Isolation, synthesis, and anti-tumor activities of a novel class of podocarpic diterpenes

Article abstract of DOI:10.1016/j.bmcl.2005.11.023

A novel unusual 17-carbon diterpenoid, named (+)-7-deoxynimbidiol, was isolated from the stalks of Celastrus hypoleucus (Oliv.) Warb. Its racemate and derivatives were synthesized, and the inhibitory activities of these compounds against four cultured human-tumor cell lines were evaluated. The structure-activity relationship was discussed.

Full text of DOI:10.1016/j.bmcl.2005.11.023

Bioorganic & Medicinal Chemistry Letters 16 (2006) 786–789
Isolation, synthesis, and anti-tumor activities of a novel class
of podocarpic diterpenes
Yi Xiong,a Kuiwu Wang,a Yuanjiang Pan,a,* Hongxiang Sunb and Jue Tub
aDepartment of Chemistry, Zhejiang University 310027, PR China
bCollege of Animal Science, Zhejiang University 310027, PR China
Received 12 September 2005; revised 26 October 2005; accepted 8 November 2005
Available online 23 November 2005
Abstract—A novel unusual 17-carbon diterpenoid, named (+)-7-deoxynimbidiol, was isolated from the stalks of Celastrus hypoleu-
cus (Oliv.) Warb. Its racemate and derivatives were synthesized, and the inhibitory activities of these compounds against four
cultured human-tumor cell lines were evaluated. The structure–activity relationship was discussed.
Ó 2005 Elsevier Ltd. All rights reserved.
Cancer remains the second leading cause of death in
most of the countries and as a result there is a need
for effective compounds. To totally synthesize and
modify structure of bioactive natural products is an
effective approach to find some potential compounds
with anti-tumor activity. The natural tri-ring diterpene
is a significant kind of potential anti-cancer agent.1–5
Some tri-ring diterpenes from natural plants have been
synthesized or modified, and their anti-tumor activities
have been investigated.6–11 During our chemical studies
on the bioactive compounds from the plants of Celastr-
aceae family, a novel podocarpic diterpene (+)-7-
deoxynimbidiol (Fig. 1)12, which attracts our intensive
interest for its good anti-tumor activity, was isolated
from Celastrus hypoleucus. We wish to find some
potent anti-tumor agents by modifying the structure
of (+)-7-deoxynimbidiol.
compounds are racemic) were synthesized, and their
anti-tumor activities against four cultured human-tumor
cell lines (HeLa, A549, CNE, and MCF) have been
investigated.
The shade-dried stalks (10 kg) were extracted with meth-
anol, and 514 g of extract was obtained, which was
partitioned with petroleum ether, EtOAc, and n-BuOH
successively. The petroleum ether extract (103 g) was
subjected to column chromatography (CC) over silica
gel (200–300 mesh, 2 kg) eluting with petroleum ether/
EtOAc (10:0–0:10, gradients) to afford 5 fractions. First
fraction was separated on silica gel CC (300–400 mesh,
100 g) repeatedly, using n-hexane/acetone (20:1) as
eluent to yield pure (+)-7-deoxynimbidiol (20.1 mg).
Synthesis of the ( )-7-deoxynimbidiol was from the
cyclization reaction of geranic acid 3. The cyclocitric
acid 4 was methylated by dimethyl sulfate in acetone
with the presence of potassium carbonate. Methyl ester
cyclocitric acid 5 was reduced by LiAlH4 in absolute
diethyl ether, and then the obtained cyclocitric alcohol
In this paper, we report a convenient and high-yield
route to totally synthesize the ( )-7-deoxynimbidiol (1)
and its diastereoisomer named cis-( )-7-deoxynimbidiol
(2)13 along with isolation of (+)-7-deoxynimbidiol. In
modifying the structure of 1, two different approaches
were designed to find out the pharmacophore including
etherification of the phenolic hydroxyl with the substi-
tuted benzyl bromide and oxidation of the C-6 or C-7.
The diastereoisomer 2 was treated in a similar manner.
A class of analogs of the (+)-7-deoxynimbidiol (all the
Keywords: Diterpene; Isolation; Synthesis; Anti-tumor activity.
*
Corresponding author. Tel.: +86 571 87951264; fax: +86 571
87951264; e-mail: panyuanjiang@css.zju.edu.cn
Figure 1. The structure of (+)-7-deoxynimbidiol.
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.11.023

