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OH
O
O
O
OCH 3
O
H
H
OH
HO
O
H
O
O
O
H 3 CO
O
O
O
FIGURE 10.10
Structure of azadirachtin from the neem tree.
Neem oil is derived by cold-pressing the seeds from the neem tree, Azadiracta
indica Juss (Meliaceae family). When the oil is obtained by solvent extraction it gives
a lower quality oil and used in soap manufacturing. The oil is mainly comprised of
triglycerides and large amounts of triterpenoid compounds (limonoids), of which aza-
dirachtin (FigureĀ 10.10) is well studied. The limonoids appear to have insecticidal prop-
erties by affecting the insect's hormonal system and they repel the insects' larvae and
adult forms.
Limonoids form a class of molecules possessing complex structures. As an exam-
ple, new limonoids belonging to the tetranortriterpenoid class of chemical compounds
possessing potent anti-RSV (respiratory syncytial virus) activity were discovered
from Dysoxylum gaudichaudianum in the Meliaceae family (FigureĀ  10.11). These
four limonoids are an example of the discovery of antiviral activity from plants used
as traditional medicine in Papua New Guinea. The bark of Dysoxylum gaudichaudia-
num (Meliaceae) was collected from a village, based on instructions from the local
shaman (medicine man). The plant was analyzed in the research laboratories using
respiratory syncytial viral cytopathic effect (CPE) inhibition and plaque reduction
assays to guide bioactivity-directed fractionation and anti-RSV activity was found
in both CPE inhibition and plaque reduction assays. Isolation of active components
was achieved by extraction of the dried bark of D. gaudichaudianum using a 1:1
isopropanol/methylene chloride mixture, and four limonoids were eventually isolated
in pure form.
As an example of a structure determination, this was achieved based on a com-
bination of spectroscopic techniques and comparison to a known terpene skeleton,
namely trichilinin E. Limonoid A has a molecular formula of C 35 H 48 O 10 that can
be determined from high-resolution mass spectrometry (HRMS) to allow an accu-
rate molecular formula determination. The values were obtained from electrospray
HRMS: M+, m/z 628.3234, and the calculated value is 628.3248, in agreement with
the proposed molecular formula. In the 13 C NMR, the following 35 carbon resonance
signals were observed and assigned as follows: 8 methyl (CH3), 6 methylene (CH2),
11 methine (CH) carbons, and therefore 10 quaternary (C) carbons in the molecule
and consistent with the proposed structure.
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