Acyclic Sesterterpenes

More than 150 sesterterpenes are known to date The acyclic representatives are derived from 3,7,1 ljl5,19-pentamethylicosane C25H52. Their biogenesis primarily yields geranylfamesyldiphosphate, involving the known pathway (section 1.3). 

The structures of terpenes or terpenoids varies widely and are classified according to various aspects, e.g. number of isoprene units (Cio monoterpenes, C15 sesquiterpenes, C20 diterpenes, C25 sesterterpenes, C30 triterpenes, C40 tetraterpenes) or division in acyclic, mono-, hi-, tri-, tetra-, pentacyclic terpenes. Often terpenes are named by their trivial names, e.g. d-limonene. 

A unique tetracyclic furanosesterterpene, wistarin (195), has been discovered in Ircinia wistarii in addition to the previously known isomer ircinianin (196) 131). 196 decomposed readily, whereas 195 was relatively stable. As wistarin (195) was not detected among the decomposition products of 196 and 196 did not undergo cyclization to 195 on treatment with dilute acid, base or Lewis acids, it was proposed that 195 was a natural product and not an artifact. The biosynthesis of these sesterterpenes must involve a different folding of the acyclic 25-precursor. 

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