Y. Xiong et al. / Bioorg. Med. Chem. Lett. 16 (2006) 786–789
787
Scheme 1. Reagents and conditions: (a) 85% phosphate acid, toluene, 100 °C, 2 h, 82%; (b) Me2SO4, K2CO3, acetone, rt, 5 h, 91%; (c) Ar, LiAlH4,
absolute diethyl ether, rt, 4 h, 85%; (d) PCC, CH2Cl2, rt, 4 h, 55%; (e) Ar, n-BuLi, 3,4-dimethyl-benzyl triphenyl phosphonium chloride, absolute
THF, rt, 6 h, 74.5%; (f) H2, 10% Pd/C, diethyl ether, 30 min, quantitative yield; (g) BF3Æ(C2H5)O, CH2Cl2, 24 h, 83.5%; (h) BBr3, CH2Cl2, 0–5 °C, 2 h.
Scheme 2. Reagents and conditions: (a) substituted benzyl bromide, NaI, K2CO3, acetone, reflux, 5 h.
6 was dissolved in methylene chloride and oxidized by
the pyridine chromium trioxide chloride (PCC) to get
cyclocitric aldehyde 7 (35% yields of total four steps).
After 7 was added to the clear solution of n-BuLi and
3,4-dimethyl-benzyl triphenyl phosphonium chloride in
absolute THF at room temperature, the mixture was
stirred for 3 h to yield the olefin 8 (74.5%). The double
bond at C6-C7 of 8 was hydrogenated with H2 and
10% Pd/C in diethyl ether to yield 9 (quantitative yield).
9 was treated with the boron trifluoride etherate
(BF3Æ(C2H5)2O) in methylene chloride at room tempera-
ture and the mixture stood for 24 h, given the product 10
and its diastereoisomer 11 (isomer ratio = 2:1). Treat-
ment of the methyl ether 10 with boron tribromide in
methylene chloride at 0–5 °C for 2 h yielded the target
molecular ( )-7-deoxynimbidiol (76.3%). The diastereo-
isomer 2 was obtained from 11 in the same manner as
described above (74.5%) (Scheme 1). Compound 1 was

