Synthesis and Biological Activity of 1H-Pyrrolo[2,3-b]pyridine Derivatives: Correlation between Inhibitory Activity against the Fungus Causing Rice Blast and Ionization Potential

Article abstract of DOI:10.1021/jf9607730

Synthesis and biological activity of a variety of 3- and 6-substituted 1H-pyrrolo[2,3-6]pyridine (7-azaindole) derivatives are described. Many of the synthesized 7-azaindoles exhibited considerable fungicidal activity toward Pyricularia oryzae, a fungus which causes rice blast, in vivo. When quantum parameters of the tested 7-azaindoles were evaluated by semiempirical molecular orbital calculations, a relationship was observed between the activity and the calculated ionization potentials of the 7-azaindole derivatives.

Full text of DOI:10.1021/jf9607730

J. Agric. Food Chem. 1997, 45, 2345
−2348
2345
Syn th esis a n d Biologica l Activity of 1H-P yr r olo[2,3-b]p yr id in e
Der iva tives: Cor r ela tion betw een In h ibitor y Activity a ga in st th e
F u n gu s Ca u sin g Rice Bla st a n d Ion iza tion P oten tia l
Satoshi Minakata, Takayuki Hamada, and Mitsuo Komatsu*
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka 2-1,
Suita, Osaka 565, J apan
Hiroyuki Tsuboi
Plant Protection Department, Biochemicals Division, Dainippon Ink and Chemicals, Inc., Sakato 631,
Sakura, Chiba 285, J apan
Hiroshige Kikuta† and Yoshiki Ohshiro‡
Department of Mathematics and Physics, Faculty of Science and Technology, and Research Institute for
Science and Technology, Kinki University, Kowakae 3-4-1, Higashi-Osaka, Osaka 577, J apan
Synthesis and biological activity of a variety of 3- and 6-substituted 1H-pyrrolo[2,3-b]pyridine (7-
azaindole) derivatives are described. Many of the synthesized 7-azaindoles exhibited considerable
fungicidal activity toward Pyricularia oryzae, a fungus which causes rice blast, in vivo. When
quantum parameters of the tested 7-azaindoles were evaluated by semiempirical molecular orbital
calculations, a relationship was observed between the activity and the calculated ionization potentials
of the 7-azaindole derivatives.
Keyw or d s: 1H-Pyrrolo[2,3-b]pyridine; fungicidal activity; ionization potential; rice blast
Ta ble 1. List of Tested 7-Aza in d ole Der iva tives
INTRODUCTION
Studies on the biological activities of 1H-pyrrolo[2,3-
b]pyridine (7-azaindole) derivatives have attracted con-
siderable interest in recent decades (Yakhontov and
Prokopov, 1980), because they are aza-analogs of indoles
whose skeletons are often found in alkaloids. Although
the biological properties of some azaindoles in animals
have been reported (Crooks et al., 1988), their activities
with respect to plants have not been the subject of
intensive study (Widholm, 1981). 7-Azaindole-3-acetic
acid, for example, stimulates the vacuolation of proto-
plasm in the roots of Allium cepa. This effect parallels
the growth-stimulating effect of the corresponding in-
dole (Pilet, 1971). In contrast, 7-azatryptophan inhibits
the growth of Nicotiana tabacum and Daucus carota
cells (Widholm, 1972). These diverse biological activi-
ties of 7-azaindole derivatives prompted us to investi-
gate 7-azaindoles as potential agrochemicals.
Structure-function relationships are of considerable
interest in the fields of pharmacology and medicinal
chemistry, as well as in agrochemistry. A number of
physicochemical parameters, such as Hammett σ value
and log P value, have been used for this purpose. In
addition to these parameters, attempts have been made
to correlate various quantum parameters with observed
activities of some pharmaceuticals (Suter et al., 1995)
and agrochemicals (Akagi et al., 1995).
compd no.
R1
R2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
H
Cl
Br
I
H
H
H
H
H
H
H
H
Cl
NO2
NH2
COMe
CHdCH2
H
H
H
H
H
H
Br
I
NH2
CtCH
CN
shown in Table 1, and a correlation of their quantum
and physicochemical parameters with inhibitory activity
against the fungus responsible for rice blast.
