Insight into dihalogenation of E-ring of podophyllotoxins, and their acyloxyation derivatives at the C4 position as insecticidal agents

Article abstract of DOI:10.1016/j.bmcl.2013.08.044

Unexpected sequential E-ring dihalogenation of podophyllotoxin analogues is reported. It demonstrated that a chlorine/bromine atom was prior introduced at the C2′ position of podophyllotoxin, and the corresponding free rotation of E-ring around the C1-C1′

Full text of DOI:10.1016/j.bmcl.2013.08.044

Bioorganic & Medicinal Chemistry Letters 23 (2013) 5592–5598
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Insight into dihalogenation of E-ring of podophyllotoxins, and their
acyloxyation derivatives at the C4 position as insecticidal agents
Zhiping Che a,b, Xiang Yu a, Lingling Fan a, Hui Xu a,
⇑
a Laboratory of Pharmaceutical Design & Synthesis, College of Sciences, Northwest A&F University, Yangling 712100, Shaanxi Province, China
b College of Forestry, Henan University of Science and Technology, Luoyang 471003, Henan Province, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Unexpected sequential E-ring dihalogenation of podophyllotoxin analogues is reported. It demonstrated
that a chlorine/bromine atom was prior introduced at the C20 position of podophyllotoxin, and the cor-
responding free rotation of E-ring around the C1–C10 bond of 20-chloro or 20-bromopodophyllotoxin
was restricted. When 20-chloro or 20-bromopodophyllotoxin reacted with N-chlorosuccinimide (NCS),
the chlorine atom was regioselectively introduced at their C60 position on the E-ring. Whereas 20-chloro
or 20-bromopodophyllotoxin reacted with NBS, the bromine atom was regioselectively introduced at their
C5 position on the B-ring. When 20-chloropodophyllotoxin reacted with different carboxylic acids in the
presence of BF3ꢀEt2O, the steric effect of its E-ring for stereoselective synthesis of 4b-acyloxy-20-chlorop-
odophyllotoxin derivatives was observed. The insecticidal activity of 20(20,60)-(di)halogen-substituted
podophyllotoxin derivatives were evaluated with Mythimna separata Walker.
Received 20 June 2013
Revised 20 July 2013
Accepted 8 August 2013
Available online 17 August 2013
Keywords:
Podophyllotoxin
Halogenation
Insecticidal activity
Botanical insecticide
Structural modification
Mythimna separata Walker
Ó 2013 Elsevier Ltd. All rights reserved.
Podophyllotoxin (1a, Scheme 1), a naturally occurring cyclolig-
nan, is the main secondary metabolite isolated from the roots and
rhizomes of Podophyllum species such as P. hexandrum and
P. peltatum. Besides its use as the lead compound for preparation
of potent anticancer drugs such as etoposide, teniposide and etopo-
side phosphate,1 compound 1a also exhibited the interesting insec-
ticidal,2 antifungal3 and antiviral activities.4 Consequently, total
synthesis5 and structural modifications6 of 1a and its derivatives
have been carried out in many research group worldwide. More re-
E-ring dihalogenation of podophyllotoxin analogues has not been
reported.
As shown in Table 1, the reaction of different podophyllotoxin
analogues such as 1a, epipodophyllotoxin (1b), picropodophyllo-
toxin (1c) and 1d, with N-chlorosuccinimide (NCS) was examined
first. When 0.4 mmol of 1a reacted with 0.46 mmol of NCS at
0–28 °C, 20-chloropodophyllotoxin (2a) and 20,60-dichloropodo-
phyllotoxin (3a) were obtained in 79% and 16% yields, respectively
(entry 1). If the amount of NCS was increased to 2 equiv of 2a, only
3a was obtained in a 90% yield (entry 2). Accordingly, when 1b or
1c reacted with NCS, the same results were also observed (entries
3–6). Interestingly, when 1d reacted with NCS (1 or 2 equiv) at
0–28 °C, only monochlorination product, 20-chloro-40-demethyle-
pipodophyllotoxin (2d), was obtained (entries 7 and 8). It is note-
worthy that when the time of 1d reacting with 2 equiv of NCS was
prolonged from 4 to 7 h, the corresponding yield of 2d was sharply
decreased from 81% to 0% (entry 8 vs 9). Because NCS is also a mild
oxidant besides its use as a chlorination agent, and 40-hydroxy
group of 2d was very sensitive to the oxidant, so 2d was further
oxidized by excessive NCS. Compound 1d reacting with 2 equiv
of NCS at 0 °C was also investigated (entry 10). Although the oxida-
tion process was delayed at 0 °C, and the reaction time was
prolonged to 12 h, its dichlorination product, 20,60-dichloro-40-
demethylepipodophyllotoxin (3d), was still not obtained, and only
2d was obtained in a 77% yield. The X-ray crystal structures of 2a,
2b, and 3a were shown in Figure 1.
