26.9 Carbon-Carbon Bond Formation in Terpene Biosynthesis example, the formation of cyclic monoterpenes. Neryl pyrophosphate, formed by an enzyme-catalyzed isomerization of the e double bond in geranyl pyrophosphate, has the proper geometry to form a six-membered ring via intramolecular attack of the double bond on the allylic pyrophosphate unit. 2 COPP Geranyl pyrophosphate Neryl pyrophosphate gives a-terpineol, also a known natural product Limonene mono The same tertiary carbocation serves as the precursor to numerous bicyclic terpenes. A carbocation having a bicyclic skeleton is formed by intramolecular attack of the T electrons of the double bond on the positively charged carbon. This bicyclic carbocation then undergoes many reactions typical of carbocation inter- mediates to provide a variety of bicyclic monoterpenes, as outlined in Figure 26.7. PROBLEM 26.10 The structure of the bicyclic monoterpene borneol is shown in Figure 26.7. Isoborneol, a stereoisomer of borneol, can be prepared in the labo- ratory by a two-step sequence. In the first step, borneol is oxidized to camphor by treatment with chromic acid. In the second step camphor is reduced with sodium borohydride to a mixture of 85% isoborneol and 15% borneol On the basis of these transformations deduce structural formulas for isoborneol and cam- Analogous processes involving cyclizations and rearrangements of carbocations derived from farnesyl pyrophosphate produce a rich variety of structural types in the sesquiterpene series. We will have more to say about the chemistry of higher terpenes Back Forward Main MenuToc Study Guide ToC Student o MHHE Website26.9 Carbon–Carbon Bond Formation in Terpene Biosynthesis 1031 example, the formation of cyclic monoterpenes. Neryl pyrophosphate, formed by an enzyme-catalyzed isomerization of the E double bond in geranyl pyrophosphate, has the proper geometry to form a six-membered ring via intramolecular attack of the double bond on the allylic pyrophosphate unit. Loss of a proton from the tertiary carbocation formed in this step gives limonene, an abundant natural product found in many citrus fruits. Capture of the carbocation by water gives -terpineol, also a known natural product. The same tertiary carbocation serves as the precursor to numerous bicyclic monoterpenes. A carbocation having a bicyclic skeleton is formed by intramolecular attack of the  electrons of the double bond on the positively charged carbon. This bicyclic carbocation then undergoes many reactions typical of carbocation inter￾mediates to provide a variety of bicyclic monoterpenes, as outlined in Figure 26.7. PROBLEM 26.10 The structure of the bicyclic monoterpene borneol is shown in Figure 26.7. Isoborneol, a stereoisomer of borneol, can be prepared in the labo￾ratory by a two-step sequence. In the first step, borneol is oxidized to camphor by treatment with chromic acid. In the second step, camphor is reduced with sodium borohydride to a mixture of 85% isoborneol and 15% borneol. On the basis of these transformations, deduce structural formulas for isoborneol and cam￾phor. Analogous processes involving cyclizations and rearrangements of carbocations derived from farnesyl pyrophosphate produce a rich variety of structural types in the sesquiterpene series. We will have more to say about the chemistry of higher terpenes, Bicyclic carbocation HO Limonene -Terpineol H H2O OPP Geranyl pyrophosphate OPP Neryl pyrophosphate Tertiary carbocation OPP Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website