Synthesis and seed germination stimulating activity of some imino analogs of strigolactones

Article abstract of DOI:10.1271/bbb.70398

Strigolactones are germination stimulants for seeds of the root parasitic weeds, Striga and Orobanche spp. The imino analog of GR24 showed moderate germination stimulating activity against the seeds of S. hermonthica. The seed germination stimulating activity of some phenyliminoacetates and phenyliminoacetonitriles was also examined. The degree of activity of the phenyliminoacetate was less than that of the phenylacrylates. On the other hand, the degree of activity of the phenyliminoacetonitrile was comparable to that of the phenylacrylonitriles. Among the tested compounds, the 3- pyridyliminoacetonitrile showed higher activity against the seeds of O. crenata than GR24. These findings demonstrate that it is not always essential to have the Michael acceptor of the C-D ring junction moiety which has been proposed to react with nucleophilic species presented at the target site to enhance the activity.

Full text of DOI:10.1271/bbb.70398

Biosci. Biotechnol. Biochem., 71 (11), 2781–2786, 2007
Synthesis and Seed Germination Stimulating Activity of Some Imino Analogs
of Strigolactones
Yuki KONDO, Eriko TADOKORO, Mayuko MATSUURA, Kyoko IWASAKI,
y
Yukihiro SUGIMOTO, Hideyoshi MIYAKE, Hirosato TAKIKAWA, and Mitsuru SASAKI
Department of Agrobioscience, Graduate School of Agricultural Science, Kobe University,
1
-1 Rokkodai, Nada-ku, Kobe 658-8501, Japan
Received June 21, 2007; Accepted July 18, 2007; Online Publication, November 7, 2007
[doi:10.1271/bbb.70398]
Strigolactones are germination stimulants for seeds of
Zwanenburg,6) as shown in Fig. 2. This mechanism
involves the addition of a nucleophilic species presented
at the target site in Michael fashion to the stimulant,
with subsequent elimination of the D ring unit. This
leads to covalent bonding of the ABC portion of the
stimulant to the receptor, a chemical change that is
claimed to be responsible for initiating or triggering
germination.
the root parasitic weeds, Striga and Orobanche spp. The
imino analog of GR24 showed moderate germination
stimulating activity against the seeds of S. hermonthica.
The seed germination stimulating activity of some
phenyliminoacetates and phenyliminoacetonitriles was
also examined. The degree of activity of the phenyl-
iminoacetate was less than that of the phenylacrylates.
On the other hand, the degree of activity of the phen-
yliminoacetonitrile was comparable to that of the phen-
ylacrylonitriles. Among the tested compounds, the 3-
pyridyliminoacetonitrile showed higher activity against
the seeds of O. crenata than GR24. These findings de-
monstrate that it is not always essential to have the
Michael acceptor of the C–D ring junction moiety which
has been proposed to react with nucleophilic species
presented at the target site to enhance the activity.
Meanwhile, strobilurin A (9), which was isolated
from mushroom, has been reported to have strong
7
,8)
antifungal activity. It was expected that, as methoxy-
acrylate 9 has an ꢀ,ꢁ-unsaturated carbonyl group in the
molecule, it might work as a Michael acceptor at the
target site in consideration of the structural similarity to
strigolactones. However, as a result of the synthesis of
strobilurin A analogs, for instance, kresoxim-methyl
(10), which no longer has the enol ether group in its
molecule, showed strong antifungal activity with the
9
)
Key words: strigolactone; seed germination stimulant;
root parasitic weed
same mode of action as that of strobilurin A. So the
predicted reaction mechanism, i.e., the Michael acceptor
mechanism, is impossible in this case (Fig. 3). In the
context of the structural modification from strobilurin A
to kresoxim-methyl, it is interesting to examine if the
imino analogs of strigolactones will show seed germi-
nation stimulating activity or not. Our interest in
modifying the chemical structure to seek a higher active
germination stimulant therefore led to the design,
synthesis and bioassay of some imino analogs of
strigolactones. We describe here the results of a study
involving the discovery of phenyliminoacetonitriles as
new germination stimulants.