788
Y. Xiong et al. / Bioorg. Med. Chem. Lett. 16 (2006) 786–789
Scheme 3. Reagents and conditions: (a) CrO3, CH2Cl2, rt, 2 h; (b) CrO3, CH2Cl2, 50 °C, 2 h; (c) BBr3, CH2Cl2, 0–5 °C, 2 h.
Table 1. Biological activities of the synthesized compounds
Compound
HeLaa IC50 (lg/ml)b
A549 IC50 (lg/ml)
CNE IC50 (lg/ml)
MCF IC50 (lg/ml)
1
24.3 ( 0.98)
10.9 ( 0.48)
41.5 ( 0.23)
13.2 ( 0.04)
>50
19.6 ( 0.22)
5.7 ( 0.24)
41.7 ( 0.09)
9.6 ( 0.07)
>50
25.1 ( 0.17)
3.6 ( 0.22)
25.1 ( 0.25)
4.5 ( 0.09)
>50
41.2 ( 0.11)
9.6 ( 0. 51)
39.5 ( 0.07)
10.9 ( 0.11)
>50
2
10
11
1a
1b
1c
>50
>50
>50
>50
>50
>50
>50
>50
1d
1e
>50
>50
>50
>50
>50
>50
>50
>50
1f
2a
>50
>50
>50
>50
>50
42.2 ( 0.07)
15.3 ( 0.97)
>50
46.9 ( 0.15)
13.7 ( 0.27)
36.4 ( 0.18)
>50
27.2 ( 0.04)
7.2 ( 0.16)
32.9 ( 0.34)
>50
2b
2c
12.4 ( 0.09)
>50
>50
2d
2e
>50
13.4 ( 0.25)
18.6 ( 0.08)
12.7 ( 0.13)
16.1 ( 0.09)
25.7 ( 0.06)
17.6 ( 0.45)
18.0 ( 0.09)
27.7 ( 0.02)
42.0 ( 1.25)
5.89 ( 0.15)
35.0 ( 0.33)
>50
17.8 ( 0.78)
>50
42.9 ( 0.05)
>50
2f
12
26.3 ( 0.08)
33.8 ( 0.06)
>50
3.1 ( 0.05)
5.6 ( 0.54)
>50
7.8 ( 0.22)
16.5 ( 0.07)
>50
13
14
15
16
>50
25.8 ( 0.11)
>50
13.5 ( 0.02)
9.1 ( 0.12)
22.1 ( 0.07)
12.1 ( 0.33)
2.34 ( 0.04)
36.2 ( 0.14)
24.8 ( 0.12)
>50
17
(+)-7-Deoxynimbidiol
CDDPc
36.1 ( 0.03)
3.58 ( 0.14)
36.5 ( 0.04)
6.53 ( 0.25)
a Cultured human-tumor cell lines: HeLa, cervical carcinoma; A549, lung adenocarcinoma; CNE, nasopharyngeal carcinoma; MCF, breast
adenocarcinoma.
b Values are means of three experiments, standard deviation is given in parentheses.
c CDDP was used as positive control.
added to the mixture of sodium iodide, potassium car-
bonate, and substituted benzyl bromide in acetone,
and then the mixture was stirred at 50–60 °C for 5 h to
give the products 1a–1f in which 12-OH or 13-OH was
etherified solely. In the same method, we obtained the
compounds 2a–2f from compound 2. The ratio of 12-
substituted products and 13-substituted products was
1:1 (Scheme 2).

