Synthesis of diaryl-azo derivatives as potential antifungal agents

Article abstract of DOI:10.1016/j.bmcl.2010.05.048

As compared with a commercially available agricultural fungicide hymexazol, some phenyl-azo phenol derivatives (e.g., 4a, 4b, 4f, 4n, 4q, 4u, and 4v) exhibited the more promising and pronounced antifungal activities in vitro against seven phytopathogenic fungi. It seemed that 4-((un)substituted phenylazo)-phenol and 4-((un)substituted phenylazo)-3-methylphenol might be considered as new lead structures for further design of agricultural fungicides.

Full text of DOI:10.1016/j.bmcl.2010.05.048

Bioorganic & Medicinal Chemistry Letters 20 (2010) 4193–4195
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis of diaryl-azo derivatives as potential antifungal agents
*
Hui Xu , Xiwen Zeng
Laboratory of Pharmaceutical Design and Synthesis, College of Sciences, Northwest A&F University, Yangling 712100, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 February 2010
Revised 8 May 2010
Accepted 13 May 2010
Available online 20 May 2010
As compared with a commercially available agricultural fungicide hymexazol, some phenyl-azo phenol
derivatives (e.g., 4a, 4b, 4f, 4n, 4q, 4u, and 4v) exhibited the more promising and pronounced antifungal
activities in vitro against seven phytopathogenic fungi. It seemed that 4-((un)substituted phenylazo)-
phenol and 4-((un)substituted phenylazo)-3-methylphenol might be considered as new lead structures
for further design of agricultural fungicides.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Phenyl-azo-phenol
Synthesis
Antifungal activity
Phytopathogenic fungi
Many phenol derivatives (I, Fig. 1) including those isolated from
the plants exhibited the good antifungal activity, and it suggested
that the hydroxyl group of phenol analogs was required for their
antifungal activity through the structure–activity relationship
(SAR) study, that is, removal of the phenolic hydroxyl group would
sharply lead to loss of activity.1–4 In the meantime, it was well-
known that azo compounds (II, Fig. 1), besides their use as dyes,
showed antibacterial and antifungal activities.5–7
Bioisosterism is an effective and useful strategy for molecular
modification and drug design.8 In light of the above interesting bio-
logical activities, and in continuation of our program aimed at the
discovery and development of bioactive molecules,9–11 we wanted
to prepare phenyl-azo-phenol derivatives (4, Fig. 1) as antifungal
agents by combining the azo moiety with the phenolic hydroxyl
group. To the best of our knowledge, little attention has been paid
to the antifungal activities of simple phenyl-azo phenol derivatives
against phytopathogenic fungi. Meanwhile, phytopathogenic fungi,
which are hard to control, easily infect many crops, therefore the
development of bioactive compounds for effective control of those
agricultural diseases is highly desirable. In this Letter 23 phenyl-
azo phenol derivatives (4a–w, Scheme 1) were designed, synthe-
sized and evaluated in vitro for their antifungal activities against
seven phytopathogenic fungi. The SAR of these compounds was
also preliminarily investigated.
purification. Then the intermediates 2a–f further reacted with phe-
nols (3a–e) at 0–5 °C for 3–6 h to produce phenyl-azo phenol
derivatives (4a–w).12 The structures of the target compounds were
well characterized by 1H NMR, MS, and mp (see Supplementary
data). The purities of the target compounds were all larger than
95% measured with RP-HPLC.
The antifungal activities of the 23 phenyl-azo phenol derivatives
(4a–w) against seven phytopathogenic fungi (i.e., Fusarium grami-
nearum, Alternaria alternata, Bipolaris sorokinianum, Pyricularia ory-
zae, Fusarium oxysporum f. sp. vasinfectum, Fusarium oxysporum
f. sp. cucumarinum, and Alternaria brassicae) were investigated
in vitro by poisoned food technique.9,13 Hymexazol, a commercially
available agricultural fungicide, was used as a positive control.