Parasitic weeds of the genera Striga and Orobanche
cause severe damage to several gramineous and legu-
1
)
minous crops in tropical and subtropical areas. The
seeds of the parasites germinate only when stimulated
by a chemical exuded from the roots of the host and
some non-host plants. These chemicals include strigol
(
(
1), sorgolactone (2), alectrol (3) and orobanchol (4)
Fig. 1). Their synthetic derivatives are also known to
2)
possess seed germination stimulation activity such as
3
)
GR24 (5), Nijmegen 1 (6) and its phenyl derivative (7).
These germination stimulants are collectively referred to
as strigolactones. 5-Deoxystrigol, one of strigolactones,
is also known as a hyphal branching factor in arbuscular
Materials and Methods
4
)
Apparatus. H- and 13C-NMR spectra were recorded
in CDCl3 with a JNM-AL300 spectrometer (Jeol), using
tetramethylsilane (TMS) as an internal standard, and
chemical shifts are shown in ꢂ (ppm). IR spectra were
obtained with an IR-408 spectrometer (Shimadzu), and
mass spectra were recorded with a QSTAR spectrometer
1
mycorrhizal fungi. Carba-GR24 (8), in which the vinyl
ether oxygen atom in GR24 has been replaced by a
methylene function, is completely inactive in the
5
)
stimulation of seeds of the parasitic weeds. Based on
these findings, a molecular mechanism for the stimula-
tion of germination has been proposed by Mangnus and
y
To whom correspondence should be addressed. Tel/Fax: +81-78-803-5919; E-mail: msasaki@kobe-u.ac.jp

2782
Y. KONDO et al.
O
O
O
O
O
OH
O
H
H
O
H
O
O
O
O
O
O
O
O
OH
Strigol (1)
Sorgolactone (2)
Alectrol (3)
O
O
O
O
O
O
O
N
H
O
O
O
O
O
O
O
O
O
OH
O
GR24 (5)
Nijmegen 1 (6)
Orobanchol (4)
O
O
O
O
O
O
O
O
O
(7)
Carba-GR24 (8)
Fig. 1. Structures of Strigolactones and Their Synthetic Analogs.
1
0)
O
O
Synthesis of the compounds. GR24 (5), Nijmegen 1
(6)11) and its phenyl derivative (7) were prepared by
the reported procedure.
O
12)
C
A
B
H
O
D
OH
O
3-(4-Methyl-5-oxo-2,5-dihydrofuran-2-yl)oxyimino-
3a,4-dihydro-3H-indeno[1,2-b]furan-2(8bH)-one (11).
Strigol (1)
1
0)
3,3a,4,8b-Tetrahydroindeno[1,2-b]furan-2-one (1.74 g,
10 mmol) was dissolved in DMF (36 ml), and the
O
O
O
O
O
O
Nu
O
O
ꢁ
Nu
resulting solution was cooled to ꢀ10 C. Potassium
Nu
+
t-butoxide (1.34 g, 12 mmol) and then isoamyl nitrite
(2.34 g, 20 mmol) were added to the solution at the same
temperature. The mixture was gradually warmed to
room temperature and kept for 2.5 h. The reaction
mixture was diluted with dil. HCl (150 ml) and extracted
with ethyl acetate (100 ml). The ethyl acetate layer was
washed with water (2 times, each 100 ml), dried over
anhydrous MgSO4 and evaporated in vacuo. The residue
was chromatographed on silica gel (hexane/ethyl ace-
tate = 10/1, v/v) to give a yellow solid of the cor-
O
O
O
O
O
O
O
Fig. 2. Proposed Action Mechanism of Strigolactones.
O
O
Nu
O
O
+
δ
ꢁ
responding oxime (1.38 g, 68%), mp 193–197 C. IR
ꢀ1
O
O
O
Strobilurin A (9)
ꢃmax (nujol) cm : 1760 (s), 1650 (m), 1290 (s), 1245
(s), 1000 (s), 940 (s). NMR ꢂ (CDCl ): 3.24–3.61 (2H,
H 3
m), 4.03–4.10 (1H, m), 6.06 (1H, d, J ¼ 7:5 Hz), 7.25–
7.55 (4H, m), 12.30 (1H, s). NMR ꢂC (CDCl3): 35.1,
38.2, 85.2, 124.9, 126.2, 127.4, 130.2, 137.9, 142.8,
149.6, 165.8.
