Dr. Solomon Derese 31
SCH 511
The monoterpenes are best known ascomponents of the volatile essences offlowers and of the essential oils of herbs andspices, in which they make up as much as 5%of plant dry weight. Monoterpenes areisolated by either distillation or extractionand find considerable industrial use inflavors and perfumes.
Monoterpenes (C10)
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SCH 511Biosynthesis of Monoterpenes (C10)Combination of DMAPP and IPP via the enzymeprenyl transferase yields Geranyl PyroPhosphate(GPP).
OPP
OPPHHR S
OPP
Geranyl Pyrophosphate (GPP)opp
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SCH 511
This produces a monoterpene diphosphate, GeranylPP (GPP), in which the new double bond is E.
Geranyl PP (GPP)
E
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SCH 511
:OPP PPO PPO
:OPP
Z
Linalyl PP (LPP)
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SCH 511
Linalyl PP (LPP) and Neryl PP (NPP) areisomers of geranyl PP, and are formedfrom geranyl PP by ionization to theallylic cation, which can thus allow achange in attachment of thediphosphate group (to the tertiarycarbon in linalyl PP) or a change instereochemistry at the double bond (toZ in neryl PP).
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SCH 511
These three compounds, by relativelymodest changes, can give rise to a rangeof linear monoterpenes found ascomponents of volatile oils used inflavoring and perfumery.
The resulting compounds may behydrocarbons, alcohols, aldehydes, orperhaps esters, especially acetates.
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SCH 511
Acyclic monoterpenesOpen chain monoterpenes are derived from GPP,LPP and NPP by:
Reduction
Oxidation
Dehydration
Using NADPH
Using NADP+
Loss of water
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SCH 511
OPP
GPP
O
Geranial(Lemon oil)
H
O
Citronellal(Citronella oil)
H
OH
(Rose oil)Citronellol
OH
GeraniolGeranium oil
H2O1. H2O
1. H2O
1. H2O
2. NADP+3. NADP+
2.NADPH
2. NADPH
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SCH 511
OPP
GPP
PPO
LPP
-Myrcene (Hops)
HO
Linalool (Coriander oil)
H2O
- HOPP
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SCH 511
OPP
GPP
H2O
O
Neral (Lemon oil)
1. H2O
2. NADP+
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SCH 511
Cyclic monoterpenes
Which of these is ready for cyclization?Cyclization would not be expected to occur with theprecursor GPP, the E stereochemistry of the doublebond being unfavorable for ring formation. NPP or LPP,however, do have favorable stereochemistry, andeither or both of these would act as precursors formonocyclic terpenes.
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SCH 511
LPP
PPO
OPP
NPP
Teritary Carbocation
Menthyl or a-terpinylCarbocation
Menthyl or a-terpinyl Carbocation isthe precursor for many cyclic monoterpenes.
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SCH 511
- H
Limonnene
OH
a-Terpineol
H2O
OHO
Cineole
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SCH 511
HH
H HH
Terpinen-4-yl Cation
Phellandryl cation
Wagner-Meerwein Shift
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SCH 511
Phellandryl cation
a-Phellandrene -Phellandrene
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SCH 511
Terpinen-4-yl Cation- H - H
H2O
a-Terpinene-Terpinene
OH
Terpinen-4-ol
Thujyl cation Sabinene
- H
[O] [H]
Thujone
O
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SCH 511
Pinyl cation
a-Pinene
- H+
- H+
-Pinene
1,2-Alk
yl Shift
Fenchylcation
OH
H2O
FencholO
NADP+
Fenchone
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SCH 511
OH
H2O
Borneol
O NADP+
Camphor
Bornyl Cation
Isocamphyl cation
Camphene
- H+
1,2-Alkyl Shift
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SCH 511
a-terpinyl cation
OPP
GPP
OH
O
OPP
OH
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SCH 511
OPP
OPP
H H
PPOPPO
O OH
Chrysanthemic acid
H2O 2NADP+
