Phenazine-1-carboxylic acid derivatives: Design, synthesis and biological evaluation against Rhizoctonia solani Kuhn

Article abstract of DOI:10.1016/j.bmcl.2010.10.050

Rhizoctonia solani Kuhn is the pathogen that causes sheath blight and results in significant yield reduction in rice and in nearly 50 other crops. In order to develop a new fungicide effective against this pathogen, a series of structurally diverse phenaz

Full text of DOI:10.1016/j.bmcl.2010.10.050

Bioorganic & Medicinal Chemistry Letters 20 (2010) 7369–7371
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Phenazine-1-carboxylic acid derivatives: Design, synthesis and biological
evaluation against Rhizoctonia solani Kuhn
Long Ye a, Hongyan Zhang b, Hong Xu a, Qi Zou a, Chao Cheng a, Dexian Dong a, Yuquan Xu a, Rongxiu Li a,
⇑
a Key Laboratory of MOE for Microbial metabolism and School of Life Sciences & Biotechnology, Shanghai Jiao Tong University, Dongchuan Road 800, Shanghai 200240, China
b Shanghai Nongle Biological Products Co., Ltd, Shanghai 201613, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Rhizoctonia solani Kuhn is the pathogen that causes sheath blight and results in significant yield reduction
in rice and in nearly 50 other crops. In order to develop a new fungicide effective against this pathogen, a
series of structurally diverse phenazine-1-carboxylic acid derivatives, 2a, 2b, 2c, 2d, 2e, 2f, 2g, 2h, 2i, 2j,
and 2k, were designed, synthesized and evaluated for their antifungal activity. The two most active com-
pounds 2i and 2j were selected as lead compounds for further antifungal research.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 21 July 2010
Revised 8 October 2010
Accepted 9 October 2010
Available online 20 October 2010
Keywords:
Phenazine-1-carboxylic acid
Rhizoctonia solani Kuhn
Antifungal activity
Rice
Sheath blight
Rice is one of the major staple crops consumed by almost half of
the world’s population, particularly in East, Southeast and South
Asia.1,2 Rhizoctonia solani Kuhn, the pathogen that causes sheath
blight, is one of the most devastating pathogenic fungi affecting
rice.3–5 In addition to rice, this fungus can infect crops of nearly
50 other species, including barley, lettuce, tomato, sorghum, and
maize. Sheath blight infection causes significant decreases in rice
yield; for example, a yield reduction of 40% was recorded in a rice
crop with the highest inoculum density.6 In the United States, a
yield loss of 50% was reported when susceptible cultivars were
planted.7 In China, sheath blight disease affects about 15–20 mil-
lion hectares and causes a yield loss of 6 million tons of rice grain
per year.4,8 For these reasons, development of an efficient method
is urgent for the suppression of R. solani infection.
In recent years, significant research effort has been directed
towards the screening of biocontrol agents and chemical com-
pounds against R. solani, many of which are now considered to
be promising.9–15 In the case of biofungicides, high efficiency and
low toxicity are necessary requirement. Phenazine-1-carboxylic
acid (PCA, 1, Scheme 1), one of the research focuses of our research
group, is produced by several plant growth-promoting rhizosphere
(PGPR) pseudomonads and has proven effective against several
soil-borne fungal phytopathogens that are of great agriculture sig-
nificance.16–18 PCA has been registered as the biofungicide ‘Shen-
qinbactin’ in China and is noted for its high fungicidal efficiency,
low toxicity to humans and animals, environmental friendliness,
and improvement of crop production.19–23 Our current aim is to
identify new and more efficient antifungal compounds. In the pres-
ent study, we describe the design and synthesis of PCA derivatives
modified on the 1-position of PCA, taking advantage of the high
chemical reactivity of carboxylic acid with other reagents such as
amines and alcohols.
We investigate the antifungal activities of the derived com-
pounds on R. solani as well as preliminary SAR studies.
PCA was isolated from metabolites extracted from genetically
modified Pseudomonas sp. M18 which produces much larger
amounts of PCA than the wild type, using the purification proce-
dure previously described.21,24,25 A series of PCA derivatives (2a–
2k, Scheme 2) were then designed with respect to structure and
chemical diversity. Different modifications, such as addition of
linear chain groups, cyclic hydrocarbons, or aromatic groups, and
changes in hydrophilicity or hydrophobicity, were all taken into
consideration.
Scheme 3 outlines the general synthesis of PCA derivatives 2a,
2b, 2c, 2d, 2e, 2f, 2g, 2h, 2i, 2j and 2k. Treatment of PCA isolated
O
OH
N
N
⇑
Corresponding author. Tel.: +86 021 34205517; fax: +86 021 34205517.
E-mail address: rxli@sjtu.edu.cn (R. Li).
Scheme 1. PCA structure.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.10.050