O
O
O
Nu
-
δ
O
N
O
N
O
Kresoxim-methyl (10)
To a cooled solution of the oxime (102 mg, 0.5 mmol)
in DMF (3 ml) were added potassium t-butoxide (67
10)
mg, 0.6 mmol) and 5-bromo-3-methylfuran-2(5H)-one
Fig. 3. Predicted Action Mechanism of Strobilurin A.
ꢁ
(106 mg, 0.6 mmol) at ꢀ78 C while stirring. The mix-
ture was warmed to room temperature and kept over-
night. The reaction mixture was diluted with water (30
ml) and extracted with ethyl acetate (30 ml). The ethyl
acetate layer was washed with water (2 times, each 30
ml), dried over anhydrous MgSO4 and evaporated in
vacuo. The residue was chromatographed on silica gel
(hexane/ethyl acetate = 10/1, v/v) to give a yellow
(
Thermoquest). Melting point (mp) data were measured
by MP-S3 apparatus (Yanaco) and are uncorrected.
Silica gel 60N (Kanto Chemical) was used for column
chromatography, and pre-coated silica gel plates (Kie-
selgel 60 F254, Merck) were used for analytical thin-
layer chromatography.

Seed Germination Stimulating Activity of Strigolactone Analogs
2783
oil of 11 (50 mg, 33%). IR ꢃmax (nujol) cm 1: 1770 (s,
ꢀ
To a cooled solution of this crude 3-hydroxy-2-
phenylacrylonitrile (1.34 g, 9.2 mmol) in DMF (20 ml),
potassium t-butoxide (1.14 g, 12 mmol) and then 5-
bromo-3-methylfuran-2(5H)-one (2.12 g, 12 mmol) were
C=O), 1650 (w), 1330 (m), 1030 (m), 945 (s). NMR ꢂH
(
CDCl ): 2.04 (3H, s, CH ), 3.18–3.62 (2H, m), 4.07–
3 3
4
6
(
1
1
3
.16 (1H, m), 6.06–6.10 (1H, m), 6.62–6.63 (1H, m),
.98–6.99 (1H, m), 7.24–7.60 (4H, m). NMR ꢂC
CDCl3): 10.8, 35.4, 39.2, 85.6, 103.2, 125.1, 126.6,
ꢁ
added at ꢀ20 C while stirring. The mixture was
warmed to room temperature and kept overnight. The
reaction mixture was diluted with water (50 ml) and
extracted with ethyl acetate (50 ml). The ethyl acetate
layer was washed with water (2 times, each 50 ml), dried
28.0, 130.8, 135.9, 137.4, 140.3, 142.2, 154.2, 163.9,
þ
70.9. HRESIMS m=z ½M þ Hꢂ : calcd. for C16H14NO5,
00.0866; found, 300.0861.
Methyl 2-[(4-methyl-5-oxo-2,5-dihydrofuran-2-yl)oxy-
over anhydrous MgSO and evaporated in vacuo. The
4
imino]-2-phenylacetate (12). To a cooled solution of
EZ)-2-hydroxyimino-2-phenylacetate (prepared from
residue was chromatographed on silica gel (hexane/
ethyl acetate = 10/1, v/v) to give a yellow oil of 13
(
ꢀ1
methyl phenylglutarate and hydroxylamine hydrochlor-
ide, 0.80 g, 4.5 mmol) in DMF (15 ml) were added
potassium t-butoxide (0.55 g, 5.6 mmol) and 5-bromo-3-
(0.63 g, 28%). IR ꢃmax (nujol) cm : 2040 (s, CN), 1760
(s, C=O). NMR ꢂH (CDCl3): 2.02–2.06 (3H, m), 6.21–
6.22 (1H, m), 6.97–6.99 (1H, m), 7.03–7.70 (6H, m).