Although 3-substituted 7-azaindoles can be easily and
conveniently synthesized by electrophilic substitution
reactions, procedures for direct functionalization at the
6-position of 7-azaindole are more difficult. We recently
developed a method which allows the facile direct
introduction of halogen atoms and cyano groups at the
6-position of 7-azaindole (Minakata et al., 1992b), and
this has enabled us to prepare an extensive series of
6-functionalized analogs.
From this point of view, we report herein the synthe-
sis of a variety of 3- and 6-substituted 7-azaindole
derivatives, which are similar in molecular size, as
* Corresponding author.
† Faculty of Science and Technology, Kinki University.
‡ Research Institute for Science and Technology.
S0021-8561(96)00773-X CCC: $14.00
© 1997 American Chemical Society

2346 J. Agric. Food Chem., Vol. 45, No. 6, 1997
Minakata et al.
EXPERIMENTAL PROCEDURES
sprayed on six rice seedlings (Oryza sativa L. variety Aichia-
sahi) at the fourth-leaf stage. The rice plants were inoculated
by spraying a spore suspension of Pyricularia oryzae contain-
ing ca. 105 spores/mL. The inoculated plants were kept in a
humid chamber at 25 °C for one day and then transferred to
a greenhouse. One week after inoculation, the number of rice
blast lesions on the fourth leaves was counted and the
protective value of the test compound was calculated using
the following formula:
Melting points were obtained using a Yanagimoto micro
melting point apparatus and are uncorrected. IR spectra were
obtained on a HITACHI 270-30 infrared spectrometer. 1H and
13C NMR were recorded on a J EOL J MH-FX-90Q or a J EOL
J MN-EX270 spectrometer with tetramethylsilane as the in-
ternal standard. Electron impact (EI) mass spectra were
obtained on a Shimadzu GCMS-QP2000 or a J EOL J MS-
DX303 mass spectrometer. Molecular orbital calculations were
performed with the MOPAC program (PM3 method) on a
CAChe Work System (SONY Tektronix). 7-Azaindole deriva-
tives, 3 (Robison and Robison, 1956), 4 (Herbert and Wibberley,
1969), 5 and 6 (Robison et al., 1959), and 7 (Ga´lvez and
Viladoms, 1982), were prepared using methods reported in the
literature. Compounds 8, 12, and 13 (Minakata et al., 1992a)
and 9 and 10 (Minakata et al., 1992b) were synthesized using
procedures developed in Osaka University.
protective value (%) )
(1 - av no. of lesions in treated plants)
(av no. of lesions in untreated plants)
× 100
RESULTS AND DISCUSSION
There are no examples of the synthetic procedures for
the tested 7-azaindole derivatives except for compounds
3 (Robison and Robison, 1956), 4 (Herbert and Wibber-
ley, 1969), 5 and 6 (Robison et al., 1959), and 7 (Ga´lves
and Viladoms, 1982). Although bromine can be used
in the 3-bromination of 7-azaindole (Robison and Rob-
ison, 1956), we employed N-chlorosuccinimide in the
3-chlorination of 7-azaindole, a procedure which gives
3-chloro-7-azaindole 2 in excellent yield. The introduc-
tion of a vinyl group to 7-azaindole at the 3-position
proceeded from the 3-formyl derivative via a two-step
reaction (Minakata et al., 1992a). For other derivatives,
however, we were able to achieve the facile and direct
introduction of functional groups such as halogeno and
cyano groups onto 6-position of 7-azaindole (Minakata
et al., 1992b). In addition, 6-bromo-7-azaindole was
conveniently converted into the 6-amino- or ethynyl-
substituted analogs by a substitution reaction using
aqueous ammonia or by a cross-coupling reaction with
(trimethylsilyl)acetylene, using palladium as the cata-
lyst, respectively.