cently, we have studied stereoselective synthesis of 4
a-acyloxy/
alkyloxy-2b-chloropodophyllotoxins and 4 -acyloxy/alkyloxy-2a/
a
b-bromopodophyllotoxins (I) [Scheme 1, Eq. (1)],7 and found some
derivatives showed more potent insecticidal activity than toosend-
anin, a commercial insecticide isolated from Melia azedarach. Based
upon the above-meantioned results, we envisioned that when the
halogen atom at the C2 position on the C-ring of I was transferred
to its C20 or C20 and C60 position on the E-ring to afford 4-acyloxy-
20(20,60)-(di)halogen-substituted podophyllotoxin derivatives (II)
[Scheme 1, Eq. (2)], whether the corresponding insecticidal activity
of II could be improved. To our knowledge, although some exam-
ples of E-ring monohalogenation of 1a and 40-demethylepipodo-
phyllotoxin (1d, Table 1) were pioneeringly reported by Ayres
et al., Hu et al., Kofod et al., and Emmenegger et al., respectively,8
⇑
Corresponding author. Tel./fax: +86 (0)29 87091952.
E-mail address: orgxuhui@nwsuaf.edu.cn (H. Xu).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.08.044

Z. Che et al. / Bioorg. Med. Chem. Lett. 23 (2013) 5592–5598
5593
Previous work
O
OR or OCOR
O
OH
OH
4
3
O
O
C
1
D
A
B
O
ROH or RCO2H
BF3.Et2O
O
ref. [7]
X = Cl, Br
(1)
O
O
2
O
X
X
O
O
O
E
MeO
OMe
MeO
OMe
MeO
OMe
OMe
OMe
1a
OMe
OH
I
This work
OCOR
OH
O
O
O
O
O
O
O
RCO2H
O
NBS or NCS
O
(2)
O
X
BF3.Et2O
O
X
1'
O
Y
Y
2'
6'
?
MeO
OMe
MeO
OMe
MeO
OMe
4'
OMe
OMe
II
OMe
X = Cl, Br; Y = H, Cl
Scheme 1. The chemical structures of compounds I and II.
Table 1
Investigation of 1a–d reacting with NCSa
R1
R1
R1
5
6
4
3
O
O
O
O
O
A
B
8
O
D
O
C
2
O
NCS/DMF
0-28 o
O
7
+
1
R2
R2
R2
C
1'
O
O
O
2'
6'
Cl
Cl
Cl
E
3'
5'
MeO
OMe
MeO
OMe
4'
OR3
MeO
OMe
OR3
2a-d
OR3
3a-d
1a-d
a
c
b
:
R1
R1
=
α−OH; R2
α−OH; R2
=
=
β−H; R3 = Me
α−H; R3 = Me
:
R1
R1
=
=
β−OH; R2
=
=
β−H; R3 = Me
β−H; R3 = H
β−OH; R2
:
=
d
:
Entry
Compd
Amount of (mmol)
t (h)
Isolated yield (%)
1
NCS
2
3
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.46
0.8
0.46
0.8
0.46
0.8
0.46
0.8
24
24
24
24
24
24
16
4
2a (79)
2a (0)
2b (81)
2b (0)
2c (78)
2c (0)
2d (85)
2d (81)
2d (0)
3a (16)
3a (90)
3b (10)
3b (91)
3c (17)
3c (94)
3d (0)
3d (0)
3d (0)
3d (0)
0.8
0.8
7
10
12b
2d (77)
a
A solution of NCS in DMF (4 mL) was added dropwise to 1 in DMF (4 mL) at 0 °C. After adding, the mixture was allowed to warm from 0 to 28 °C.