Y. Xiong et al. / Bioorg. Med. Chem. Lett. 16 (2006) 786–789
789
Acknowledgment
The generous financial support of Natural Science
Foundation of China (20472073) was gratefully
acknowledged.
1
2
References and notes
Figure 2. Comparison of the backbone structures of compounds
1 and 2.
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Treatment of the compound 11 with chromium trioxide
in acetic acid at room temperature for 2 h yielded the C-
7 oxidized compound 12 (81.3%). When the temperature
was increased to 50 °C, compound 13 (66.5%) in which
both C-7 and C-6 were oxidized appeared. But for the
compound 10, we didnot find the further oxidized prod-
uct besides compound 14 (85.6%) obtained by oxidizing
C-7. The compounds 12, 13, and 14 were demethylated
with boron tribromide in methylene chloride at 0–5 °C
for 2 h to yield 15 (58.2%), 16 (44.7%), and the known
nature product ( )-nimbidiol 17 (62.4%), respectively
(Scheme 3).
10. Hang, S. C.; Ho, C. T.; Lin-Shiau, S. Y. Biochemical
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The biologic activities of (+)-7-deoxynimbidiol and all
synthesized compounds were assessed against cultured
human-tumor cell lines using the MTT assay14 (Table
1). Compounds 2, 11, 2b, 2e, and 12 showed potential
activities against HeLa; compounds 2, 11, and 2b
showed potential activities against A549; compounds
2, 11, 2b, 12, 13, and 16 showed potential activities
against CNE; compounds 2, 11, 2b, and 12 showed po-
tential activities against MCF. As shown in Table 1, the
racemate 1 showed better activity than the (+)-enantio-
mer against HeLa and A549. The compounds in which
13-OH was etherified (2e, 2d, and 2f) had as much po-
tent activity as those in which 12-OH was etherified
(2a, 2b, and 2c). Comparing the methylated compounds
with those demethylated, the anti-cancer activity had no
obvious contrast. We obtained an attractive matter from
Table 1 that the bioactivities of the compounds with
5-b-H were prominently better than those of the analogs
12. Physical characters and spectral data for the (+)-7-
deoxynimbidiol. White powder; mp (petroleum ether/
20
EtOAc (5:1)) 90–92°; ½aꢀD = +49.44ꢁ (c 0.10, MeOH);
UV (MeOH) kmax: 288, 212 nm; IR (KBr) cmꢂ1: 3362,
1607, 1515; 1H NMR (CDCl3, 500 MHz) d 0.90 (s, 3 H, H-
C(15)), 0.93 (s, 3H, H-C(16)), 1.14 (s, 3H, H-C(17)), 1.17–
1.75 (m, 7H, H-C(1,2,3,5,6)), 1.85 (m, 1H, H-C(6)), 2.15
(m, 1H, H-C(1)), 2.78 (m, 2H, H-C(7)), 6.52 (s, 1H, H-
C(14)), 6.75 (s, 1H, H-C(11)); 13C NMR (CDCl3,
125 MHz) d 19.33 (t, C-2), 19.58 (t, C-6), 21.81 (q, C-
16), 25.04 (q, C-17), 30.02 (t, C-7), 33.52 (q, C-15), 33.62
(s, C-4), 37.59 (s, C-10), 39.31 (t, C-1), 41.89 (t, C-3), 50.81
(d, C-5), 111.70 (d, C-11), 115.47 (d, C-14), 128.31 (s, C-8),
141.19 (s, C-13), 141.52 (s, C-12), 143.58 (s, C-9); HRMS
(ESI) calcd for C17H24O2Na [M+Na]+ 283.1669 found
283.1667.
13. Spectral data for the key compounds 1 and 2. Compound
1, 1H NMR (CDCl3, 500 MHz) d 0.92 (s, 3 H), 0.95 (s,
3H), 1.15 (s, 3H), 1.19–1.76 (m, 7H), 1.86 (m, 1H), 2.16
(m, 1H), 2.79 (m, 2H), 5.08 (b, 2H), 6.53 (s, 1H), 6.77 (s,
1H); 13C NMR (CDCl3, 125 MHz) d 19.31, 19.56, 21.82,
25.13, 30.06, 33.54, 33.62, 37.60, 39.29, 41.89, 50.73,
111.71, 115.41, 128.25, 141.19, 141.54, 143.55; HRMS
(ESI) calcd for C17H24O2Na [M+Na]+ 283.1669 found
283.1673. Compound 2, 1H NMR (CDCl3, 500 MHz) d
0.41 (s, 3H), 0.91 (s, 3H), 1.11 (s, 3H), 1.25–1.47 (m, 6H),
1.92 (m, 1H), 2.14 (m, 1H), 2.28 (m, 1H), 2.76 (m, 2H),
4.92 (b, 2H), 6.53 (s, 1H), 6.79 (s, 1H); 13C NMR (CDCl3,
125 MHz) d 18.28, 19.53, 22.95, 25.85, 32.98, 34.65, 34.75,
36.99, 38.51, 43.41, 50.21, 111.59, 115.71, 130.41, 137.01,
141.00, 141.39; HRMS (ESI) calcd for C17H24O2Na
[M+Na]+ 283.1669 found 283.1671.
with 5-a-H. With the purpose of developing
a
preliminary SAR concept, the major conformations of
compounds 1 and 2 were investigated with MM2 energy
minimization by Chem3D program (Fig. 2). The skele-
ton of the tricycle of the compound 1 was found close
to planar configuration, but that of the compound 2
showed distinctly a hooked configuration. Further
research of the relation between this structural diversity
and the bioactivity is in progress.
In summary, a novel podocarpic diterpene (+)-7-deoxy-
nimbidiol was isolated and synthesized conveniently,
whose structure was modified as a lead compound. 2,
11, and 2b were identified to have potent anti-cancer
activity. Further SAR studies to find more potent antag-
onists are being pursued.
14. Araki, T.; Enokido, Y.; Inamura, N.; Aizawa, S.; Reed,
J. C.; Hatanaka, H. Brain Res. 1998, 794, 239.