3-Ch lor o-1H-p yr r olo[2,3-b]p yr id in e (2). A solution of
7-azaindole (1) purchased from Aldrich Chemical Co., Inc. (118
mg, 1 mmol) and N-chlorosuccinimide (150 mg, 1.1 mmol) in
CCl4 (20 mL) and CHCl3 (10 mL) was stirred for 4 h at room
temperature under an atmosphere of nitrogen. After the
solvent was removed, 70 mL of ether was added and the
solution was washed with saturated aqueous NaHCO3 (20 mL
× 2), dried (MgSO4), and concentrated in vacuo to give 2 (152
mg, 99%) as a colorless powder: mp 169-170 °C; IR (KBr) ν
1592, 1292, 1002, and 764 cm-1; 1H NMR (CDCl3) δ 7.18 (1H,
dd, J ) 5.0, 8.0 Hz, H-5), 7.36 (1H, s, H-2), 8.00 (1H, d, J )
8.0 Hz, H-4), 8.38 (1H, d, J ) 5.0 Hz, H-6), 11.0-11.5 (1H,
brs, H-1); 13C NMR (CDCl3) δ 104.3, 116.2, 118.6, 122.0, 127.2,
143.4, 147.2; MS (DEI) m/z (relative intensity) 154 (M+ + 2,
33), 152 (M+, 100). Anal. Calcd for C7H5N2Cl: C, 55.10; H,
3.30; N, 18.36; Cl, 23.24. Found: C, 54.89; H, 3.22; N, 18.42;
Cl, 23.08.
6-Iod o-1H-p yr r olo[2,3-b]p yr id in e (11). 6-Iodo-1-meth-
oxycarbonyl-7-azaindole (302 mg, 1.0 mmol) (Minakata et al.,
1992b) was dissolved in MeOH (30 mL) and 1 N NaOH (10
mL). After the solution was stirred for 20 h at room temper-
ature, the solvent was removed and the residue was extracted
with CHCl3 (20 mL × 3), dried (MgSO4), and concentrated in
vacuo. The crude product was purified by chromatography on
a silica gel column using hexane/EtOAc (8:2) to give 11 (232
mg, 95%) as colorless needles: mp 196-197 °C; IR (KBr) ν
1594, 1564, 1402, 1086, and 754 cm-1; 1H NMR (CDCl3) δ 6.51
(1H, dd, J ) 1.6, 3.3 Hz, H-3), 7.41 (1H, dd, J ) 3.3, 3.4 Hz,
H-2), 7.46 (1H, d, J ) 8.3 Hz, H-5), 7.66 (1H, d, J ) 8.3 Hz,
H-4), 10.0-11.0 (1H, brs, H-1); 13C NMR (CDCl3) δ 100.8,
107.8, 119.7, 125.6, 126.1, 130.7, 149.0; MS (EI) m/z (relative
intensity) 244 (M+, 100), 117 (M+ - I, 80). Anal. Calcd for
C7H5N2I: C, 34.45; H, 2.07; N, 11.48; I, 52.0. Found: C, 34.69;
H, 2.13; N, 11.37; I, 51.89.
Some of the synthesized 7-azaindoles exhibited con-
siderable fungicidal activity in vivo toward P. oryzae, a
fungus which causes rice blast. In order to correlate
structure with activity, several physicochemical param-
eters of the tested 7-azaindoles were evaluated. The
hydrophobic parameter π for each substituent is defined
as follows:
π ) log PX - log PH
PH is the partition coefficient of benzene between
1-octanol and water, and PX is that for monosubstituted
benzene (Fujita et al., 1964). Dipole moments and
ionization potentials were calculated using a semi-
empirical molecular orbital calculation with the
MOPAC-PM3 method. The parameters and the protec-
tive values against rice blast are summarized in Table
2.