The reaction temperature was 0 °C.
b
Subsequently9, 1a reacting with N-bromosuccinimide (NBS)
chemical structure of which was determined by the X-ray crystal-
lography (Fig. 3),9 was obtained in a 40% yield. On the contrary,
when 2a reacted with 1 equiv of NBS at 0–16 °C for 24 h, the ex-
pected 20-chloro-60-bromopodophyllotoxin (9) was not obtained.
However, besides the oxidized product, 20-chloropodophyllotox-
one (8, 43% yield), which was confirmed by the X-ray crystallogra-
phy (Fig. 3),9 the unexpected 20-chloro-5-bromopodophyllotoxin
(7, 12% yield) was obtained. The configuration of 7 was confirmed
according to the X-ray crystallography9 of its ester,
was examined in Scheme 2. When 1a reacted with NBS (1 equiv)
at 0–16 °C for 20 h, 20-bromopodophyllotoxin (2a0) and 20-bromop-
odophyllotoxone (4), the chemical structures of which were deter-
mined by the X-ray crystallography (Fig. 2),9 were obtained in 88%
and 7% yields, respectively. Interestingly, when 1a reacted with 2
equiv of NBS at 0–16 °C for 46 h, besides 2a0 (33% yield) and 4
(28% yield), an unusual 20,5-dibromopodophyllotoxin (5, 15% yield,
Fig. 2)9 was obtained. However, the expected 20,60-dibromopodo-
phyllotoxin (50) was not produced. We assumed that the cavity be-
tween the B-ring and E-ring of 2a0 was not big enough for
introduction of a second bromine atom at the C60 position. There-
fore, the second bromine atom was regioselectively introduced at
the C5 position on the B-ring of 2a0.
20-chloro-5-bromo-4
a
-propionyloxypicropodophyllotoxin
(70,
Fig. 3), which was prepared by the reaction of 7 with propionic acid
in the presence of N,N0-dicyclohexylcarbodiimide (DCC) and 4-
N,N0-dimethylaminopyridine (DMAP) at 28 °C for 29 h. It also dem-
onstrated that the trans-lactone of 7 was easily transferred into the
cis-lactone in the presence of DMAP (a weak base). Although the E-
ring of podophyllotoxin is free to rotate around its C1–C10 bond, we
here, based upon the above results, demonstrated for the first time
The cross-reaction of 2a0 with NCS, or 2a with NBS, was further
investigated (Scheme 3). When 2a0 reacted with 1 equiv of NCS at
0–16 °C for 24 h, only 20-bromo-60-chloropodophyllotoxin (6), the

5594
Z. Che et al. / Bioorg. Med. Chem. Lett. 23 (2013) 5592–5598
that free rotation of E-ring around the C1–C10 bond of 20-chloro or
20- bromopodophyllotoxin is restricted. On the other hand, it fur-
ther suggested that (a) a chlorine or bromine atom was preferen-
tially introduced at the C20 position of podophyllotoxin according
to X-ray crystallography; (b) once a bromine atom was firstly
introduced at the C20 position of podophyllotoxin, its cavity be-
tween the B-ring and E-ring of 2a0 was big enough for sequential
introduction of a chlorine atom at the C60 position to afford 6
(Scheme 3), and not big enough for introduction of a bromine atom
at the C60 position (Scheme 2); (c) similarly, once a chlorine atom
was firstly introduced at the C20 position of podophyllotoxin, its
cavity between the B-ring and E-ring of 2a was also big enough
for sequential introduction of a chlorine atom at the C60 position
to give 3a (Table 1), and not big enough for introduction of a bro-
mine atom at the C60 position (Scheme 3); (d) when 20-chloro or
20-bromopodophyllotoxin further reacting with NBS, the bromine
atom was regioselectively introduced at their C5 position on the
B-ring.
