by chlorophyUase as an integral part of plant catabolism in fruit ripening and yellowing of leaves. Microbial decomposition of phytol creates some unusual branched fatty acids, such as pristanic and phytanic acids (see Section 3.2.3.3.2). Acyclic diterpenic alcohol is also retinol (vitamin Aj) (see Section 5.2.1). 

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