10.5.1 Hemiterpenes
Hemiterpenes are the simplest among the terpenoids. They consist of only one
isoprene unit. Isoprene itself is considered as hemiterpene, but oxygen-containing
isoprene compounds like angelic acid extracted from Angelica archangelica, isovaleric acid isolated from Vaccinium myrtillus and prenol obtained from citrous
fruit, tomato, cranberry are hemiterpenoids (Fig. 10.21) (Seigler 1995; Perveen
2018).
10.5.2 Monoterpenes
Monoterpenes are made up of two isoprene units and 10 carbon backbone structure
having molecular formula C 10 H 16 . They are important constituents of essential oils
or volatile oils of plant origin. They are found in certain plant families, such as
Lamiaceae, Rutaceae, Apiaceae, and Pinaceae. Few compounds, like geraniol, are
almost universal and are available in minute quantity in the volatile excretions of
C
H 3
COOH
CH 3
Angelic acid
C
H 3
COOH
CH 3
Isovaleric acid ( 3-Methylbutanoic acid}
C
H 3
CH 2 OH
CH 3
Prenol
Fig. 10.21 The structure of hemiterpenes derived from plant sources
Table 10.4 The formula and examples of different classes of terpenoids of plant sources
Number of isoprene
units
Class ( formula)
Examples of compounds/drugs
1
Hemiterpene (C 5 H 8 )
Isoprene, prenol, isovaleric acid,
tiglic acid
2
Monoterpenes
(C 10 H 16 )
Limonene, eucalyptol, pinene
3
Sesquiterpenes
(C 15 H 24 )
Abscisic acid ( ABA)
4
Diterpenes (C 20 H 32 )
Gibberellin
5
Sesterterpenes
(C 25 H 40 )
Dysidiolide
6
Triterpenes (C 30 H 48 )
Brassinosteroids, squalene, lanosterol
8
Tetraterpenes
(C 40 H 64 )
Carotenoids, lycopen
> 8
Polyterpenes
(C >40 H >64 )
Ubuquinones, rubber, cytokonines,
vitamine E
10 Secondary Metabolites from Plant Sources
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