On the other hand, 4b-acyloxypodophyllotoxins (10a and 10b)
reacting with NCS was also examined. As described in Table 2,
when 0.2 mmol of 10a reacted with 0.23 mmol of NCS at 28 °C or
40 °C for 48 h, no product was obtained. When 0.2 mmol of 10a
reacted with 0.23 mmol of NCS at 50 °C for 48 h,
20-chloro-4b-n-butanoyloxypodophyllotoxin (11a) was obtained
in a 41% yield. When at 60 °C for 24 h, besides 11a (72% yield),
the dichlorination product, 20,60-dichloro-4b-n-butanoyloxypodo-
phyllotoxin (12a), was also obtained (12% yield). When 0.2 mmol
of 10a reacted with 0.4 mmol of NCS at 60 °C for 24 h, the yields
of 11a and 12a were 31% and 47%, respectively. The configuration
of 11a was determined by the X-ray crystallography (Fig. 4). Sim-
ilarly, when 0.2 mmol of 10b reacted with 0.23 mmol of NCS at
60 °C for 24 h, 20-chloro-4b-n-octanoyloxypodophyllotoxin (11b)
and 20,60-dichloro-4b-n-octanoyloxypodophyllotoxin (12b) were
obtained in 68% and 8% yields, respectively. Obviously, the reaction
conditions for chlorination of 4b-acyloxypodophyllotoxins in the
presence of NCS would be more harsh as compared with those
for 1a–d (Table 1).
Meanwhile, as shown in Table 3, monochlorination products
11a and 11b further reacting with NCS was also investigated. For
example, when 0.2 mmol of 11a reacted with 0.23 mmol of NCS
at 50 °C for 48 h, 12a was obtained only in a 9% yield, and when
at 60 °C for 48 h, 12a was obtained in a 31% yield. When 0.2 mmol
of 11a reacted with 0.3 mmol of NCS at 60 °C for 48 h, the yield of
12a was improved to 50%. Interestingly, when the molar ratio of
NCS and 11a was 2/1 at 60 °C for 24 h, 12a was smoothly obtained
in a 77% yield; whereas at 50 °C for 48 h, 12a was afforded only in a
47% yield. In addition, when 0.2 mmol of 11b reacted with
0.4 mmol of NCS at 60 °C for 24 h, 12b was given in a 72% yield.
Figure 1. X-ray crystal structures of 2a (top), 2b (middle) and 3a (bottom).
O
OH
O
O
O
O
O
O
O
O
+
Br
Br
MeO
OMe
MeO
OMe
NBS (1 equiv)
20 h
OMe
7%
OMe
88%
4
2a'
DMF
0-16 o
Br OH
OH
1a
C
O
O
O
NBS (2 equiv)
46 h
D
O
O
A
B
C
2a'
+
4
+
O
33%
28%
O
O
Br
Br
Br
E
X
MeO
OMe
MeO
OMe
5
OMe
15%
OMe
5'
Scheme 2. Investigation of 1a reacting with NBS.

Z. Che et al. / Bioorg. Med. Chem. Lett. 23 (2013) 5592–5598
5595
Figure 2. X-ray crystal structures of 2a0 (top), 4 (middle) and 5 (bottom).
OH
Figure 3. X-ray crystal structures of 6 (top), 70 (middle), and 8 (bottom).
O
O
NCS (1 equiv)/DMF
O
2a'
0-16 oC/24 h
40%
O
Therefore, the present dichlorination reaction depended on the
reaction temperature and the amount of NCS.
Cl
Br
MeO
OMe
As shown in Table 4, when 2a reacted with different carboxylic
OMe
6
.
acids (except R as Me or Et) in the presence of BF3 Et2O, 4b-acyloxy-
O
20-chloropodophyllotoxin derivatives (13c–n) were stereoselec-
tively obtained in 41–82% yields. It may be due to the steric
hindrance of E-ring of 20-chloropodophyllotoxin, 13c–n were stere-
oselectively synthesized by the SN1 reaction. The assignment of
configuration of C4 position of 13a–b was based on J3.4 coupling
constants: The C4b-substituted compounds have a J3.4 ꢁ4.0 Hz
due to a cis relationship between H3 and H4. If J3.4 =10.0 Hz, it indi-
cates that H3 and H4 is trans relationship, and the substituent at the
OH
Br
O
O
O
O
O
O
O
O
O
Cl
Br
Cl
MeO
OMe
MeO
OMe
EtCO2H
OMe
OMe
64%
DCC/DMAP
NBS (1 equiv)
DMF
X
7'
9
28 oC/29 h
O
a
configuration.10 For example, as
Br
OH
O
C4 position of podophyllotoxin is
2a
0-16 oC/24 h
O
O
described in Table 4, there were two H4 chemical shifts for 13a, and
two corresponding J3.4 values of H4 were 9.5 and 3.0 Hz, respec-
O
O
+
O
O
O
tively, so 13a was a mixture of
ing to the peak areas of two H4, the ratio of
was 1/1. Similarly, the ratios of and b isomers of 13b were 1/1.27.
