1030 CHAPTER TWENTY-SIX Lipids OPP Geranyl pyrophosphate Geranyl pyrophosphate is an allylic pyrophosphate and, like dimethylallyl pyrophosphate, can act as an alkylating agent toward a molecule of isopentenyl pyrophosphate. A 15-carbon carbocation is formed, which, on deprotonation, gives far OPP OPP Geranyl pyrophosphate Isopentenyl pyrophosphate OPP OPP Farnesyl pyrophosphate Hydrolysis of the pyrophosphate ester group converts farnesyl pyrophosphate to the cor responding alcohol farnesol(see Figure 26.6 for the structure of farnesol) repetition of the process just shown produces the diterpene geranylgeraniol from farnesyl pyrophosphate Geranylgeraniol TPROBLEM 26.9 Write a sequence of reactions that describes the formation of geranylgeraniol from farnesyl pyrophosphate The higher terpenes are formed not by successive addition of Cs units but by the coupling of simpler terpenes. Thus, the triterpenes(C3o) are derived from two molecules of farnesyl pyrophosphate, and the tetraterpenes( Cao) from two molecules of geranyl geranyl pyrophosphate. These carbon-carbon bond-forming processes involve tail-to-tail couplings and proceed by a more complicated mechanism than that just described. The enzyme-catalyzed reactions that lead to geraniol and farnesol(as their pyrophosphate esters)are mechanistically related to the acid-catalyzed dimerization of alkenes discussed in Section 6.21. The reaction of an allylic pyrophosphate or a carbo- cation with a source of T electrons is a recurring theme in terpene biosynthesis and is nvoked to explain the origin of more complicated structural types. Consider, for Back Forward Main MenuToc Study Guide ToC Student o MHHE Website1030 CHAPTER TWENTY-SIX Lipids Geranyl pyrophosphate is an allylic pyrophosphate and, like dimethylallyl pyrophosphate, can act as an alkylating agent toward a molecule of isopentenyl pyrophosphate. A 15-carbon carbocation is formed, which, on deprotonation, gives far￾nesyl pyrophosphate. Hydrolysis of the pyrophosphate ester group converts farnesyl pyrophosphate to the cor￾responding alcohol farnesol (see Figure 26.6 for the structure of farnesol). A repetition of the process just shown produces the diterpene geranylgeraniol from farnesyl pyrophosphate. PROBLEM 26.9 Write a sequence of reactions that describes the formation of geranylgeraniol from farnesyl pyrophosphate. The higher terpenes are formed not by successive addition of C5 units but by the coupling of simpler terpenes. Thus, the triterpenes (C30) are derived from two molecules of farnesyl pyrophosphate, and the tetraterpenes (C40) from two molecules of geranyl￾geranyl pyrophosphate. These carbon–carbon bond-forming processes involve tail-to-tail couplings and proceed by a more complicated mechanism than that just described. The enzyme-catalyzed reactions that lead to geraniol and farnesol (as their pyrophosphate esters) are mechanistically related to the acid-catalyzed dimerization of alkenes discussed in Section 6.21. The reaction of an allylic pyrophosphate or a carbo￾cation with a source of  electrons is a recurring theme in terpene biosynthesis and is invoked to explain the origin of more complicated structural types. Consider, for OH Geranylgeraniol OPP Geranyl pyrophosphate OPP Isopentenyl pyrophosphate H H OPP H OPP Farnesyl pyrophosphate H2O Geraniol OH Geranyl pyrophosphate OPP Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website