26.9 Carbon-Carbon Bond Formation in Terpene Biosynthesis Isopentenyl pyrophosphate is the biological isoprene unit; it contains five carbon atoms ord Isopentenyl pyrophosphate undergoes it, in an equilibrium process, to 3-methyl-2-butenyl pyrophosphate (dimethylallyl OPP OPP Carbocation intermediate Isopentenyl pyrophosphate and dimethylallyl pyrophosphate are structurally lar--both contain a double bond and a pyrophosphate ester unit--but the chemical tivity expressed by each is different. The principal site of reaction in dimethylallyl pyrophosphate is the carbon that bears the pyrophosphate group. Pyrophosphate is a rea- sonably good leaving group in nucleophilic substitution reactions, especially when, as in dimethylallyl pyrophosphate, it is located at an allylic carbon. Isopentenyl pyrophos phate, on the other hand, does not have its leaving group attached to an allylic carbon and is far less reactive than dimethylallyl pyrophosphate toward nucleophilic reagents The principal site of reaction in isopentenyl pyrophosphate is the carbon-carbon double bond, which, like the double bonds of simple alkenes, is reactive toward electrophile 26.9 CARBON-CARBON BOND FORMATION IN TERPENE BIOSYNTHESIS The chemical properties of isopentenyl pyrophosphate and dimethylallyl pyrophosphate are complementary in a way that permits them to react with each other to form a car- bon-carbon bond that unites two isoprene units. Using the T electrons of its double bond, isopentenyl pyrophosphate acts as a nucleophile and displaces pyrophosphate from dimethylallyl pyrophosphate. -(OPP OPP Ten-carbon carbocation The tertiary carbocation formed in this step can react according to any of the various reaction pathways available to carbocations. One of these is loss of a proton to give a double bond OPP OPP The product of this reaction is gerany pyrophosphate. Hydrolysis of the pyrophosphate ester group gives geraniol, a naturally occurring monoterpene found in rose oil Back Forward Main MenuToc Study Guide ToC Student o MHHE Website26.9 Carbon–Carbon Bond Formation in Terpene Biosynthesis 1029 Isopentenyl pyrophosphate is the biological isoprene unit; it contains five carbon atoms connected in the same order as in isoprene. Isopentenyl pyrophosphate undergoes an enzyme-catalyzed reaction that converts it, in an equilibrium process, to 3-methyl-2-butenyl pyrophosphate (dimethylallyl pyrophosphate): Isopentenyl pyrophosphate and dimethylallyl pyrophosphate are structurally simi￾lar—both contain a double bond and a pyrophosphate ester unit—but the chemical reac￾tivity expressed by each is different. The principal site of reaction in dimethylallyl pyrophosphate is the carbon that bears the pyrophosphate group. Pyrophosphate is a rea￾sonably good leaving group in nucleophilic substitution reactions, especially when, as in dimethylallyl pyrophosphate, it is located at an allylic carbon. Isopentenyl pyrophos￾phate, on the other hand, does not have its leaving group attached to an allylic carbon and is far less reactive than dimethylallyl pyrophosphate toward nucleophilic reagents. The principal site of reaction in isopentenyl pyrophosphate is the carbon–carbon double bond, which, like the double bonds of simple alkenes, is reactive toward electrophiles. 26.9 CARBON–CARBON BOND FORMATION IN TERPENE BIOSYNTHESIS The chemical properties of isopentenyl pyrophosphate and dimethylallyl pyrophosphate are complementary in a way that permits them to react with each other to form a car￾bon–carbon bond that unites two isoprene units. Using the  electrons of its double bond, isopentenyl pyrophosphate acts as a nucleophile and displaces pyrophosphate from dimethylallyl pyrophosphate. The tertiary carbocation formed in this step can react according to any of the various reaction pathways available to carbocations. One of these is loss of a proton to give a double bond. The product of this reaction is geranyl pyrophosphate. Hydrolysis of the pyrophosphate ester group gives geraniol, a naturally occurring monoterpene found in rose oil. H H H OPP Geranyl pyrophosphate OPP ( OPP) OPP Dimethylallyl pyrophosphate OPP Isopentenyl pyrophosphate OPP Ten-carbon carbocation OPP Isopentenyl pyrophosphate OPP Carbocation intermediate OPP Dimethylallyl pyrophosphate H H H H Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website