748
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Journal of Natural Prodwts
Vol. 58, No. 5,pp. 748-750, May 1995
AN INSECT ANTIFEEDANT LIMONOID FROM
TURRAEA NILOTICA
MICHAEL
D. BENTLEY,*
GEOFFREY
0 .ADUL,
Department of Chemistty, University of Maine, Orono, Maine 04469
A. RANDALL
ALFORD,
Department of Applied Ecology and Environmental Scimes, Uniwrsity of Maine, Orono, Maine 04469
FLI-YUNGHUANG,
Department of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332
&SUE
GELBAUM,
Research Centerfw Biotechnology, Georgia Institute of Technology, Atlanta, Gewgia 30332
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and AHMEDHASANMI
ICIPE, P.O. Box 30772, Nairobi, Kenya
Assmm.-Nilotin 111,a new limonoid, has been isolated from the rootbark of Tuwaea
nilotica and its structure established by spectroscopic methods. It displayed significant activity
as an antifeedant against larvae of the Colorado potato beetle, Lptinotarsa deremliwta.
The genus Tuwaea (Meliaceae)is comprised of a group of 60-70 species of
shrubs and small trees occurring in the
Indian Ocean region. Several speciesstudied have been shown to produce limonoids
of the priurianin and havanensin classes
(1-3). Tuwaea nilotica Kotschy et Peyr. is
found in East'Africa and, in traditional
medicine, adecoction of the roots is taken
for upset stomachs (4).From an earlier
study of the stem components of this
plant, protolimonoids, but not limonoids,
were reported (5). In continuing our investigations on the limonoid chemistry
of this genus, we have examined a MeOH
extract of the root bark of T. nilotica and
here report the isolation of a new limonoid
active as an insect antifeedant.
Chromatography of the MeOH extract of T. nilotica root bark was performed on Si gel using a hexane/EtOAc
gradient to obtain a fraction for which the
presence of a limonoid was confirmed by
'H-nmr spectroscopy. This fraction was
further purified by Si gel cc (toluene/
methyl ethyl ketone eluent) to obtain a
new limonoid, nilotin {l], C4&&.
The 'H-nmr spectrum of 1 (see Table 1)
indicated the presence of four tertiary
methyls, acarbomethoxy, four acetates, a
I
tiglate, a P-substituted furan, and an
epoxide, and was consistent with the
presence of a limonoid of the havanensin
class. Substitution patterns and stereochemistry were determined with the aid
of 'H COSY and 'H NOESY nmr experiments. Important proton connectivities
observed in the COSY spectrum were H1 and H-3 with H-2; H-5 and H-7 with
H-6; H-9 with H-11; and H-11 with H12. The small coupling constants of H- 1
and H-3 with H-2 were consistent with
the commonly observed equatorial stereochemistry of the 1- and 3-protons. Fur-
May 19951
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Bentley et al. : Bioactive Limonoid from Tuwaea
TABLE
1. 'H- and I3C-NmrData for 1.
'C 6 (ppm)
Position
1 ........
2 ........
3 ........
4 ........
5 ........
6 ........
7 ........
8 ........
9 ........
10 . . . . . . .
11 . . . . . . .
12 . . . . . . .
13 . . . . . . .
14 . . . . . . .
15 . . . . . . .
16 . . . . . . .
17 . . . . . . .
18 . . . . . . .
19 . . . . . . .
20 . . . . . . .
21 . . . . . . .
22 . . . . . . .
23 . . . . . . .
29 . . . . . . .
30 . . . . . . .
2' . . . . . . . .
3' . . . . . . . .
4' . . . . . . . .
5' . . . . . . . .
Me(Ac).
...
COfiMe. . . .
c = o . .. . .
4.65 t (3)
2.20 m
4.97 t (3)
3.13 dd (12, 2)
1.70 m
4.70 t (2.5)
3.38 d (3.5)
5.16 t (3.5)
4.85 d (3.5)
3.61 s
2.34 m
2.80 m
1.06 s
1.25 s
7.10 m
6.40 m
7.27 m
1.22 s
1.34 s
6.83 qq ( 7 2 )
1.7-1.76 m
1.7-1.76 m
1.94 s
2.05 s
2.10 s
2.14 s
3.49 s
-
73.9
24.8'
72.4
40.1
33.4
24.2'
74.0
49.6b
40.3
40.1
74.4
79.6
48.7b
73.7
63.2
32.5
40.3
17.9
16.5
128.0'
140.6
112.3
142.3
749
the usual a-stereochemistry of the furan.
Correlation of H-17 and H-12 further
supported these stereochemical assignments. Correlation of H-7 with the 30methyl corroborated the assignment of
equatorial stereochemistry for H-7 indicated by its coupling constant with H-6.
13
C-Nmr assignments were made with
the aid ofAPT and HETCOR techniques
and are presented in Table 1. The structure of nilotin 111 has the same skeleton
and substitution pattern as several
limonoids previously isolated from
Tuwaeafizbud (3),but differs in groups
attached at C-3, C-11, and C-12.
