Unexpectedly convenient and stereoselective synthesis of 4α-acyloxy-2-chloropodophyllotoxins in the presence of BF 3·Et2O

Article abstract of DOI:10.1016/j.bmcl.2011.05.004

Twenty-one 4α-acyloxy-2-chloropodophyllotoxin derivatives (5a-u), whose C-4 spatial configuration was mainly stereocontrolled by the configuration of C-2 chlorine atom, were unexpectedly prepared by the reaction of 2-chloropodophyllotoxin with carboxylic

Full text of DOI:10.1016/j.bmcl.2011.05.004

Bioorganic & Medicinal Chemistry Letters 21 (2011) 4008–4012
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Unexpectedly convenient and stereoselective synthesis
of 4
a
-acyloxy-2-chloropodophyllotoxins in the presence of BF3ꢀEt2O
Hui Xu a, , Xiao Xiao a, Xue-Fei Zhao b, Yong Guo a, Xiao-Jun Yao b
⇑
a Laboratory of Pharmaceutical Design and Synthesis, College of Sciences, Northwest A&F University, Yangling 712100, China
b Department of Chemistry, Lanzhou University, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Twenty-one 4a-acyloxy-2-chloropodophyllotoxin derivatives (5a–u), whose C-4 spatial configuration
Received 22 December 2010
Revised 12 April 2011
Accepted 2 May 2011
Available online 13 May 2011
was mainly stereocontrolled by the configuration of C-2 chlorine atom, were unexpectedly prepared
by the reaction of 2-chloropodophyllotoxin with carboxylic acids in the presence of BF3ꢀEt2O. Compared
with ordinary esterifications of carboxylic acids mediated by the condensation agent, for example,
N,N’-diisopropylcarbodiimide (DIC), the present method made the procedure for the preparation of 4a-
acyloxy-2-chloropodophyllotoxins more convenient, practical and easy. Meanwhile, the insecticidal
activity of compounds 5a–u was preliminarily evaluated against the pre-third-instar larvae of Mythimna
separata Walker in vivo at the concentration of 1 mg/mL.
Keywords:
Podophyllotoxin
2-Chloropodophyllotoxin
Ó 2011 Elsevier Ltd. All rights reserved.
4a-Acyloxy-2-chloropodophyllotoxin
Stereoselective
Insecticidal activity
Podophyllotoxin (1, Fig. 1) and its derivatives exhibited pro-
nounced biological activities as antiviral and antineoplastic agents.
Especially, two semisynthetic derivatives of podophyllotoxin, that
is, etoposide (VP-16, 2, Fig. 1) and teniposide (VM-26, 3, Fig. 1), are
currently used in the chemotherapy for various types of solid tu-
mor.1–3 On the other hand, podophyllotoxin derivatives also dis-
played the interesting insecticidal activity.4–9 More recently, we
have reported the insecticidal activity of some derivatives of
2-chloropodophyllotoxin (4, Fig. 1), and found several 4a-acyloxy-
2-chloropodophyllotoxins (5, Fig. 2) showed more pronounced
insecticidal activity than toosendanin, a commercial insecticide de-
rived from Melia azedarach.10 In continuation of our program aimed
at the discovery and development of podophyllotoxin derivatives
with potent insecticidal activity,10,11 and to study the relationship
between the chemical structures and the insecticidal activity of 4-
acyloxy-2-chloropodophyllotoxins, we further designed the corre-
H
O
H
O
O
O
Me
O
S
O
HO
O
OH
HO
OH
O
OH
OH
O
O
O
O
O
O
O
4
3
O
O
O
O
2
1
O
Cl
H
O
O
H
H
O
O
6'2'
MeO
OMe
MeO
OMe
MeO
OMe
MeO
OMe
4'
OMe
OH
OMe
OH
2
4
1
3
Figure 1. Chemical structures of podophyllotoxin (1), etoposide (VP-16, 2) teniposide (VM-26, 3), and 2-chloropodophyllotoxin (4).
⇑
Corresponding author. Tel./fax: +86 29 87091952.
E-mail address: orgxuhui@nwsuaf.edu.cn (H. Xu).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.05.004

