Natural products-based insecticidal agents 7. Semisynthesis and insecticidal activity of novel 4α-alkyloxy-2-chloropodophyllotoxin derivatives against Mythimna separata Walker in vivo

Article abstract of DOI:10.1016/j.bmcl.2010.07.050

In continuation of our program aimed at the discovery and development of natural products-based insecticidal agents, 16 novel 4α-alkyloxy-2- chloropodophyllotoxin derivatives were semisynthesized from podophyllotoxin, and preliminarily evaluated for their

Full text of DOI:10.1016/j.bmcl.2010.07.050

Bioorganic & Medicinal Chemistry Letters 20 (2010) 5009–5012
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Natural products-based insecticidal agents 7. Semisynthesis and insecticidal
activity of novel 4a-alkyloxy-2-chloropodophyllotoxin derivatives against
Mythimna separata Walker in vivo
*
Hui Xu , Xiao Xiao, Qing-tian Wang
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:
In continuation of our program aimed at the discovery and development of natural products-based insec-
Received 5 May 2010
Revised 19 June 2010
Accepted 13 July 2010
Available online 16 July 2010
ticidal agents, 16 novel 4a-alkyloxy-2-chloropodophyllotoxin derivatives were semisynthesized from
podophyllotoxin, and preliminarily evaluated for their insecticidal activity against the pre-third-instar
larvae of Mythimna separata Walker in vivo. Among all the tested derivatives, especially compounds
4b, 4e, 4g, and 4p exhibited more promising and pronounced insecticidal activity than toosendanin, a
commercial insecticide derived from Melia azedarach. Generally, it was obviously demonstrated that
the length of straight-chain or branched-chain alkyloxy, and heteroatom-containing cycloalkyloxy at
the C-4 position of 2-chloropodophyllotoxin were very important for the insecticidal activity.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Alkyloxy
Podophyllotoxin
2-Chloropodophyllotoxin
Semisynthesis
Insecticidal activity
The routine use of a wide variety of synthetic insecticides in
agriculture has now become an accepted practice, however, the
application of those chemicals over the years has led to the devel-
opment of resistance in insect pest populations and environmental
problems. Meanwhile, plant secondary metabolites result from the
interaction between plants and environment (life and non-life)
during the long period of evolution in plants. Consequently, the
discovery and development of new insecticidal compounds from
plant secondary metabolites, followed by using them as the lead-
compounds for further modification has recently been one of the
important ways for the research and development of new pesti-
cides. Podophyllotoxin (1, Fig. 1), a naturally occurring aryltetralin
lignan, besides its use as the lead-compound for the preparation of
potent anticancer drugs,1 also exhibited the interesting insecticidal
activity.2–4
More recently, the insecticidal activity of 4b-benzenesulfona-
mides of podophyllotoxin,5 40-aromatic esters/substituted ben-
zenesulfonates of 4-deoxypodophyllotoxin,6,7 and 4
a-acyloxy-2-
O
OH
O
O
R
O
O
O
3
O
4
O
O
O
O
O
O
O
1
O
O
O
O
2
1'
O
Cl
Cl
2'
O
O
O
6'
H CO
OCH
3
3
4'
OCH
H CO
3
OCH
3
H CO
3
OCH
H CO
3
OCH
3
3
3
OCH
4a
OCH
2
OCH
3
3
3
3
1
Figure 1. Chemical structures of podophyllotoxin (1), 4
pyranylpodophyllotoxin (4a).
a-acyloxy-2-chloropodophyllotoxins (2), 4-O-tetrahydropyranylpodophyllotoxin (3), and 2-chloro-4-O-tetrahydro-
* Corresponding author. Tel./fax: +86 29 87091952.
E-mail address: orgxuhui@nwsuaf.edu.cn (H. Xu).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.07.050