Nilotin 111 showed significant
antifeedant activity in no-choice feeding
assays using 4th instar Colorado potato
beetles. The dose required for 50% feeding reduction (ED,,) was 7kg/ml, and
was thus comparable to that determined
earlier for the citrus limonoid, limonin
(ED,,=8 p.g/ml) (6).This is structurally
consistent with our previous findings in
structure-activity studies on citrus
limonoids as Colorado potato beetle
antifeedants that demonstrated that the
furan and epoxide, both present in 1,are
primarily responsible for the observed
activity (6).
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16.5
23.6
128.5'
137.8
11.6
14.3
21.4
21.9
20.9
20.9
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EXPERIMENTAL
174.0
170.7
169.7
169.5
169.2
165.7
a<
Values with the same superscripts are interchangeable.
ther correlations were H- 11 with H- 12,
H-17 with H-16, H-5' with H-3', and
H-22 with H-21 and H-23. Using the
NOESY technique, correlations between
H-9 and H-11 confirmed the p stereochemistry of the ester at C-11. Correlation of the 18-methyl protonsand a tiglate
methyl placed the tiglate at the 12a
position, while correlation of the 18methyl with the furan H-23 confirmed
GENERALEXPERIMENTALPROCEDURES.-M~S
were determined on a Fisher hot-stage apparatus
and are uncorrected. Ir spectra were recorded on a
Bio-Rad FTS-60 spectrometer. 'H- and "C-nmr
spectra were obtained on Varian XL-200 and XL400 instruments, respectively, using CDCI, as
solvent and TMS as internal standard.
PLANT mnRlhL.-The
roots of T. ndorica
were collected in May 1989, from Shimba Hills,
Kwale District, near Mombasa, Kenya. The plant
was authenticated by S.G. Mathenge of the Herbarium of the Department of Botany, University
of Nairobi, and a reference specimen is on file in
that department.
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EXTRACTIONANDISOLATION.-The root bark
of T. nilotzcu was airdried for a week and ground
into a powder. T h e powder (103 g) was soaked in
MeOH (1 liter) for 4 weeks at room temperature,
filtered, and the extract concentrated in vawo.
Water was added and the aqueous MeOH extract
partitioned with petroleum ether. The aqueous
750
Journal of Natural Products
MeOH phase was evaporated to yield an oil (20
g), of which 18 g were chromatographed on
600 g of Si gel (Merck Kieselgel, 230-400
mesh) using a hexaneiEtOAc gradient. A
limonoid-containing fraction was collected. Cc
of this fraction (680 rng) on Si gel eluted with
toluene-methyl ethyl ketone (8:2) led to the
isolation of 1 (180 rng) as a white solid: rnp
138-140'; hrfabrns mlz CMfLi]' 763.3530,
C4,H5,0,,Li, calcd 763.3493; ir (KBr) v rnax
2951, 1737 (br), 1365, 1232, 1126, 1032
-1
1
crn ; H- and "C-nmr spectral data, see
Table 1.
{Vol. 58, No. 5
%FR=61 (SEM=15) (30 insects); and for 3 pgi
disk, %FR=20 (SEM=11) (8 insects).
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ANTIFEEDANT
BIOASSAY.-compound 1 was
subjected to no-choice antifeedant assays at three
dose levels (30, 10, and 3 pgiml) on potato leaf
disks using 4th instar Colorado potato beetles
according to our previously published procedure
(6). Percent feeding reduction (%FR) was calculated by the equation %FR=[l-treatment consumption/control consurnption)X 100 and standard errors (SEM) were calculated (6). For an
applied concentration of 30 pgiml leaf disk,
%FRZ78(SEM=19)(38 insects);for 10 pgidisk,
ACKNOWLEDGMENTS
We are grateful to Mr. Edward Nyandat for
the plant collection.
LITERATURE CITED
1.
2.
3.
4.
5.
6.
M.D. Bentley, F. Gaul, M.S. Rajab, and A.
Hassanali,]. "a.Prod., 55, 84 (1992).
M.S. Rajab, M.D. Bentley, A. Hassanali, and
A. Chapya, PhpchemistryJ 2 7 , 2353 (1988).
J . A . Akinniyi, J . D . Connolly, D . A .
Mulholland, D.S. Rycroft, and D.A.H. Taylor, Phytochemistry, 2 5 , 2187 (1986).
J.O. Kokwaro, "Medicinal Plants of East
Africa," East Africa Literature Bureau,
Nairobi, Kenya, 1976, p. 168.
D.A. Mulholland and D.A.H. Taylor, Phytochemistry, 2 7 , 1220 (1988).
M.D. Bentley,M.S. Rajab,A.R. Alford, M.J.
Mendel, and A. Hassanali, Entomol. Exp. Appl,,
49, 189 (1988).
Received 26 August 1994