NMR ꢂC (CDCl3): 10.6, 10.7, 100.0, 100.4, 117.9,
124.8, 125.4, 126.3, 127.4, 127.9, 128.6, 128.7, 128.9,
129.2, 129.4, 129.5, 129.9, 135.7, 136.2, 140.7, 141.2,
ꢁ
methylfuran-2(5H)-one (1.06 g, 6.0 mmol) at ꢀ60 C
while stirring. The mixture was warmed to room
temperature and kept overnight. The reaction mixture
was diluted with water (50 ml) and extracted with ethyl
acetate (50 ml). The ethyl acetate layer was washed with
water (2 times, each 50 ml), dried over anhydrous
MgSO4 and evaporated in vacuo. The residue was chro-
matographed on silica gel (hexane/ethyl acetate = 10/1,
v/v) to give a yellow oil of 12 (0.21 g, 17%) as an (EZ)
mixture with regard to the CN double bond. NMR ꢂH
þ
152.3, 152.4, 153.6, 170.0. HRESIMS m=z ½M þ Hꢂ :
calcd. for C14H12NO3, 242.0811; found, 242.0806.
2-[(4-Methyl-5-oxo-2,5-dihydrofuran-2-yl)oxyimino]-
2-phenyl-acetonitrile (14). To a stirred solution of phen-
ylacetonitrile (5.0 g, 40 mmol) in DMF (35 ml), potas-
sium t-butoxide (9.0 g, 80 mmol) and isoamyl nitrite
ꢁ
(9.4 g, 80 mmol) were added at 0 C. The resulting
(
6
CDCl ): 1.95 and 1.99 (3H, s), 3.87 and 3.93 (3H, s),
3
.47–6.54 (1H, m), 6.83–6.92 (1H, m), 7.39–7.59 (5H,
þ
mixture was kept at room temperature for 4 h. The
reaction mixture was diluted with dil. HCl (30 ml) and
extracted with ethyl acetate (30 ml). The ethyl acetate
layer was washed with water (2 times, each 30 ml), dried
over anhydrous MgSO4 and concentrated in vacuo to
give a solid (5.4 g, crude 2-hydroxyimino-2-phenylace-
tonitrile). NMR ꢂH (CDCl3): 7.42–7.53 (3H, m), 7.79–
7.83 (2H, m), 9.07 (1H, s).
m). HRESIMS m=z ½M þ Hꢂ : calcd. for C14H14NO5,
2
76.0872; found, 276.0860.
Part of this mixture was further purified by silica gel
column chromatography to give the pure E- and Z-
isomers.
E-Isomer (12E): Yellow oil. IR ꢃmax (nujol) cm 1:
775 (s, C=O), 1725 (s, C=O), 1660 (m), 1340 (s),
220 (s), 1080 (s). NMR ꢂ (CDCl ): 1.99 (3H, s, CH ),
ꢀ
1
1
3
7
1
1
To a cooled solution of this crude 2-hydroxyimino-2-
phenyl-acetonitrile (1.50 g, 10.3 mmol) in DMF (20 ml),
potassium t-butoxide (1.27 g, 10.2 mmol) and then 5-
bromo-3-methylfuran-2(5H)-one (2.24 g, 12.7 mmol)
H
3
3
.93 (3H, s, OCH3), 6.47 (1H, m), 6.92 (1H, m), 7.39–
.59 (5H, m). NMR ꢂC (CDCl3): 10.6, 52.6, 102.5,
26.7, 128.7, 128.8, 129.0, 135.3, 140.9, 153.8, 162.9,
71.3.
ꢁ
were added at ꢀ20 C while stirring. The mixture was
warmed to room temperature and kept overnight. The
reaction mixture was diluted with water (50 ml) and
ethyl acetate (50 ml). The ethyl acetate layer was
washed with water (2 times, each 50 ml), dried over
ꢁ
ꢀ1
Z-Isomer (12Z): Mp 98–102 C. IR ꢃmax (nujol) cm
:
770 (s, C=O), 1725 (s, C=O), 1300 (s), 1200 (m),
1
1
3
7
1
1
160 (m), 950 (s). NMR ꢂ (CDCl ): 1.95 (3H, s, CH ),
H
3
3
.89 (3H, s, OCH ), 6.54 (1H, m), 6.84 (1H, m), 7.41–
3
anhydrous MgSO and evaporated in vacuo. The residue
4
was chromatographed on a silica gel (hexane/ethyl
acetate = 10/1, v/v) to give a white solid of 14 (1.16 g,
.44 (5H, m). NMR ꢂC (CDCl3): 10.6, 53.2, 103.0,
28.1, 128.4, 129.0, 130.2, 135.5, 140.5, 152.6, 163.3,
71.1.
