Terpenoid fragments containing several iso-prene units are found as alkyl substituents in shikimate-derived quinones (see page 158). Thus ubiquinones typically have C40-C50 side-chains, rubber = 103-10:
plastoquinones usually C45, and menaquinones up to C65. The alkylating agents are polyprenyl diphosphates, formed simply by an extension of the prenyltransferase reaction (see page 172), repeatedly adding IPP residues. Even longer polyiso-prene chains are encountered in some natural polymers, especially rubber and gutta percha. Rubber (Figure 5.74), from the rubber tree Hevea brasiliensis (Euphorbiaceae), is unusual in possessing an extended array of cis (Z) double bonds rather than the normal trans configuration. Gutta percha, from Palaquium gutta (Sapotaceae) on the other hand, has trans (E) double bonds. The cis double bonds in rubber are known to arise by loss of the pro-S proton (HS) from C-2 of IPP (contrast loss of H*, which gives a trans double bond) (Figure 5.75). However, a small (up to C20) trans-allylic diphosphate initiator is actually used for the beginning of the chain before the extended cis chain is elaborated.
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