H. Xu et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4008–4012
4009
OCOR
Cl
OCOR
O
O
O
O
O
O
Cl
O
O
MeO
OMe
MeO
OMe
OMe
OMe
5
6
Figure 2. Chemical structures of 4a-acyloxy-2-chloropodophyllotoxins (5), and
4b-acyloxy-2-chloropodophyllotoxins (6).
OCOR
4β
O
O
O
H
O
Reference 12
OH
O
O
R1 = H
MeO
OMe
O
RCO2H
OMe
R1
7
BF3.Et2O
O
?
OCOR
R1 = Cl
O
MeO
OMe
4β
Cl
O
in this paper
to be investigated
OMe
O
O
MeO
OMe
Figure 3. The X-ray crystallography of compound 5a.
OMe
6
Due to the steric hindrance of the substituent (30,40,50-trime-
thoxyphenyl group) on the C-1 position of podophyllotoxin,
4b-acyloxypodophyllotoxins (7) could be stereoselectively
obtained from 1 and carboxylic acids by SN1 reaction in the pres-
ence of BF3ꢀEt2O (Scheme 1).12 Therefore, in this Letter we wanted
Scheme 1. The strategy for the preparation of 6.
sponding stereoisomer of 5, that is, 4b-acyloxy-2-chloropodo-
phyllotoxins (6, Fig. 2), as insecticidal agents.
OCOR
OH
OCOR
4α
4β
O
RCO H
2
O
O
O
O
RCO H
2
4
1
O
3
2
.
O
.
O
BF Et O/DCM
3
2
BF Et O/DCM
O
3
2
Cl
O
o
Cl
-15 C to rt
Cl
o
O
O
-15 C to rt
MeO
OMe
MeO
OMe
MeO
OMe
OMe
OMe
OMe
5a-u
6a-u
4
a:
f:
k: O N
2
q:
Cl
O N
2
b:
c:
r:
l:
MeO
Cl
g:
O N
2
s:
Cl
m:
n:
Me
O N
2
h:
CH
3
t:
i:
j:
o:
p:
d:
e:
Me
Me
O
u:
Cl
Scheme 2. The synthetic routes of 4 reacting with carboxylic acids in the presence of BF3ꢀEt2O.

4010
H. Xu et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4008–4012
to investigate whether compounds 6 could also be stereoselective-
ly prepared from 4 with carboxylic acids in the presence of
BF3ꢀEt2O (Scheme 1).
Supplementary data), however, compounds 6a–u were not ob-
tained at all. The assignment of configuration of C-4 position was
based on J3.4 coupling constants. The C-4b-substituted compounds
have a J3.4 ꢁ 4.0 Hz due to a cis relationship between H-3 and H-4.
If J3.4 = 9.5 Hz, it indicates that H-3 and H-4 is trans relationship,
As described in Scheme 2, interestingly, besides the unreacted
starting material, 4, we obtained only 5a–u, which were well char-
acterized by 1H NMR, MS or HR–MS, optical rotation, and IR (see
and the substituent on the C-4 position of podophyllotoxin is
a
Table 1
Stereoselective synthesis of 4a-acyloxy-2-chloropodophyllotoxins (5a–u) in the presence of BF3ꢀEt2O (Route A) as compared with in the presence of DIC (Route B)
RCO H
2
OCOR
4α
OH
.
BF Et O/DCM
3
2
o
O
O
-15 C to rt
O
O
4
3
Route A
Route B
O
O
2
4-48 h, 26-70%
1
Cl
Cl
O
O
RCO H
2
DIC/DCM/rt
1-7 h, 28-84%
MeO
OMe
MeO
OMe
OMe
OMe
4
_
5a u
Compound
Route A
Route B
Isolated yield (%)
Time (h)
Isolated yield (%)
Time (h)
5a
5b
5c
5d
5e
5f
5g
5h
5i
51
40
59
44
52
50
62
33
33
36
35
26
34
49
70
44
52
45
41
26
42
24
24
24
24
24
48
24
24
10
18
10
11
11
10
4
63
28
56
50
54
72
67
77
81
61
84
63
63
57
57
66
64
66
44
58
53
2
7
4
4
4
5
2
3
4
4
4
5j
5k
5l
4.5
2.5
1
1.5
1
1.5
1
4
5m
5n
5o
5p
5q
5r
5s
5t
16.5
8
9
26
8
9
4
3
5u
steric hindrance
X
O
Cl
a
e
3
O
4
O
2
1
a
e
Route 1
Route 2
4β
O
O
Route 1
Cl
e
O
O
3
O
.
O
HX
2
BF Et O
3
2
1
a
e
O
6
4
O
X = OCOR
Route 2
O
O
Cl
O
e
X
a
e
1
O
3
O
O
2
e
8
O
O
4α
O
O
5
Scheme 3. The possible mechanism for the synthesis of 5a–u by the reaction of 4 with carboxylic acids.