Additionally, as the J3.4 values of H4 of 13c–n were 2.5 or 3.0 Hz, so
the acyloxy groups at the C4 position of 13c–n were all b
configuration.
a
and b isomers. Meanwhile, accord-
Cl
Cl
a
and b isomers of 13a
MeO
OMe
7
MeO
OMe
a
OMe
12%
OMe
43%
8
Scheme 3. Investigation of reaction of 2a0 with NCS, and 2a with NBS, respectively.

5596
Z. Che et al. / Bioorg. Med. Chem. Lett. 23 (2013) 5592–5598
Table 2
Investigation of 10a and 10b reacting with NCS
R
R
R
O
O
O
O
O
O
O
O
O
O
O
O
O
NCS/DMF
O
O
O
+
O
O
O
Cl
Cl
Cl
MeO
OMe
MeO
OMe
MeO
OMe
OMe
Me
OMe
12a,b
10a,b
11a,b
a: R = n-C3H7; b: R = n-C7H15
Amount of (mmol)
T (°C)
t (h)
Isolated yield (%)
10
NCS
11
12
10
0.2 (10a)
0.2 (10a)
0.2 (10a)
0.2 (10a)
0.2 (10a)
0.2 (10b)
0.23
0.23
0.23
0.23
0.4
28
40
50
60
60
60
48
48
48
24
24
24
11a (0)
11a (0)
11a (41)
11a (72)
11a (31)
11b (68)
12a (0)
12a (0)
12a (0)
12a (12)
12a (47)
12b (8)
10a (100)
10a (100)
10a (40)
10a (0)
10a (0)
10b (0)
0.23
When 3a reacted with different carboxylic acids in the presence
of BF3ꢀEt2O as shown in Table 5, only 14g–j and 14l were stereose-
lectively obtained. However, the relationships between the substit-
uents introduced at the C4 position of 14a–m and their
configuration at the C4 position is not very obvious. The assign-
ment of configuration of C4 position of 14a–m was based on the
above-mentioned Lee0s rule.10 The configuration of the substitu-
ents at the C4 position of 14a–m and their corresponding J3.4 cou-
pling constants were reported in Table 5.
Finally, the insecticidal activity of some E-ring monohalo-
genation or dihalogenation products of podophyllotoxin against
the pre-third-instar larvae of Mythimna separata Walker in vivo
was tested by the leaf-dipping method at the concentration of
1 mg mLꢂ1 2b
Toosendanin, a commercial insecticide isolated
.
from Melia azedarach, was used as the positive control. Leaves
treated with acetone alone were used as a blank control group.
For podophyllotoxin derivatives showed delayed insecticidal
activity in our previous papers,2b,2f,7 the insecticidal activity
of the tested compounds was recorded as the final mortality
rate. As shown in Table 6, many compounds exhibited equal
or higher insecticidal activity than toosendanin. Especially
compounds 14d and 14f showed the highest insecticidal activ-
ity. In general, introduction of halogen atom on the E-ring of
podophyllotoxins could lead to the more potent compounds
(1a–d vs 2a–d vs 3a–c). The trans-lactone of podophyllotoxin
derivatives is usually considered to be important for their
insecticidal activity. But the final mortality rate of 70 containing
cis-lactone was 59.3%, so introduction of bromine and chlorine
atoms on the B- and E-rings could improve the insecticidal
activity. The insecticidal activity of 3a containing 20,60-dichloro
Figure 4. X-ray crystal structure of 11a.
Table 3
Investigation of 11a and 11b reacting with NCS
R
R
O
O
O
O
O
O
O
O
O
O
NCS/DMF
O
O
Cl
atoms, was more pronounced than that of
6 containing
Cl
Cl
20-bromo and 60-dichloro atoms. Similarly, the insecticidal
activity of 12b containing 20,60-dichloro atoms, was more
promising than that of 11b containing 20-chloro atom. To
alkylacyloxy series, the proper length of the side chain at the
C-4 position is very important for the insecticidal activity.