ꢁ
ꢀ1
47%), mp 158–161 C. IR ꢃmax (nujol) cm : 1760 (s,
C=O), 1340 (s), 1205 (m), 950 (s). NMR ꢂH (CDCl3):
2.04–2.06 (3H, m), 6.58–6.59 (1H, m), 6.99–7.00 (1H,
m), 7.47–7.57 (3H, m), 7.81–7.84 (2H, m). NMR ꢂC
3
ylacrylo-nitrile (13). To a stirred solution of phenyl-
acetonitrile (5.0 g, 40 mmol) in methyl formate (30 ml)
and DMF (10 ml), sodium hydride (60% in oil; 2.1 g,
-[(4-Methyl-5-oxo-2,5-dihydrofuran-2-yl)oxy]-2-phen-
(CDCl ): 10.8, 102.7, 108.7, 126.8, 128.2, 129.2, 132.3,
3
ꢁ
þ
5
0 mmol) was portionwise added at 0 C. The resulting
135.6, 136.1, 140.1, 170.8. HRESIMS m=z ½M þ Hꢂ :
mixture was kept at room temperature overnight. The
mixture was diluted with dil. HCl (30 ml) and extracted
with dichloromethane (30 ml). The dichloromethane
layer was dried over anhydrous MgSO4 and concen-
trated in vacuo to give a solid (2.7 g, crude 3-hydroxy-2-
phenylacrylonitrile). NMR ꢂ (CDCl ): 7.20–7.78 (5H,
calcd. for C13H11N2O3, 243.0764; found, 243.0751.
N-[(4-Methyl-5-oxo-2,5-dihydrofuran-2-yl)oxy]nico-
tinimidoylcyanide (15). In a similar manner, 15 was
prepared from 2-hydroxyimino-2-(3-pyridyl)acetonitrile
ꢁ
ꢀ1
(0.68 g, 47%), mp 72–76 C. IR ꢃmax (nujol) cm : 1760
(s, C=O), 1660 (m), 1580 (m), 1340 (s), 1310 (s), 1200
H
3
m), 7.66 (1H, s), 11.00 (1H, s).
(m), 1090 (s). NMR ꢂH (CDCl ): 1.89–1.92 (3H, m),
3

2784
Y. KONDO et al.
O
O
O
O
O
N
O
O
N
O
O
N
O
O
O
O
O
O
(
12Z)
(
11)
(12E)
CN
CN
N
X
N
O
O
O
O
O
O
(
(
13, X = CH)
14, X = N)
(15)
Fig. 4. Chemical Structure of the Synthesized Compounds.
6
.47–6.53 (1H, m), 6.91–6.96 (1H, m), 7.24–7.38 (1H,
m), 7.97–8.10 (1H, m), 8.60–8.64 (1H, m), 8.88–9.03
1H, m). NMR ꢂC (CDCl3): 10.4, 31.0, 36.1, 102.6,
room temperature. A disk with conditioned seeds was
placed on top of each treated disk and moistened with
40 ml of distilled water. The treated seeds were incubated
(
ꢁ
1
1
1
02.8, 107.7, 112.7, 123.2, 123.4, 123.6, 124.2, 132.8,
33.6, 135.3, 135.8, 135.9, 136.7, 139.8, 139.9, 147.4,
50.0, 152.3, 152.5, 162.2, 170.3, 170.4. HRESIMS m=z
at 30 C and microscopically evaluated for their germi-
nation (radicle protrusion) 24 h later. O. crenata and
ꢁ
O. minor seeds were conditioned at 23 C for 6 days,
treated with each test solution, incubated for 5 days at
þ
½
M þ Hꢂ : calcd. for C12H10N3O3, 244.0716; found,
ꢁ
2
44.0707.