H. Xu et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4008–4012
4011
configuration.13 For example, the J3.4 values of H-4 of 5b and 5c
were 8.8 and 9.2 Hz, respectively, therefore, the acyloxy groups
5a and 6a was 11.57 kcal/mol. It was clearly indicated that 5a was
energetically more stable than 6a.
on the C-4 position of 5b and 5c were
a
configuration.
Meanwhile as shown in Table 2, the insecticidal activity of
compounds 5a–u against the pre-third-instar larvae of Mythimna
separata Walker was tested at the concentration of 1 mg/mL by
leaf-dipping method (see Supplementary data). Toosendanin, a
commercial insecticide derived from M. azedarach, was used as a
positive control at 1 mg/mL. The corrected mortality rates of M. sep-
arata caused by 5a–u with the advance of time were described in
To obtain the precise three-dimensional structural information
and absolute configuration of 5a–u, the single-crystal structure of
5a was determined by X-ray crystallography (Fig. 3).14 It was obvi-
ously demonstrated that the C-2 chlorine atom and the C-4 naph-
thalene acetyloxy group of 5a adopted the b and
respectively.
a configuration,
The results in the presence of BF3ꢀEt2O (Route A) were summa-
rized in Table 1. Meanwhile, the results, obtained from ordinary
esterifications of carboxylic acids by using of the condensation
agent, N,N’-diisopropylcarbodiimide (DIC), were also presented as
the control (Route B, Table 1).10 Compounds 5a–u were stereose-
lectively obtained in 26–70% yields for 4–48 h in the presence of
BF3ꢀEt2O. The electronic and steric effects of carboxylic acids,
including aromatic acids and aliphatic acids, on this reaction were
not very clear. Although the reaction time on Route A was
generally prolonged as compared with that of Route B (28–84%
yields for 1–7 h), the workup for obtaining pure target products
was very convenient. It is well-known that on Route B complete re-
moval of N,N’-diisopropylurea (DIU, the hydrate of DIC) from the
product is always troublesome.
Additionally, the possible mechanism for the synthesis of 5a–u
from 4 with carboxylic acids was described in Scheme 3. Firstly,
benzylic cation (8) was produced from 4 in the presence of
BF3ꢀEt2O; then 8 was further allowed to react with carboxylic acids
to give 5 (C-4 acyloxy group in the equatorial position), not 6 (C-4
acyloxy group in the axial position), the steric hindrance of 6 be-
tween C-2 chlorine atom and C-4 acyloxy group was bigger than
that of 5, that is, introduction of the acyloxy group to the carbocat-
ion species proceeded from a-side reflecting the steric hindrance of
2b-chlorine atom. On the other hand, to determine the relative sta-
bilities of compounds 4–6, the theoretical total energies of 4, 5a and
6a were typically calculated at the B3LYP/6-311G level of theory by
using the suite of computational programs in Gaussian 09 (Revi-
sion-A.01).15 As shown in Figure 4, total energy differences between
Table 2
Insecticidal activity of
separata in vivo
4
a
-acyloxy-2-chloropodophyllotoxins (5a–u) against M.
Compound
Corrected mortality rate (%)
10 d
20 d
35 d
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
5o
5p
5q
5r
5s
5t
5u
1
4
14.3 ( 8.2)
21.4 ( 4.7)
25.0 ( 0.0)
14.3 ( 8.2)
3.6 ( 8.2)
28.6 ( 9.4)
25.0 ( 8.2)
42.9 ( 4.7)
21.4 ( 4.7)
7.1 ( 4.7)
70.4 ( 4.7)
44.4 ( 16.3)
59.3 ( 4.7)
44.4 ( 8.2)
48.1 ( 9.4)
44.4 ( 14.1)
63.0 ( 12.5)
40.7 ( 12.5)
37.0 ( 9.4)
37.0 ( 4.7)
55.6 ( 12.5)
40.7 ( 4.7)
55.6 ( 8.2)
40.7 ( 4.7)
40.7 ( 4.7)
37.0 ( 4.7)
40.7 ( 4.7)
37.0 ( 9.4)
40.7 ( 12.5)
40.7 ( 9.4)
37.0 ( 4.7)
44.4 ( 14.1)
48.1 ( 12.5)
48.1 ( 4.7)
10.7 ( 4.7)
28.6 ( 9.4)
14.3 ( 0.0)
25.0 ( 8.2)
14.3 ( 0.0)
28.6 ( 4.7)
21.4 ( 4.7)
25.0 ( 8.2)
17.9 ( 4.7)
17.9 ( 9.4)
25.0 ( 8.2)
14.3 ( 8.2)
3.6 ( 8.2)
10.7 ( 4.7)
25.0 ( 8.2)
17.9 ( 4.7)
17.9 ( 9.4)
10.7 ( 4.7)
21.4 ( 4.7)
17.9 ( 4.7)
32.1 ( 4.7)
17.9 ( 4.7)
28.6 ( 9.4)
17.9 ( 4.7)
39.3 ( 8.2)
28.6 ( 4.7)
32.1 ( 4.7)
28.6 ( 4.7)
25.0 ( 8.2)
32.1 ( 4.7)
25.0 ( 8.2)
17.9 ( 4.7)
25.0 ( 8.2)
35.7 ( 8.2)
28.6 ( 4.7)
21.4 ( 4.7)
14.3 ( 8.2)
32.1 ( 4.7)
Toosendanin
Figure 4. The graphical depiction of relative total energies of 4, 5a and 6a.