However, to alkylacyloxy series, the effect of the configuration
of acyloxy at the C-4 position on the insecticidal activity was
not very obvious.
In summary, unexpected sequential E-ring double halogena-
tion of podophyllotoxin analogues was investigated. Dichlori-
nation of 40-demethylepipodophyllotoxin in the presence of
NCS was affected by its 40-hydroxyl group. It should be pointed
MeO
OMe
MeO
OMe
OMe
OMe
a
:
b
: R = n-C7H15
R = n-C3H7;
T (°C)
11a,b
12a,b
Amount of (mmol)
NCS
t (h)
Isolated yield (%)
12 11
11
0.2 (11a)
0.2 (11a)
0.2 (11a)
0.2 (11a)
0.2 (11a)
0.2 (11b)
NCS (0.23)
NCS (0.23)
NCS (0.3)
NCS (0.4)
NCS (0.4)
NCS (0.4)
50
60
60
50
60
60
48
48
48
48
24
24
12a (9)
11a (67)
11a (43)
11a (20)
11a (30)
11a (0)
12a (31)
12a (50)
12a (47)
12a (77)
12b (72)
11b (0)

Z. Che et al. / Bioorg. Med. Chem. Lett. 23 (2013) 5592–5598
5597
Table 4
Investigation of 2a reacting with carboxylic acids in the presence of BF3.Et2O
OH
OCOR
O
O
O
3
O
O
BF3.Et2O/CH2Cl2
4
O
-15 to 28 o
C
2
1
RCO2H
+
O
O
1~2 h
41~82%
Cl
E
Cl
MeO
OMe
MeO
OMe
OMe
2a
OMe
13a-n
Compound
R
dH-4 (ppm)
J3,4 (Hz)
Configuration
Isolated yield (%)
13a
13b
13c (11a)
13d (11b)
13e
13f
13g
13h
13i
13j
13k
13l
13m
13n
Me
Et
5.92/6.18
5.93/6.18
6.18
6.18
6.18
6.18
6.17
6.25
6.47
6.46
6.42
6.43
6.41
9.5/3.0
9.0/3.0
3.0
2.5
2.5
2.0
2.5
2.0
2.0
2.5
2.5
3.0
2.0
a
a
b
b
b
b
b
b
b
b
b
b
b
b
:b = 1:1
:b = 1:1.27
79
82
82
81
67
60
47
73
76
41
63
78
68
75
n-Propyl
n-Heptyl
n-(CH2)10CH3
n-(CH2)14CH3
(Z)-9-n-C17H33
CCl3
(m-NO2)Ph
(p-NO2)Ph
(m-Cl)Ph
Ph
(p-Me)Ph
(m-Me)Ph
6.43
3.0
Table 5
Investigation of 3a reacting with carboxylic acids in the presence of BF3.Et2O
OH
OCOR
O
O
O
3
O
BF3.Et2O/CH2Cl2
4
O
-15 to 28 o
C
O
2
1
RCO2H
+
O
O
1~2 h
43~74%
Cl
Cl
Cl
Cl
MeO
OMe
MeO
OMe
OMe
3a
OMe
14a-m
Compound
R
dH-4 (ppm)
J3,4 (Hz)
Configuration
Isolated yield (%)
14a
14b
14c
14d
14e
14f
14g
14h
14i
Me
Et
5.97/6.12
5.97/6.13
5.97/6.13
5.97/6.12
5.97/6.12
5.97/6.12
6.20
6.42
6.39
6.37
6.22/6.37
6.35
9.0/2.0
9.0/2.5
10.0/2.5
10.0/2.0
10.5/2.5
10.5/1.5
2.0
2.0
1.5
2.0
9.5/2.0
2.0
a
a
a
a
a
a
b
b
b
b
a
b
a
:b = 1:3.35
:b = 1:3
:b = 1:2.7
:b = 1:2.45
:b = 1:2.33
:b = 1:2.13
71
70
74
57
73
54
71
71
43
59
72
67
72
n-propyl
n-heptyl
n-(CH2)10CH3
(Z)-9-n-C17H33
CCl3
(m-NO2)Ph
(p-NO2)Ph
(m-Cl)Ph
Ph
14j
14k
14l
:b = 1:4.28
:b = 1:3.2
(p-Me)Ph
(m-Me)Ph
14m
6.22/6.37
10.0/2.5
out that no free rotation of E-ring around the C1–C10 bond of
20-chloro or 20-bromopodophyllotoxin was demonstrated here.