5010
H. Xu et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5009–5012
chloropodophyllotoxins (2, Fig. 1)8 has been studied in our re-
search group, and some derivatives have showed the potent insec-
ticidal activity. During investigation of structure–insecticidal
activity relationships of 2, interestingly, 2-chloro-4-O-tetrahydro-
pyranylpodophyllotoxin (4a, Fig. 1), an intermediate of 2, exhibited
more potent insecticidal activity than 4-O-tetrahydropyranylpodo-
phyllotoxin (3, Fig. 1) and toosendanin, a commercial insecticide
derived from Melia azedarach.8 That is, introduction of chlorine
atom at the 2b position of 3 led to the more potent compound
4a. This encouraging result, therefore, prompted us in present Let-
ter to further study other 4-alkyloxy derivatives of 2-chloropodo-
phyllotoxin as insecticidal agents.
Sixteen novel 4a-alkyloxy-2-chloropodophyllotoxin derivatives
(4b–q) were synthesized from podophyllotoxin (1) as outlined in
Scheme 1. The 4-OH group of 1 was firstly protected by a tetrahy-
dropyranyl (THP) group in the presence of phosphorus oxychloride
(POCl3) and dihydropyran (DHP) at room temperature to give 4-O-
tetrahydropyranylpodophyllotoxin (3) in a 92% yield.9 2-Chloro-4-
O-tetrahydropyranylpodophyllotoxin (4a) was then prepared by
treatment of 3 with lithium diisopropylamide (LDA) at ꢀ78 °C in
dry THF, followed by the stereoselective reaction with hexachloro-
ethane. Subsequently, hydrolysis of the THP group of 4 afforded 2-
chloropodophyllotoxin (5).10 Finally, 16 novel 4
a-alkyloxy-2-chlo-
Figure 2. The X-ray crystallography of compound 4h.
ropodophyllotoxin derivatives (4b–q) were obtained in 13–78%
yields by reaction of 5 with the corresponding alcohols in the pres-
ence of BF3ꢁEt2O. The structures of the target compounds were well
characterized by 1H NMR, HRMS, mp, and IR (see Supplementary
data).
ture of 4h was determined by X-ray crystallography as illustrated
in Figure 2.12 It was clearly demonstrated that the 2-chloro and
the 4-n-butoxy groups of 4h adopted the b and
a configuration,
The assignment of configuration of C-4 position was based on
J3.4 coupling constants. The C-4b-substituted compounds have a
J3.4 ꢂ 4.0 Hz due to a cis relationship between H-3 and H-4. If
J3.4 = 10.0 Hz, it indicates that H-3 and H-4 is trans relationship,
respectively.
The insecticidal activity of 4b–q against the pre-third-instar lar-
vae of Mythimna separata Walker in vivo was screened by the leaf-
dipping method at the concentration of 1 mg/mL. Compound 4a,
and the substituent on the C-4 position of podophyllotoxin is
a
and toosendanin,
a commercial insecticide derived from M.
configuration.11 For example, the J3.4 value of H-4 of 4d was
azedarach, were used as positive controls.8
9.2 Hz, therefore, the hydroxylethoxy group on the C-4 position
The corrected mortality rates of M. separata caused by 4b–q
with the advance of time were shown in Figure 3. The correspond-
ing mortality rates after 35 d were far higher than those after 10 d
and 20 d. That is, these compounds, different from those conven-
of 4d was
a configuration.
In order to obtain precise three-dimensional structural informa-
tion and absolute configuration of 4b–q, the single-crystal struc-
O
O
O
O
O
O
O
C2Cl6 (1.4 equiv)
-78 oC/4 h
DHP/POCl3
r.t./3 h
O
LDA (1.2 equiv)/THF
-78 oC/15 min
O
O
1
O-Li+
O
92%
H3CO
OCH3
OCH3
H3CO
OCH3
OCH3
3
O
O
OH
OR
O
O
O
O
O
O
O
O
ROH (4 equiv)
.
conc. HCl/ THF
(v/v:1/9), r.t./2 h
O
BF3 Et2O/DCM
Cl
Cl
O
O
-15 oC to r.t.
3-72 h
13-78%
Cl
O
59% from 3 to 5
H3CO
OCH3
OCH3
4a
H3CO
OCH3
H3CO
OCH3
OCH3
4b-q
OCH3
5
Scheme 1. The synthetic route of 4a-alkyloxy-2-chloropodophyllotoxins (4b–q).