As outlined in Table 1, among of all the derivatives, compounds
4a, 4b, 4e, 4f, 4i, 4j, 4n, 4q, 4u, and 4v exhibited a good and broad
spectrum of antifungal activities against the seven phytopatho-
genic fungi tested at the concentration of 100 lg/mL. It demon-
strated that the hydroxyl group at the 4-position on the right
R2
Bioisosterism
R1
R2
N
N
OH
As shown in Scheme 1, phenylamine derivatives (1a–f) firstly
reacted with concentrated hydrochloric acid and sodium nitrite
at 0–5 °C to give the corresponding benzenediazonium chlorides
(2a–f), which were used directly for the next step without further
OH
I
R3
4
N
N
R1
II
* Corresponding author. Tel./fax: +86 29 87091952.
E-mail address: orgxuhui@nwsuaf.edu.cn (H. Xu).
Figure 1. Design strategy of the target compounds 4.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.05.048

4194
H. Xu, X. Zeng / Bioorg. Med. Chem. Lett. 20 (2010) 4193–4195
R1
R2
N
N
OH
4a, 4b, 4e, 4f, 4i, 4j, 4n, 4q, 4u-w
4a: R1 = R2 = H; 4b: R1 = H, R2 = 2-Me;
4e: R1 = 4-Me, R2 = H; 4f: R1 = 4-Me, R2 = 2-Me;
4i: R1 = 4-NO2, R2 = H; 4j: R1 = 4-NO2, R2 = 2-Me;
4n: R1 = 2-Cl, R2 = 2-Me; 4q: R1 = 2-NO2, R2 = H;
4u: R1 = 3-Cl, R2 = H; 4v: R1 = 3-Cl, R2 = 2-Me;
4w: R1 = 4-Me, R2 = 2-OH;
OH
R2
NH2
N2+Cl-
R1
NaNO2/HCl
º
3a-e
R1
0-5
C
NaOH/EtOH
R1
R2
N
1a-f
2a-f
0-5 o
57-73%
C
N
1a: R1 = H
HO
3a: R2 = H
1b: R1 = 4-Me
1c: R1 = 4-NO2
1d: R1 = 2-Cl
1e: R1 = 2-NO2
1f: R1 = 3-Cl
3b: R2 = 3-Me
3c: R2 = 4-Cl
3d: R2 = 4-Bu(t)
3e: R2 = 3-OH
4c, 4d, 4g, 4h, 4k-m, 4o, 4p, 4r-t
4c: R1 = H, R2 = 5-Cl; 4d: R1 = H, R2 = 5-Bu(t);
4g: R1 = 4-Me, R2 = 5-Cl; 4h: R1 = 4-Me, R2 = 5-Bu(t);
4k: R1 = 4-NO2, R2 = 5-Cl; 4l: R1 = 4-NO2, R2 = 5-Bu(t);
4m: R1 = 2-Cl, R2 = 5-Cl; 4o: R1 = 2-Cl, R2 = 5-Bu(t);
4p: R1 = 2-NO2, R2 = 5-Cl; 4r: R1 = 2-NO2, R2 = 5-Bu(t);
4s: R1 = 3-Cl, R2 = 5-Cl; 4t: R1 = 3-Cl, R2 = 5-Bu(t)
Scheme 1. The synthetic route of compounds 4a–w.