It is noteworthy that a chlorine or bromine atom prior intro-
duced at the C20 position of podophyllotoxin was confirmed
by X-ray crystallography. When 20-chloro or 20-bromopodo-
phyllotoxin further reacted with NCS, the chlorine atom was
regioselectively introduced at their C60 position on the E-ring.
Whereas 20-chloro or 20-bromopodophyllotoxin further reacted
with NBS, the bromine atom was regioselectively introduced
at their C5 position on the B-ring. To our delight, it is the first
time that regioselective bromination on the B-ring of podo-
phyllotoxin is reported. Moreover, the reaction of 20-chloropod-
ophyllotoxin or 20,60-dichloropodophyllotoxin with different
carboxylic acids in the presence of BF3ꢀEt2O was also investi-

5598
Z. Che et al. / Bioorg. Med. Chem. Lett. 23 (2013) 5592–5598
Table 6
References and notes
Insecticidal activity of some E-ring monohalogenation or dihalogenation products of
podophyllotoxin against M. separata on leaves treated with a concentration of 1 mg/
1. (a) Gupta, S.; Das, L.; Datta, A. B.; Poddar, A.; Janik, M. E.; Bhattacharyya, B.
Biochemistry 2006, 45, 6467; (b) You, Y. J. Curr. Pharm. Des. 2005, 11, 1695; (c)
Xu, H.; Lv, M.; Tian, X. Curr. Med. Chem. 2009, 16, 327; Gordaliza, M.; Garcia, P.
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441.
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Chem. 1999, 47, 5108; (b) Xu, H.; Wang, J.; Sun, H.; Lv, M.; Tian, X.; Yao, X.;
Zhang, X. J. J. Agric. Food Chem. 2009, 57, 7919; (c) Di, X.; Liu, Y.; Liu, Y.; Yu, X.;
Xiao, H.; Tian, X.; Gao, R. Pestic. Biochem. Physiol. 2007, 89, 81; (d) Gao, R.; Gao,
C.; Tian, X.; Yu, X.; Di, X.; Xiao, H.; Zhang, X. Pest Manag. Sci. 2004, 60, 1131; (e)
Wang, Y.; Shao, Y.; Wang, Y.; Fan, L.; Yu, X.; Zhi, X.; Yang, C.; Qu, H.; Yao, X.; Xu,
H. J. J. Agric. Food Chem. 2012, 60, 8435.
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4. (a) Castro, M. A.; del Corral, J. M. M.; Gordaliza, M.; Gomez-Zurita, M. A.; de la
Puente, M. L.; Betancur-Galvis, L. A.; Sierra, J.; Feliciano, A. S. Eur. J. Med. Chem.
2003, 38, 899; (b) VanVliet, D. S.; Tachibana, Y.; Bastow, K. F.; Huang, E. S.; Lee,
K. H. J. Med. Chem. 2001, 44, 1422.
5. (a) Stadler, D.; Bach, T. Angew. Chem., Int. Ed. 2008, 47, 7557; (b) Engelhardt, U.;
Sarkar, A.; Linker, T. Angew. Chem., Int. Ed. 2003, 42, 2487; (c) Reynolds, A. J.;
Scott, A. J.; Turner, C. I.; Sherburn, M. S. J. Am. Chem. Soc. 2003, 125, 12108; (d)
Mingoia, F.; Vitale, M.; Madec, D.; Prestat, G.; Poli, G. Tetrahedron Lett. 2008, 49,
760; (e) Clayden, J.; Kenworthy, M. N.; Helliwell, M. Org. Lett. 2003, 5, 831.
6. (a) Passarella, D.; Peretto, B.; Yepes, R. B.; Cappelletti, G.; Cartelli, D.; Ronchi, C.;
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Cappelletti, G.; Cartelli, D.; Peretto, B.; Ventura, M.; Riccioli, M.; Colombo, F.;
Snaith, J. S.; Borrelli, S.; Passarella, D. Tetrahedron 2011, 67, 7354; (c) Xiao, Z.;
Bastow, K.; Vance, J. R.; Sidwell, R. S.; Wang, H.; Chen, M.; Shi, Q.; Lee, K. H. J.
