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26.7 Terpenes: The Isoprene Rule

26.7 Terpenes: The Isoprene Rule. CH 3. H 2 C. CH. CH 2. C. Terpenes. Terpenes are natural products that are structurally related to isoprene. or. Isoprene (2-methyl-1,3-butadiene). CH 3. CH 2. CH 2. CHCH 2 CH 2 CCH. CH 3 C. Terpenes.

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26.7 Terpenes: The Isoprene Rule

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  1. 26.7Terpenes: The Isoprene Rule

  2. CH3 H2C CH CH2 C Terpenes • Terpenes are natural products that are structurally related to isoprene. or Isoprene(2-methyl-1,3-butadiene)

  3. CH3 CH2 CH2 CHCH2CH2CCH CH3C Terpenes • Myrcene (isolated from oil of bayberry) is a typical terpene. or

  4. The Isoprene Unit • An isoprene unit is the carbon skeleton of isoprene (ignoring the double bonds) Myrcene contains two isoprene units.

  5. The Isoprene Unit • The isoprene units of myrcene are joined "head-to-tail." head tail tail head

  6. Table 26.2 Classification of Terpenes • Class Number of carbon atoms • Monoterpene 10 • Sesquiterpene 15 • Diterpene 20 • Sesterpene 25 • Triterpene 30 • Tetraterpene 40

  7. OH O H Figure 26.6 Representative Monoterpenes a-Phellandrene(eucalyptus) Menthol(peppermint) Citral(lemon grass)

  8. OH O H Figure 26.6 Representative Monoterpenes a-Phellandrene(eucalyptus) Menthol(peppermint) Citral(lemon grass)

  9. Figure 26.6 Representative Monoterpenes a-Phellandrene(eucalyptus) Menthol(peppermint) Citral(lemon grass)

  10. Figure 26.6 Representative Sesquiterpenes H a-Selinene(celery)

  11. Figure 26.6 Representative Sesquiterpenes H a-Selinene(celery)

  12. Figure 26.6 Representative Sesquiterpenes a-Selinene(celery)

  13. OH Figure 26.6 Representative Diterpenes Vitamin A

  14. OH Figure 26.6 Representative Diterpenes Vitamin A

  15. Figure 26.6 Representative Diterpenes Vitamin A

  16. Figure 26.6 Representative Triterpene tail-to-tail linkage of isoprene units Squalene(shark liver oil)

  17. 26.8Isopentenyl Pyrophosphate:The Biological Isoprene Unit

  18. The Biological Isoprene Unit • The isoprene units in terpenes do not come from isoprene. • They come from isopentenyl pyrophosphate. • Isopentenyl pyrophosphate (5 carbons) comes from acetate (2 carbons) via mevalonate (6 carbons).

  19. O CH3 O CH3COH HOCCH2CCH2CH2OH OH CH3 O O H2C CCH2CH2OPOPOH The Biological Isoprene Unit 3 Mevalonic acid Isopentenyl pyrophosphate

  20. CH3 O O H2C CCH2CH2OPOPOH OPP Isopentenyl Pyrophosphate or Isopentenyl pyrophosphate

  21. OPP OPP Isopentenyl and Dimethylallyl Pyrophosphate Isopentenyl pyrophosphate is interconvertible with2-methylallyl pyrophosphate. • Dimethylallyl pyrophosphate has a leaving group (pyrophosphate) at an allylic carbon; it is reactive toward nucleophilic substitution at this position. Isopentenyl pyrophosphate Dimethylallyl pyrophosphate

  22. 26.9Carbon-Carbon Bond Formation in Terpene Biosynthesis

  23. OPP + OPP Carbon-Carbon Bond Formation • The key process involves the double bond of isopentenyl pyrophosphate acting as a nucleophile toward the allylic carbon of dimethylallyl pyrophosphate.

  24. OPP + OPP – OPP + OPP Carbon-Carbon Bond Formation

  25. + OPP After C—C Bond Formation... • The carbocation can lose a proton to give a double bond.