7370
L. Ye et al. / Bioorg. Med. Chem. Lett. 20 (2010) 7369–7371
O
H
N
H
H
O
COOH
O
N
O
N
O
N
H
N
N
N
N
N
N
2c
2b
2a
O
N
N
H
N
H
H
O
O
N
N
O
N
N
N
N
2f
N
N
N
N
2e
2d
F
H
N
H
N
H
O
N
O
O
N
N
N
N
N
F
N
N
2i
2g
2h
H
H
H
N
O
N
N
O
O
O
O
N
N
N
N
N
N
2k
Scheme 2. PCA derivatives: 2a, 2b, 2c, 2d, 2e, 2f, 2g, 2h, 2i, 2j, and 2k.
2j
H
O
OH
O
N
R
O
Cl
a
b
N
N
N
N
N
N
1
2a-2k
3
Scheme 3. Preparation of target compounds 2a, 2b, 2c, 2d, 2e, 2f, 2g, 2h, 2i, 2j, and 2k. Reagents and conditions: (a) SOCl2, reflux, 6 h; (b) RNH2 (2a–2j 1 equiv, 2k 0.5 equiv),
triethylamine (4 equiv), CH2Cl2, 0 °C to room temperature, overnight.
from genetically modified Pseudomonas sp. M18 with SOCl2 at re-
flux temperature afforded intermediate 3 after the evaporation of
SOCl2. The target compounds were then synthesized by adding cor-
responding amines to intermediate 3 as a CH2Cl2 solution, utilizing
triethylamine as a base at 0 °C (analysis data of target compounds
are presented in Supplementary data).
phenyl group (2i, IC50 = 0.008
l
M) by a pyridinyl group (2g,
IC50 = 0.023
lM) led to weaker inhibition (compared with 2i, but
Table 1
Antifungal evaluation of PCA derivatives
All target compounds were screened for their antifungal activity
against R. solani. Concentration effective for growth inhibition was
tested by a plate method (PDA medium, procedures are described
in Supplementary data). Authentic PCA was used as a positive con-
Compound
Rhizoctonia solani Kuhn
IC50 M)
(l
1 (positive control)
0.068
0.079
0.123
0.246
0.087
0.074
0.084
0.023
0.075
0.008
0.003
0.051
2a
2b
2c
2d
2e
2f
2g
2h
2i
trol. The inhibitory concentration (IC50) values (in
sented in Table 1.
lM) are pre-
For R. solani, the biological results were relatively heteroge-
neous (0.003 <IC50 M) <0.246). Compounds bearing a cyclic side
(l
chain with high hydrophobicity (2i and 2j) exhibited a significant
level of activity, with IC50 values 8- to 23-fold lower than that of
PCA itself (1, IC50 = 0.068
chain with high hydrophilicity (2b and 2c) were less active (IC50
values of 0.123 and 0.246 M, respectively). Replacement of
lM). Compounds bearing a linear side
2j
2k
l

L. Ye et al. / Bioorg. Med. Chem. Lett. 20 (2010) 7369–7371
7371
still more active than PCA). Other derivatives (2a, 2d, 2e, 2f, 2h,
and 2k) showed nearly the same IC50 values (0.079, 0.087, 0.074,
0.084, 0.075 and 0.051 M, respectively) as that of PCA (1,
IC50 = 0.068 M).
Supplementary data
l
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2010.10.050.
l
Compound 2i and 2j had the highest activity while 2b and 2c
had the lowest. Since 2i and 2j have the highest hydrophobicity,
while 2b and 2c have the lowest, hydrophobicity can be assumed
to be one of the most important factors for enhancing the anti-
fungal activity of PCA derivatives against R. solani. The supposition
is supported by the comparison of 2g and 2i, because 2i is more ac-
tive (three times) than 2g, even though they differ structurally by
only one atom (carbon vs nitrogen) on the aromatic group of the
side chain. At the same time, we can speculate from the biological
results that compounds bearing a cyclic group are more active than
those with a linear group (activity: 2j, 2h > 2a, 2b, 2c; other com-
pounds bear both linear and cyclic groups).
In conclusion, we have successfully prepared a series of struc-
turally diverse PCA derivatives, some of which exhibited signifi-
cantly higher levels of antifungal activity against Rhizoctonia
solani Kuhn than did PCA itself. Compound 2i and 2j were screened
as lead compounds for further antifungal research on PCA deriva-
tives and their activities indicate them to be potential candidates
for development as fungicides.
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We thank for the financial support from National S & T Major
Projects of China (Key Innovative Drug Development, no.
2009ZX09306-008), National Basic Research Program of China
(973 Program, nos. 2007CB936004 and 2009CB118906), National
High Technology Research and Development Program of China
(863 Program, no. 2007AA100506), Natural Science Foundation of
China (no. 30630012), Shanghai Leading Academic Discipline Pro-
ject (no. B203) and Shanghai Science and Technology Innovation
Action Program (nos. 072312048 and 08DZ1204400). We also
thank the Instrumental Analysis Center of Shanghai Jiao Tong Uni-
versity for HRMS, 13C NMR and IR data.
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