23 C, and then examined for germination.
Bioassay of the seed germination stimulating activity.
Results
Striga hermonthica seeds were collected from mature
plants parasitizing sorghum at the Gezira Research
Station in Sudan and provided by Prof. A. G. T. Babiker
of Agricultural Research Corporation in Sudan. S. ges-
nerioides seeds were collected from mature plants
parasitizing cowpea in Kano, Nigeria, and were pro-
vided by Prof. A. M. Emechebe of International Institute
of Tropical Agriculture. Orobanche crenata seeds were
collected from mature plants parasitizing vetch in Kfar
Massarick, Israel, and provided by Dr. R. Vasey of
Sheffield University, UK. O. minor seeds were collected
from mature plants parasitizing red clover in the
Watarase basin of Japan.
Synthesis of the imino analogs
The structures of the synthesized compounds are
illustrated in Fig. 4. The tricyclic lactone, 3,3a,4,8b-
tetrahydroindeno[1,2-b]furan-2-one, was converted into
the corresponding oxime by nitrosation with isoamyl
nitrite in a 68% yield. Coupling of this oxime with 5-
bromo-3-methylfuran-2(5H)-one gave GR24 imino-ana-
log 11 in a 33% yield, probably as an (EZ) mixture with
regards to the CN double bond and used for the
subsequent bioassay without further purification. Com-
pounds 12–15 were smoothly obtained as a mixture of
their geometrical isomers. The (EZ) isomers of 12 were
separable by silica gel column chromatography. (EZ)-2-
Hydroxyimino-2-phenylacetate prepared from methyl
phenylglutarate and hydroxylamine hydrochloride con-
tained geometrical isomers with regard to the CN double
bond in a 3:1 ratio, this being estimated from each peak
The seeds of these parasitic weeds were surface
sterilized by immersion in 0.5% (w/v) NaOCl contain-
ing a few drops of Tween 20, and sonicated for 3 min in
an ultrasonic cleaner, after having been rinsed 3 times
ꢁ
with distilled water and surface-dried at 27 C under a
laminar hood. S. hermonthica and S. gesnerioides seeds
1
area of two OH groups in the H-NMR spectrum. The
ꢁ
were pretreated (conditioned) for 10 to 12 days at 30 C
signal of the major isomer appeared at ꢂH 9.27 (1H, s,
OH), while that of the minor isomer was at ꢂH 8.52 (1H,
s, OH). The lower chemical shift for the OH group of the
major isomer is thought to have been due to intra-
molecular hydrogen bonding between the OH and CO
groups in the molecule. Based on this observation, the
geometry with regard to the CN double bond of the
major isomer was assigned to be of (Z) configuration.
The chromatographically purified (Z)-2-hydroxyimino-
2-phenylacetate was reacted with 5-bromo-3-methylfur-
on 8-mm glass fiber filter paper disks (ca. 50 seeds each)
placed on distilled water-saturated filter paper. For each
bioassay, distilled water and an aq. solution of GR24
were included as negative and positive controls, respec-
tively. A dilution series were prepared for each sample.
Each aqueous solution was assayed directly by applying
2
0-ml aliquots of the respective test solution to condi-
tioned seeds on 8-mm disks. For those solutions con-
taining an organic solvent, aliquots (20 ml each) of the
test solution were applied to 8-mm disks of glass fiber
filter paper which were then allowed to dry for 1 h at
1
an-2(5H)-one to give a product whose H-NMR pattern
was completely consistent with that of 12Z. Thus, the

Seed Germination Stimulating Activity of Strigolactone Analogs
2785
Table 1. Seed Germination Stimulating Activity of Some Imino Analogs of Strigolactones
Germination (%)
No.