  26. + – H + OPP After C—C Bond Formation... • The carbocation can lose a proton to give a double bond. OPP

  27. After C—C Bond Formation... • This compound is called geranyl pyrophosphate. It can undergo hydrolysis of its pyrophosphate to give geraniol (rose oil). OPP

  28. OH After C—C Bond Formation... OPP H2O Geraniol

  29. OPP + OPP Geranyl pyrophosphate + OPP From 10 Carbons to 15

  30. OPP + – H + OPP From 10 Carbons to 15

  31. OPP From 10 Carbons to 15 • This compound is called farnesyl pyrophosphate. • Hydrolysis of the pyrophosphate ester gives the alcohol farnesol (Figure 26.6).

  32. OPP OPP From 15 Carbons to 20 • Farnesyl pyrophosphate is extended by another isoprene unit by reaction with isopentenyl pyrophosphate.

  33. + OPP OPP Cyclization • Rings form by intramolecular carbon-carbon bond formation. E double bond Z double bond

  34. + – H + Limonene OH H2O a-Terpineol

  35. + Bicyclic Terpenes + + + a-Pinene b-Pinene

  36. 26.10The Pathway from Acetate to Isopentenyl Pyrophosphate

  37. O CH3 O CH3COH HOCCH2CCH2CH2OH OH CH3 O O H2C CCH2CH2OPOPOH Recall 3 Mevalonic acid Isopentenyl pyrophosphate

  38. O O CH3CCH2CSCoA Biosynthesis of Mevalonic Acid • In a sequence analogous to the early steps of fatty acid biosynthesis, acetyl coenzyme A is converted to S-acetoacetyl coenzyme A. S-Acetoacetylcoenzyme A

  39. O O O CH3CCH2CSCoA CH3CSCoA Biosynthesis of Mevalonic Acid • In the next step, S-acetoacetyl coenzyme A reacts with acetyl coenzyme A. • Nucleophilic addition of acetyl coenzyme A (probably via its enol) to the ketone carbonyl of S-acetoacetyl coenzyme A occurs. +

  40. O O O CH3CCH2CSCoA CH3CSCoA HO O CH3CCH2CSCoA CH2COH O Biosynthesis of Mevalonic Acid +

  41. HO O CH3CCH2CSCoA CH2COH O Biosynthesis of Mevalonic Acid • Next, the acyl coenzyme A function is reduced. • The product of this reduction is mevalonic acid.

  42. HO CH3CCH2CH2OH CH2COH O HO O CH3CCH2CSCoA CH2COH O Mevalonicacid

  43. 2– HO OPO3 CH3CCH2CH2OH CH3CCH2CH2OPP CH2COH CH2COH O O Conversion of Mevalonic Acid to Isopentenyl Pyrophosphate • The two hydroxyl groups of mevalonic acid undergo phosphorylation.

  44. 3– OPO3 2– OPO3 CH3CCH2CH2OPP – •• CH2 O C •• •• O C O O Conversion of Mevalonic Acid to Isopentenyl Pyrophosphate • Phosphorylation is followed by a novel elimination involving loss of CO2 and PO43–. CH3CCH2CH2OPP CH2

  45. CH3CCH2CH2OPP CH2 Conversion of Mevalonic Acid to Isopentenyl Pyrophosphate • The product of this elimination is isopentenyl pyrophosphate.

  46. O CH3 O CH3COH HOCCH2CCH2CH2OH OH CH3 O O H2C CCH2CH2OPOPOH Biosynthetic pathway is based on experiments with 14C-labeled acetate Mevalonic acid Isopentenyl pyrophosphate

  47. CH3 O O O CH3COH H2C CCH2CH2OPOPOH • • O • • • • H Biosynthetic pathway is based on experiments with 14C-labeled acetate • Citronellal biosynthesized using 14C-labeled acetate as the carbon source had the labeled carbons in the positions indicated.

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