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H. Xu et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4008–4012
Figure 5. The corrected mortality rates of M. separata caused by 5a–u with the increase of time.
Figure 5. It suggested that these compounds, in a time-dependent
manner, different from those conventional neurotoxic insecticides,
such as organophosphates, carbamates and pyrethroids, exhibited
delayed insecticidal activity. The symptoms of the tested M. separa-
ta were described as the same way as our previous reports.10,11
Compounds 5a, 5c, 5g, 5k, and 5m exhibited more potent insecti-
cidal activity than or comparable to toosendanin. Some preliminar-
ily results about the chemical structures and the insecticidal
activity (SAR) were also found as follows: (i) in general, introduc-
tion of aryloyloxy groups at the C-4 position of 4 could lead to the
more potent compunds than those with alkyloyloxy groups at the
C-4 position of 4; (ii) the influence of the length of alkyloyloxy
groups at the C-4 position of 4 on the insecticidal activity was not
very obvious (5n–r); (3) the methylene group between the phenyl
and carbonyl groups was completely important for the insecticidal
activity. For example, the final mortality rate of 5g was 63%, while
the final mortality rates of 5i, 5t, and 5u were only 37%, 40.7%,
and 37%, respectively.
China (No. 31071737), the Fok Ying Tong Education Foundation
for Young Talents (No. 121032), and the Special Funds of Central
Colleges Basic Scientific Research Operating Expenses
(QN2009045). We also thank to Professor H.L. Zhang for the NMR
experiments.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2011.05.004.
References and notes
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14. Crystallographic data (excluding structure factors) for the structure of 5a in
this paper has been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication number CCDC 800347. 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].
In summary, we have investigated the reactions between 2-chlo-
ropodophyllotoxin and carboxylic acids in the presence of BF3ꢀEt2O
for the first time. 4a-Acyloxy-2-chloropodophyllotoxins, whose
C-4 spatial configuration was mainly stereocontrolled by the config-
uration of C-2 chlorine atom, were stereoselectively obtained.
Compared with ordinary esterifications of carboxylic acids by using
of the condensation agent DIC, the present method made the proce-
dure for the preparation of 4a-acyloxy-2-chloropodophyllotoxins
more convenient, practical and easy. Especially compounds 5a and
5g exhibited the more potent and promising insecticidal activity
than toosendanin at 1 mg/mL. It clearly demonstrated that introduc-
tion of the arylacetyloxy groups at the C-4 position of 4 could lead to
the more potent compounds.
Acknowledgments
This work was financially in part supported by the Program for
New Century Excellent University Talents, State Education Ministry
of China (NCET-06-0868), National Natural Science Foundation of
15. Gaussian 09, Revision-A.01, Gaussian, Inc., Wallingford CT, USA, 2009.