Chemistry Reference
In-Depth Information
Figure 1.7
Bicyclic cyclopentane monoterpenoids.
Its enantiomer, (
)-fenchone (
G6
), has been isolated from the oil of fennel
(
Foeniculum vulgare
, Umbelliferae).
+
1.4.3 Sesquiterpenoids
The C
15
terpenoids known as sesquiterpenoids are the most chemically diverse
group of terpenoids known in nature. Like monoterpenoids, many sesquiter-
penoids contribute to the flavor and fragrances of a variety of plant products.
To date about 10,000 sesquiterpenoids are known (32), and in the plant kingdom
they commonly occur as hydrocarbons, alcohols, aldehydes, ketones, carboxylic
acids, lactones, and oxiranes. The acyclic sesquiterpene hydrocarbons
-
farnesenes (
H1
and
H2
, respectively) (Fig. 1.8) are constituents of the oils of
orange (
Citrus sinensis
, Rutaceae) and mandarin (
Citrus aurantium
and
C. reticu-
lata
, Rutaceae). The parent alcohol (
E
,
E
)-farnesol (
H3
) occurs in
Acacia farnen-
siana
(Mimosaceae), and its regiosiomer (
S
)-(
+
)-nerolidol (
H4
) is a constituent
of the oil of neroli obtained from orange flowers. Some farnesane derivatives
that contain furan rings also occur in nature, and these include dendrolasin (
H5
)
from sweet potato (
Ipomoea batatas
, Convolvulaceae) and longifolin (
H6
) from
the leaves of
Actinodaphe longifolia
(Lauraceae).
The vast chemical diversity of cyclic sesquiterpenoids compared with
monoterpenoids results from the number of possible cyclization modes that
is enhanced because of increased chain length and the presence of additional
double bonds in the acyclic precursor farnesyl diphosphate (FPP). As depicted
in Fig. 1.9, these cyclization modes lead to sesquiterpenoids with mono-,
bi-, and tricyclic structures. The cyclofarnesane, (
S
)-(
+
)-abscisic acid (
I1
)
α
-and
β