phenyl ring of 4 was a very important factor for their antifungal
activities. Once the hydroxyl group was introduced at the 2-posi-
tion on the right phenyl ring of 4, the antifungal activities of the
corresponding derivatives were reduced even if other substituents
were present on the left phenyl ring of 4. For example, compounds
4c, 4d, 4g, 4h, 4k–m, 4o, 4p and 4r–t, all bearing 2-hydroxyl group
on the right phenyl ring, showed inhibition rates against the seven
phytopathogenic fungi less than 45%. Meanwhile, if methyl and
hydroxyl groups were introduced at the 2- and 4-positions on
the right phenyl ring of 4, respectively, the corresponding
Table 1
Antifungal activities of compounds 4a–w
Compd
Concn (
lg/mL)
Antifungal activities (inhibition%)
F. graminearum
P. oryzae
F. oxysporium
A. alternata
A. brassicae
B. sorokinianum
F. oxysporium
f. sp. vasinfectum
f. sp. cucumarinum
4a
4b
100
50
100
50
72.37 ( 0.67)
63.03 ( 0.38)
71.02 ( 0.67)
65.82 ( 0.77)
44.47 ( 0.39)
21.16 ( 0.67)
45.01 ( 1.70)
56.06 ( 0.39)
54.78 ( 0)
8.36 ( 0)
36.93 ( 0.39)
43.40 ( 0)
41.64 ( 1.03)
0.26 ( 0)
3.64 ( 0)
100 ( 0)
87.27 ( 1.16)
93.73 ( 0)
86.67 ( 0)
95.43 ( 0)
83.51 ( 1.61)
100 ( 0)
90.06 ( 1.22)
5.25 ( 1.14)
9.82 ( 1.14)
79.45 ( 0)
93.27 ( 0.96)
76.22 ( 1.49)
94.23 ( 0)
86.84 ( 1.20)
77.65 ( 1.18)
91.15 ( 1.38)
83.53 ( 1.18)
1.44 ( 1.83)
12.20 ( 0.69)
70.57 ( 1.38)
79.19 ( 0.69)
69.41 ( 0)
15.55 ( 0.69)
1.44 ( 0.69)
68.18 ( 1.38)
58.61 ( 1.38)
13.88 ( 1.20)
4.78 ( 0.69)
3.59 ( 0.69)
86.36 ( 1.38)
80.47 ( 1.36)
0.72 ( 0)
97.38 ( 0)
73.89 ( 2.61)
93.46 ( 0)
85.62 ( 1.11)
85.33 ( 1.33)
83.85 ( 1.28)
0 ( 1.54)
3.20 ( 1.54)
70.67 ( 0)
82.33 ( 1.33)
75.00 ( 1.28)
0 ( 1.54)
92.06 ( 0.67)
7.89 ( 1.20)
21.53 ( 1.38)
79.19 ( 1.38)
86.84 ( 1.20)
80.79 ( 1.34)
2.63 ( 0.69)
16.75 ( 0.69)
66.99 ( 0.69)
70.81 ( 0.69)
3.11 ( 1.20)
0.72 ( 1.20)
0.72 ( 1.20)
88.04 ( 0)
89.67 ( 1.13)
6.73 ( 0.96)
19.81 ( 0.56)
73.08 ( 0.56)
78.85 ( 1.11)
75.05 ( 1.13)
2.88 ( 0.96)
7.12 ( 0.56)
63.46 ( 0.56)
68.27 ( 0)
2.88 ( 0.96)
0.96 ( 0.96)
8.08 ( 0.56)
72.12 ( 0.96)
70.71 ( 0.97)
5.38 ( 0.56)
19.81 ( 0.96)
78.85 ( 0.56)
73.68 ( 0.97)
10.19 ( 1.11)
16.10 ( 1.52)
13.92 ( 1.52)
83.12 ( 0.99)
79.92 ( 1.13)
74.55 ( 0.57)
73.68 ( 0.97)
37.97 ( 1.52)
55.34 ( 0.96)
46.98 ( 0.56)
90.08 ( 0.75)
8.38 ( 1.31)
14.14 ( 0.76)
77.74 ( 1.31)
92.15 ( 1.31)
87.47 ( 0.75)
10.47 ( 0.76)
14.40 ( 1.51)
80.37 ( 1.31)