6-Cya n o-1H-p yr r olo[2,3-b]p yr id in e (14). 1-Benzoyl-6-
cyano-7-azaindole (247 mg, 1.0 mmol) (Minakata et al., 1992b)
was dissolved in MeOH (30 mL) and 1 N NaOH (10 mL). After
the solution was stirred for 24 h at room temperature, MeOH
was removed and the residue was extracted with CHCl3 (20
mL × 3), dried (MgSO4), and concentrated in vacuo. The crude
product was separated by chromatography on a silica gel
column using hexane/EtOAc (9:1) to give 14 (109 mg, 76%) as
a colorless powder: mp 175-177 °C; IR (KBr) ν 2228 (CN),
1580, 1412, 1112, and 756 cm-1; 1H NMR (CDCl3) δ 6.65 (1H,
dd, J )1.9, 3.6 Hz, H-3), 7.51 (1H, d, J ) 8.0 Hz, H-5), 7.66
(1H, dd, J ) 3.6, 3.7 Hz, H-2), 8.08 (1H, d, J ) 8.0 Hz, H-4),
11.0-12.0 (1H, brs, H-1); 13C NMR (CDCl3) δ 101.6, 118.7,
120.1, 123.8, 124.2, 129.5, 130.2, 148.2; MS (DEI) m/z (relative
intensity) 143 (M+, 100), 116 (M+ - HCN, 43). Anal. Calcd
for C8H5N3: C, 67.12; H, 3.52; N, 29.36. Found: C, 67.18; H,
3.47; N, 29.45.
F u n gicid a l Tests a ga in st Rice Bla st. For these experi-
ments, the above compounds were formulated into a 20%
wettable powder and then made up in an aqueous suspension
at a concentration of 250 ppm. [Formulation: 20% azaindole
derivative, 4% R-(p-nonylphenyl)-ω-hydroxypoly(oxyethylene)
(Matsumoto Yushi-Seiyaku Co. Ltd.), 8% hydrated silica
(Shionogi & Co., Ltd.), and 68% bentonite.] They were then
The following equations (eqs 1-3) and r2 values are
calculated using least squares without the compounds
1, 9, and 11, which did not exhibit the activity.
Y ) 4.06X + 37.7
Y ) -10.3X + 61.1
Y ) -46.3X + 443.7
r2 ) 0.019
r2 ) 0.384
r2 ) 0.629
(1)
(2)
(3)
Figure 1 shows the relation between the π value and
the protective value of 7-azaindole derivatives, where
no clear correlation is observed.

Synthesis and Activity of 7-Azaindoles
J. Agric. Food Chem., Vol. 45, No. 6, 1997 2347
Ta ble 2. P r otective Activity a ga in st Rice Bla st a n d
P a r a m eter s of 3- a n d 6-Su bstitu ted 7-Aza in d oles
compd protective
no.
dipole momentb
(debye)
ionization
valuea (%)
π
potentialb (eV)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0
29
24
92
1
72
15
47
0
26
0
45
49
19
0
0.71
0.86
1.35
0.97
0.97
1.10
5.14
1.82
3.14
1.05
2.29
2.67
2.21
1.20
1.38
5.06
8.76
8.59
8.86
8.49
9.71
8.06
9.01
8.43
8.58
8.98
8.78
8.33
8.75
9.14
1.22
-0.28
-1.23
-0.55
0.82
0.71
0.86
1.22
-1.23
0.47
-0.57
a
F igu r e 3. Ionization potential of 7-azaindole versus protective
activity against rice blast.
Methods used for estimating these data are cited in Experi-
b
mental Procedures. Calculated by the PM3 method.
These results suggest that 7-azaindole derivatives may
be involved in one of the electron transfer processes in
P. oryzae (rice blast fungus), although it cannot be
concluded that these fungicidal compounds act in this
fashion in a complex organism. The data are of interest
because it provides fundamental information which may
be useful for the design of agricultural fungicides for
rice blast by using 7-azaindole as the basic core struc-
ture.
LITERATURE CITED
Akagi, T.; Mitani, S.; Komyoji, T.; Nagatani, K. Quantitative
structure-activity relationships of fluazinum and related
fungicidal N-phenylpyridinamines: Preventive activity
against Botrytis cinerea. J . Pestic. Sci. 1995, 20, 279-290.