S. hermonthica
S. gesnerioides
O. minor
O. crenata
0.1 mM
10 mM
0.1 mM
10 mM
0.1 mM
10 mM
0.1 mM
10 mM
1
1
0.0
0.0
0.0
28:3 ꢃ 7:2
4:7 ꢃ 2:5
0.0
—
0.0
—
0.0
—
—
0.0
—
0.0
—
1:7 ꢃ 2:3
—
—
1
2E
2Z
1
2:6 ꢃ 3:4
—
0.0
—
—
—
1
3
4
5
8:5 ꢃ 5:8
4:6 ꢃ 2:2
1:4 ꢃ 1:3
68:2 ꢃ 5:2
0.0
65:2 ꢃ 11:0
46:9 ꢃ 6:4
63:7 ꢃ 5:7
66:9 ꢃ 5:6
37:4 ꢃ 3:3
24:2 ꢃ 5:0
0:2 ꢃ 0:5
0.0
0.0
27:5 ꢃ 13:5
1:8 ꢃ 2:6
0:9 ꢃ 1:9
24:6 ꢃ 10:4
—
70:8 ꢃ 10:7
72:2 ꢃ 14:1
35:2 ꢃ 11:2
57:8 ꢃ 6:9
—
6:0 ꢃ 3:2
0:4 ꢃ 0:9
43:3 ꢃ 8:5
21:9 ꢃ 6:8
—
46:3 ꢃ 14:9
28:3 ꢃ 10:6
51:6 ꢃ 19:1
32:2 ꢃ 18:6
—
1
0.0
0.0
1
5
0:9 ꢃ 1:3
—
—
0:5 ꢃ 0:8
—
—
6
7
0.0
—
—
—
—
Each data value is the mean germination percentage ꢃ standard error from five replicate tests. Compounds 12E, 12Z, 6 and 7 were tested only on S. hermonthica.
geometry with regard to the CN double bond in 12 was
tentatively assigned without considering (EZ) isomer-
ization during the reaction of the (Z)-hydroxyiminophe-
nylacetate with 5-bromo-3-methylfuran-2(5H)-one. On
the other hand, the isomers of 13, 14 and 15 were not
separable by column chromatography.
designed phenyliminoacetate 12 as the imino analog of
7. We also designed phenylacrylonitrile 13 and phenyl-
iminoacetonitrile 14 in order to examine if the seed
germination stimulating activity was influenced or not
by electronic factors concerning the C ring moiety in the
molecule of strigolactones. Interestingly, the degree of
activity of phenyliminoacetonitrile 14 was comparable
to that of phenylacrylonitrile 13. These findings dem-
onstrate that it is not always essential to have the
Michael acceptor moiety of the C–D ring junction in the
molecule of strigolactones to enhance the seed germi-
nation stimulating activity. It thus appears that a critical
structure fitting to the target site is one of the requisites
for exhibiting seed germination stimulating activity.
Moreover, 3-pyridyl derivative 15, in which a nitrogen
atom was introduced into the benzene ring in 14,
resulted in an increase in the degree of seed germination
stimulating activity toward O. crenata. This also indi-
cates the possibility of broadening the seed germination
stimulating activity spectrum by chemical modification.
A further study on designing new germination stimu-
lants by this type of modification is in progress.
Structure–seed germination stimulating activity rela-
tionships
The imino analog of GR24 (11) showed moderate
germination stimulating activity against the seeds of
S. hermonthica in comparison with that of GR24 (5), as
shown in Table 1. This finding encouraged us to design
further imino analogs of strigolactones. We decided to
make phenyliminoacetates and phenyliminoacetoni-
triles, as the corresponding phenylacrylates have been
reported to possess moderate seed germination stimulat-
ing activity.1 A bioassay of the synthesized compounds
was conducted on the germination stimulating activity
of the seeds of S. hermonthica, S. gesnerioides, O. mi-
nor, and O. crenata. The degree of activity of both
geometrical isomers in phenyliminoacetate 12 was less
than that of corresponding phenylacrylate 7. On the
other hand, phenylacrylonitrile 13 and phenyliminoace-
tonitrile 14 showed somewhat stronger activity than that
of phenylacrylate 7. The activity of compounds 13 and
2)
References
1
)
Hamphrey, A. J., Galster, A. M., and Beale, M. H.,
Strigolactones in chemical ecology: waste products or
vital allelochemicals? Nat. Prod. Rep., 23, 592–614
1
4 was stronger than that of Nijmegen 1 (6), although 6
was only tested on S. hermonthica. Among the tested
compounds, 3-pyridyliminoacetonitrile 15 had higher
activity against O. crenata than that of GR24 (5). No
compound showed seed germination stimulating activity
toward S. gesnerioides.