Med. Chem. 2004, 47, 5140; (d) Xiao, Z.; Bastow, K. F.; Vance, J. R.; Lee, K. H.
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mLa
Compound Final mortality rate (%) Compound
Final mortality rate (%)
1a
1b
1c
1d
2a
2b
2c
2d
3a
3b
3c
2a0
4
5
6
7
8
70
11a
11b
12a
12b
13a
13b
34.6 ( 3.3)
46.4 ( 0)
32.1 ( 0)
35.7 ( 0)
65.4 ( 0)
53.6 ( 3.3)
53.6 ( 3.3)
46.4 ( 0)
61.5 ( 6.7)
64.3 ( 3.3)
60.7 ( 3.3)
55.6 ( 0)
13e
13f
13g
13h
13i
13j
13k
13l
13m
13n
14a
14b
14c
14d
14e
14f
64.3 ( 3.3)
53.6 ( 3.3)
65.4 ( 5.8)
46.4 ( 0)
67.9 ( 0)
53.6 ( 3.3)
64.3 ( 3.3)
57.1 ( 0)
46.4 ( 0)
42.9 ( 3.3)
50.0 ( 3.3)
32.1 ( 3.3)
40.7 ( 3.3)
71.7 ( 3.3)
53.6 ( 3.3)
69.2 ( 3.3)
64.3 ( 3.3)
57.1 ( 0)
50.0 ( 3.3)
46.4 ( 0)
53.6 ( 3.3)
42.9 ( 3.3)
50.0 ( 3.3)
67.9 ( 0)
55.6 ( 0)
48.1 ( 3.3)
59.3 ( 3.3)
60.7 ( 3.3)
59.3 ( 3.3)
57.1( 0)
14g
14h
14i
14j
14k
14l
42.3 ( 0)
55.6 ( 6.7)
59.3 ( 3.3)
39.3 ( 3.3)
50.0 ( 3.3)
14m
Toosendanin 53.8 ( 0)
7. (a) Xu, H.; Wang, Q.; Guo, Y. Chem. Eur. J. 2011, 17, 8299; (b) Xu, H.; Xiao, X.;
Zhao, X.; Guo, Y.; Yao, X. Bioorg. Med. Chem. Lett. 2011, 21, 4008; (c) Xu, H.; Xiao,
X.; Wang, Q. Bioorg. Med. Chem. Lett. 2010, 20, 5009; (d) Guo, Y.; Fan, L.; Wang,
J.; Yang, C.; Qu, H.; Xu, H. Tetrahedron 2013, 69, 774.
a
Values are means S.D. of three replicate.
8. (a) Ayres, D. C.; Lim, C. K. J. Chem. Soc., Perkin Trans. I 1972, 1350; (b) Hu, H.;
Wang, Z. Q.; Liu, S. Y.; Cheng, Y. C.; Lee, K. H. J. Med. Chem. 1992, 35, 866; (c)
Kofod, H.; Jorgensen, C. Acta. Chem Scand 1955, 9, 1327; (d) Emmenegger, H.
Arzneim. Forsch. 1961, 11, 459.
9. Ten crystallographic data (excluding structure factors) for the structures of 2a,
2b, 3a, 2a0, 4–6, 70, 8, and 11a in this Letter have been deposited with the
Cambridge Crystallographic Data Centre as Supplementary publication number
CCDC 894211, 901563, 894294, 901564, 900813, 901328, 901327, 901562,
901329, and 900812, respectively. Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax:
+44 (0)1223 336033 or e-mail: deposit@ccdc.cam.ac.uk].
gated. The steric effect of E-ring for stereoselective synthesis of
4b-acyloxy-20-chloropodophyllotoxin derivatives was observed.
Especially some derivatives exhibited the more promising
insecticidal activity as compared with toosendanin.
Acknowledgments
10. Lee, K. H.; Imakura, Y.; Haruna, M.; Beers, S. A.; Thursto, L. S.; Dai, H. J.; Chen, C.
H.; Liu, S. Y.; Cheng, Y. C. J. Nat. Prod. 1989, 52, 606.
The present research was partly supported by National Natural
Science Foundation of China (No. 31071737), and the Special Funds
of Central Colleges Basic Scientific Research Operating Expenses
(YQ2013008).
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.bmcl.2013.08.044.