Crooks, P. A.; Godin, C. S.; Damani, L. A. Formation of
quaternary amines by N-methylation of azaheterocycles
with homogeneous amine N-methyltransferases. Biochem.
Pharmacol. 1988, 37, 1673-1677.
F igu r e 1. π value of 7-azaindole versus protective activity
against rice blast.
Fujita, T.; Iwasa, J .; Hansch, C. A new substituent constant,
π, derived from partition coefficients. J . Am. Chem. Soc.
1964, 86, 5175-5180.
Ga´lvez, C.; Viladoms, P. Reactivity of 1H-pyrrolo[2,3-b]pyri-
dine. I. Synthesis of 3-acetyl-7-azaindole and related com-
pounds. J . Heterocycl. Chem. 1982, 19, 665-667.
Herbert, R.; Wibberley, D. G. Syntheses and properties of 1H-
pyrrolo[2,3-b]pyridines. J . Chem. Soc. (C) 1969, 1505-1514.
Minakata, S.; Itoh, S.; Komatsu, M.; Ohshiro, Y. Functional-
ization of 1H-pyrrolo[2,3-b]pyridine. Bull. Chem. Soc. J pn.
1992a , 65, 2992-2997.
Minakata, S.; Komatsu, M.; Ohshiro, Y. Regioselective func-
tionalization of 1H-pyrrolo[2,3-b]pyridine via its N-oxide.
Synthesis 1992b, 661-663.
Nakayama, A.; Hagiwara, K.; Hashimoto, S.; Shimoda, S.
QSAR of fungicidal ∆3-1,2,4-thiadiazolines. Reactivity-
activity correlation of SH-inhibitors. Quant. Struct.-Act.
Relat. 1993, 12, 251-255.
Pilet, P. E. Effect of auxins on root protoplasts. C. R. Acad.
Sci., Ser. D 1971, 273, 2253-2256.
F igu r e 2. Dipole moment of 7-azaindole versus protective
activity against rice blast.
Robison, M. M.; Robison, B. L. 7-Azaindole. III. Synthesis of
7-aza analogs of some biologically significant indole deriva-
tives. J . Am. Chem. Soc. 1956, 78, 1247-1251.
Robison, M. M.; Robison, B. L.; Butler, F. P. 7-Azaindole. VI.
Preparation of 5-and 6-substituted 7-azaindoles. J . Am.
Chem. Soc. 1959, 81, 743-747.
Suter, H. U.; Maric, D. M.; Weber, J .; Thomson, C. Quantum
chemistry and drug design. Chimia 1995, 49, 125-127.
Widholm, J . M. Tryptophan biosynthesis in Nicotiana tabacum
and Daucus carota cell cultures: Site of action of inhibitory
tryptophan analogs. Biochim. Biophys. Acta 1972, 261, 44-
51.
The relation between the calculated dipole moment
and activity is illustrated in Figure 2, which also shows
that there is little correlation between them.
On the other hand, a more clear relationship was
observed between activity and the calculated ionization
potentials. These results show that higher activity is
correlated with a decrease in the value of the ionization
potential in the range 8-10 eV (Figure 3). Based on
our experience, this type of relationship between bio-
logical activity and ionization potential has rarely been
reported for agrochemicals (Nakayama et al., 1993).

2348 J. Agric. Food Chem., Vol. 45, No. 6, 1997
Minakata et al.
Widholm, J . M. Utilization of indole analogs by carrot and
tobacco cell tryptophan synthase in vivo and in vitro. Plant
Physiol. 1981, 67, 1101-1104.
Willete, R. E. In Monoazaindoles: The pyrrolopyridines;
Katritzky, A. R., Boulton, A. J ., Eds.; Advances in Hetero-
cyclic Chemistry 9; Academic Press: New York, 1968; pp
27-105.
Yakhontov, L. N.; Prokopov, A. A. Advances in the chemistry
of azaindoles. Russ. Chem. Rev. 1980, 49, 428-444.
Received for review October 15, 1996. Revised manuscript
received February 17, 1997. Accepted March 7, 1997.X
J F9607730
X Abstract published in Advance ACS Abstracts, May
15, 1997.