(
2006).
2) Butler, L. G., Chemical communication between the
parasitic weed Striga and its crop host. A new dimension
in allelochemistry. In ‘‘Allelopathy: Organisms, Proc-
esses, and Applications,’’ eds. Inderjit Dakshini, K. M.
M., and Einhellig, F. A., American Chemical Society,
Washington DC, pp. 158–168 (1995).
Discussion
3
)
Thuring, J. W. J. F., Bitter, H. H., De Kok, M. M.,
Nefkens, G. H. L., Van Riel, A. M. D. A., and
Zwanenburg, B., N-Phthaloylglycine-derived strigol an-
alogues. Influence of the D-ring on seed germination
activity of the parasitic weeds Striga hermonthica and
Orobanche crenata. J. Agric. Food Chem., 45, 2284–
2290 (1997).
We firstly designed imino analog 11 of GR24,
expecting that 11 would show seed germination stim-
ulating activity, even if the enol ether group was
converted into the imino ether group, just like kresox-
im-methyl (10), as already described. As a result, imino
analog 11 showed moderate germination stimulating
activity toward the seeds of S. hermonthica. We then
4) Akiyama, K., Matsuzaki, K., and Hayashi, H., Plant

2786
Y. KONDO et al.
sesquiterpenes induce hyphal branching in arbuscular
mycorrhizal fungi. Nature, 435, 824–827 (2005).
Thuring, J. W. J. F., Nefkens, G. H. L., and Zwanenburg,
B., Synthesis and biological evaluation of the strigol
analogue carba-GR24. J. Agric. Food Chem., 45, 1409–
490F—a broad-spectrum fungicide with a new mode of
action. In ‘‘Brighton Crop Protection Conference—Pests
and Diseases,’’ Conference Proceedings, Brighton,
pp. 403–410 (1992).
5)
6)
7)
8)
10) Mangnus, E. M., Dommerholt, F. J., De Jong, R. L. P.,
and Zwanenburg, B., Improved synthesis of strigol
analog GR24 and evaluation of the biological activity
of its diastereomers. J. Agric. Food Chem., 40, 1230–
1235 (1992).
11) Nefkens, G. H. L., Thuring, J. W. J. F., Beenakkers, M.
F. M., and Zwanenburg, B., Synthesis of a phthaloyl-
glycine-derived strigol analog and its germination-
stimulatory activity toward seeds of the parasitic weeds
Striga hermonthica and Orobanche crenata. J. Agric.
Food Chem., 45, 2273–2277 (1997).
12) Mangnus, E. M., Van Vliet, L. A., Vandenput, D. A. L.,
and Zwanenburg, B., Structural modifications of strigol
analogs. Influence of the B and C rings on the bioactivity
of the germination stimulant GR24. J. Agric. Food
Chem., 40, 1222–1229 (1992).
1414 (1997).
Mangnus, E. M., and Zwanenburg, B., Tentative mo-
lecular mechanism for germination stimulation of Striga
and Orobanche seeds by strigol and its synthetic
analogues. J. Agric. Food Chem., 40, 1066–1070 (1992).
Anke, T., Oberwinkler, F., Steglich, W., and Schramm,
G., The strobilurins—new antifungal antibiotics from the
basidiomycete Strobilurus tenacellus strain No. 21602.
J. Antibiotics, 30, 806–810 (1977).
Anke, T., Schramm, G., Schwalge, B., Steffan, B., and
Steglich, W., Antibiotics from basidiomycetes, XX.
Synthesis of strobilurin A and revision of the stereo-
chemistry of natural strobilurins. Liebigs Ann. Chem.,
1616–1625 (1984).
9)
Ammermann, E., Lorenz, G., Schelberger, K.,
Wenderoth, B., Sauter, H., and Rentzea, C., BAS