67.80 ( 2.00)
12.30 ( 1.31)
10.99 ( 1.31)
14.40 ( 1.51)
90.31 ( 2.00)
84.33 ( 1.31)
3.93 ( 1.51)
1.31 ( 0.76)
78.27 ( 0.76)
71.28 ( 0)
6.54 ( 1.51)
25.39 ( 1.31)
7.59 ( 0.76)
89.70 ( 0.76)
88.77 ( 1.51)
85.60 ( 1.31)
84.33 ( 0)
25.39 ( 1.31)
65.18 ( 1.51)
52.48 ( 1.51)
4c
4d
4e
4f
100
100
100
100
50
100
100
100
100
100
100
100
100
50
100
100
100
50
100
100
100
100
50
92.81 ( 0)
86.32 ( 0)
4g
4h
4i
4j
4k
4l
11.42 ( 0.66)
5.25 ( 1.14)
69.63 ( 1.14)
69.86 ( 0.66)
2.97 ( 1.14)
1.14 ( 1.32)
2.51 ( 1.32)
92.12 ( 0.66)
90.91 ( 0.61)
1.83 ( 1.14)
31.05 ( 0.66)
79.45 ( 0)
0 ( 1.33)
52.53 ( 0.77)
45.86 ( 0.77)
0 ( 1.33)
0 ( 1.33)
0 ( 1.33)
4m
4n
9.03 ( 0.67)
61.50 ( 0.67)
60.11 ( 0.66)
9.03 ( 0)
82.33 ( 0.77)
80.30 ( 1.11)
0 ( 1.33)
8.53 ( 0.77)
68.80 ( 0.77)
56.86 ( 1.11)
0 ( 1.54)
22.82 ( 1.12)
2.68 ( 1.12)
89.91 ( 1.29)
88.27 ( 1.28)
82.55 ( 0.65)
77.87 ( 0)
27.96 ( 1.71)
61.74 ( 1.33)
58.63 ( 0.64)
86.81 ( 1.34)
10.29 ( 1.20)
27.03 ( 1.20)
85.65 ( 0)
4o
4p
4q
33.02 ( 0.67)
61.77 ( 1.03)
53.12 ( 1.33)
24.93 ( 0.78)
31.75 ( 0.39)
7.46 ( 1.04)
71.51 ( 0.68)
65.69 ( 1.02)
61.74 ( 0.39)
50.74 ( 0.77)
40.98 ( 1.36)
66.54 ( 0.39)
54.45 ( 1.15)
13.40 ( 1.38)
62.44 ( 1.83)
55.29 ( 0)
69.91 ( 2.31)
12.68 ( 1.20)
16.96 ( 0.73)
7.59 ( 0.73)
89.30 ( 0.73)
87.27 ( 1.16)
90.57 ( 1.46)
79.86 ( 1.34)
27.09 ( 1.46)
74.22 ( 1.20)
53.70 ( 2.31)
68.29 ( 0)
4r
4s
4t
4u
12.10 ( 1.14)
23.18 ( 0.64)
8.83 ( 1.69)
90.41 ( 1.27)
89.01 ( 0.61)
89.62 ( 1.69)
84.14 ( 1.06)
35.98 ( 1.91)
71.61 ( 0.66)
64.29 ( 2.20)
11.96 ( 1.38)
17.46 ( 1.20)
4.78 ( 1.38)
88.12 ( 0.69)
86.35 ( 0.68)
83.97 ( 0.69)
77.65 ( 1.18)
25.12 ( 1.38)
56.46 ( 0.69)
43.53 ( 2.35)
4v
100
50
100
100
50
4w
Hym

H. Xu, X. Zeng / Bioorg. Med. Chem. Lett. 20 (2010) 4193–4195
4195
Table 2
The EC50 values of seven potential compounds
a
Compd
EC50
(l
g/mL)
F. graminearum
P. oryzae
F. oxysporium
A. alternata
A. brassicae
B. sorokinianum
F. oxysporium
f. sp. vasinfectum
f. sp. cucumarinum
4a
4b
4f
4n
4q
4u
4v
29.51
25.02
21.50
25.77
47.51
28.62
30.93
36.12
15.15
9.72
18.41
14.98
19.58
21.85
49.14
18.14
14.15
33.50
12.51
9.32
8.11
11.67
20.78
6.96
11.19
25.10
18.46
9.42
11.95
13.52
14.60
9.46
15.01
13.08
28.14
17.36
35.66
8.54
26.89
23.05
16.00
18.12
24.18
13.57
18.85
44.47
10.51
13.56
27.25
12.38
13.45
22.34
11.88
65.71
17.80
54.07
Hym
a
50% Effective concentration: concentration of compound that inhibits the fungi growth by 50%.