H. Xu et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5009–5012
5011
Table 1
Insecticidal activity of novel 4
separata in vivo
a-alkyloxy-2-chloropodophyllotoxins (4b–q) against M.
R
O
O
O
O
Cl
O
H CO
3
OCH
3
OCH
3
4a-q
Compounds
R
Corrected mortality rate (%)
10 d
Tetrahydropyranyl 23.3
( 12.5)
20 d
35 d
4a
33.3 ( 4.7)
66.7 ( 4.7)
Figure 3. The corrected mortality rates of M. separata caused by 4b–q with the
increase of time.
4b
4c
4d
4e
4f
4g
4h
4i
Me
Et
CH2CH2OH
CH2CH2OMe
n-Pr
i-Pr
n-Bu
MeCHEt
31.0 ( 9.4)
17.2 ( 0)
13.8 ( 4.7)
41.4 ( 4.7)
6.9 ( 8.2)
13.8 ( 4.7)
17.2 ( 0)
13.8
42.9 ( 4.7)
28.6 ( 9.4)
25.0 ( 0)
39.3 ( 4.7)
25.0 ( 8.2)
28.6 ( 4.7)
21.4 ( 4.7)
17.9 ( 9.4)
63.0 ( 4.7)
44.4 ( 0)
48.2 ( 9.4)
59.3 ( 4.7)
40.7 ( 4.7)
63.0 ( 4.7)
44.4 ( 0)
tional neurotoxic insecticides, such as organophosphates, carba-
mates, and pyrethroids, showed delayed insecticidal activity.5–8
For example, the corrected mortality rate of 4g against M. separata
after 10 d was only 13.8%, after 20 d the corresponding mortality
rate was increased to 28.6%, but after 35 d the corresponding mor-
tality rate was rapidly increased to 63%, which was more than four
times of the mortality rate after 10 d (Table 1). Meanwhile, the
symptoms of the tested M. separata were also characterized by
the same way as our previous reports.5–8 For example, the pupa-
tion of the larvae and the adult emergence of M. separata were
inhibited by these compounds, therefore, the stage from the larvae
to adulthood of M. separata was prolonged as compared with the
control group. Moreover, many larvae of the treated groups molted
to abnormal pupae, which could not reach adulthood and died dur-
ing the stage of pupation because they were not able to remove
their pupal skin.
48.2 ( 4.7)
( 12.5)
4j
4k
(CH2)2CH(Me)2
n-Octyl
10.3 ( 9.4)
10.3
14.3 ( 8.2)
28.6 ( 4.7)
40.7 ( 4.7)
51.9 ( 9.4)
( 12.5)
4l
(CH2)5CH(Me)2
17.2
17.9 ( 4.7)
48.2 ( 4.7)
( 16.3)
4m
4n
4o
4p
4q
n-C18H37
20.7 ( 9.4)
27.6 ( 8.2)
20.7 ( 17)
17.2 ( 8.2)
13.8
21.4 ( 4.7)
25.0 ( 8.2)
28.6 ( 9.4)
17.9 ( 4.7)
17.9 ( 9.4)
40.7 ( 4.7)
48.2 ( 9.4)
44.4 ( 8.2)
55.6 ( 8.2)
40.7
Cyclopentyl
Cyclohexyl
CH2Ph
CH2Ph(p-NO2)
( 12.5)
( 12.5)
48.2 ( 9.4)
1
—
—
—
20.7
( 12.5)
10.7 ( 9.4)
25.0 ( 8.2)
3
14.3
( 14.1)
35.7 ( 8.2)
51.9
( 12.5)
51.9 ( 9.4)
Through a comparative study on the relationship between the
chemical structures of 4b–q and the insecticidal activity as out-
lined in Table 1, some interesting results were found as follows:
(1) Generally, when methoxy, methoxyethoxy, isopropyloxy, and
benzyloxy moieties were introduced at the C-4 position of 2-chlo-
ropodophyllotoxin, respectively, the corresponding compounds 4b,
4e, 4g, and 4p, the activity of which could comparable to that of
Toosendanin
25.0 ( 0)
Introduction of benzyloxy moiety at the C-4 position of 2-chlorop-
odophyllotoxin gave the more potent compound than that bearing
(p-nitro)benzyloxy one at the C-4 position (4p 55.6% vs 4q 40.7%).
some 4a
-acyloxy derivatives of 2b-chloropodophyllotoxin,8 dis-
In conclusion, 16 novel 4a-alkyloxy-2-chloropodophyllotoxin
derivatives were semisynthesized from podophyllotoxin, and pre-
liminarily evaluated for their insecticidal activity against the pre-
third-instar larvae of M. separata in vivo. Among all the tested
derivatives, especially compounds 4b, 4e, 4g, and 4p exhibited
more promising and pronounced insecticidal activity than too-
sendanin. Generally, it was clearly confirmed that the length of
straight-chain or branched-chain alkyloxy, and heteroatom-con-
taining cycloalkyloxy at the C-4 position of 2-chloropodophyllo-
toxin were very important for the insecticidal activity.
played more promising and pronounced insecticidal activity than
toosendanin. (2) Interestingly, the length of the side chain (for n-
alkyloxy and isopropyloxy series) at the C-4 position of 2-chlorop-
odophyllotoxin was very essential for its insecticidal activity. In
general, as the length of the side chain at the C-4 position in-
creased, the corresponding activity was reduced (4b vs 4c, 4f, 4h,
4k, and 4m; 4g vs 4i, 4j, and 4l). For example, to isopropyloxy ser-
ies, the final mortality rates of 4i, 4j, and 4l were 48.2%, 40.7%, and
48.2%, respectively, while the final mortality rate of 4g was 63%. (3)
Introduction of methoxyethoxy moiety at the C-4 position of 2-
chloropodophyllotoxin produced the more potent compound than
that bearing hydroxylethoxy one at the C-4 position (4e 59.3% vs
4d 48.2%). It implied that the free hydroxyl group on the side chain
was not necessary for the insecticidal activity. (4) No significant
differences in the activity were observed between 4n and 4o,
which contained cyclopentyloxy and cyclohexyloxy groups at the
C-4 position of 2-chloropodophyllotoxin, respectively. However,
introduction of oxygen atom at the cyclohexyl ring of 4o led to
the more potent compound 4a (4a 66.7% vs 4o 44.4%). This
interesting result will encourage us to further investigate the het-
Acknowledgments
This work was financially supported by the Program for New
Century Excellent University Talents, State Education Ministry of
China(NCET-06-0868), the Research Fund for the Doctoral Program
of Higher Education of China (No. 20070712025), and the Fok Ying
Tong Education Foundation for Young Talents (No. 121032).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2010.07.050.
eroatom-containing 4
odophyllotoxin derivatives as the insecticidal agents in future. (5)
a-cyclopentyloxy/cyclohexyloxy-2-chlorop-

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H. Xu et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5009–5012
7. Xu, H.; Wang, J. J. Bioorg. Med. Chem. Lett. 2010, 20, 2500.
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