the left phenyl ring of 4b gave less potent compound 4j. Therefore,
according to the preliminary SAR study, the potential lead struc-
tures (III and IV, R = H, Me, and Cl) for phenyl-azo phenol deriva-
tives were shown in Figure 2.
In conclusion, 23 phenyl-azo phenol derivatives were synthe-
sized and evaluated in vitro for their antifungal activities against
seven phytopathogenic fungi. Among of all the derivatives, some
compounds showed good antifungal activities at the concentra-
R
R
N
N
N
N
Me
IV
OH
OH
R = H, Me, Cl
Figure 2. The potential lead structures III and IV.
III
tions of 100 and 50 lg/mL. Especially compounds 4a, 4b, 4f, 4n,
4q, 4u, and 4v exhibited the more promising and pronounced anti-
fungal activities than hymexazol, a commercially available agricul-
tural fungicide. It indicated that 4-((un)substituted phenylazo)-
phenol (III) and 4-((un)substituted phenylazo)-3-methylphenol
(IV) might be considered as new promising lead candidates for fur-
ther design and synthesis of agricultural fungicides.
compounds still exhibited potent activities (e.g., 4b, 4f, 4j, 4n, and
4v). On the contrary, introduction of another hydroxyl group at the
2-position on the right phenyl ring of 4e gave 4w, the correspond-
ing antifungal activities of which were decreased sharply as com-
pared with 4e. For example, the inhibition rates of 4e and 4w at
100 lg/mL against F. graminearum, P. oryzae, F. oxysporum f. sp. vas-
infectum, A. alternata, A. brassicae, B. sorokinianum, and F. oxysporum
f. sp. cucumarinum were 45.01%/40.98%, 79.19%/27.09%, 70.67%/
27.96%, 79.45%/35.98%, 73.08%/37.97%, 70.57%/25.12%, and
77.74%/25.39%, respectively. Subsequently, compounds 4a, 4b, 4f,
Acknowledgments
This work was financially supported in part by Grants from the
Program for New Century Excellent University Talents, State Edu-
cation Ministry of China (NCET-06-0868), the Key Project of Chi-
nese Ministry of Education (No. 107105), and the Research Fund
for the Doctoral Program of Higher Education of China (No.
20070712025).
4n, 4q, 4u, and 4v showing the higher inhibition rates at 100
lg/
mL were further bioassayed at 50 g/mL. The results showed that
l
the seven compounds still displayed the more potent antifungal
activities than the commercially available agricultural fungicide
hymexazol (Table 1).
Finally, EC50 values of 4a, 4b, 4f, 4n, 4q, 4u, and 4v were calcu-
lated for seven phytopathogenic fungi. As shown in Table 2, com-
pounds 4a, 4b, 4f, 4n, 4u, and 4v were all more potent than
hymexazol. Compound 4f showed the most potent antifungal
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2010.05.048.
activity against F. graminearum with EC50 value of 21.50
compound 4b exhibited the most potent antifungal activity against
P. oryzae with EC50 value of 9.72 g/mL; compound 4v showed the
most potent antifungal activity against F. oxysporum f. sp. vasinfec-
tum with EC50 value of 14.15 g/mL; compound 4u was the most
potent derivative against A. alternata with EC50 value of 6.96 g/
lg/mL;
l
References and notes
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