Design and synthesis of brazilin-like compounds

Article abstract of DOI:10.1055/s-0030-1259511

A general and flexible synthetic route, which leads to the synthesis of brazilin-like compounds, was developed. The aza-brazilin derivatives show strong anticancer activities in MTT assay towards a number of human cancer cell lines including HT29, A549, HL60, and K562. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0030-1259511

LETTER
425
Design and Synthesis of Brazilin-Like Compounds
D
esign and Synth
h
e
sis of Braz
i
lin-Like Co
n
mpounds gxue Pan, Xianghui Zeng, Yifu Guan, Xiangliu Jiang, Liang Li, Hongbin Zhang*
Key Laboratory of Medicinal Chemistry for Natural Resource (Yunnan University), Ministry of Education,
School of Chemical Science and Technology, Yunnan University, Kunming, Yunnan 650091, P. R. of China
Fax +86(871)5035538; E-mail: zhanghb@ynu.edu.cn
Received 20 October 2010
The brazilin skeleton is quite interesting in terms of its di-
verse biological activities and strongly attracts our atten-
tion. Natural products and its mimics are important
resources for the development of new pharmaceuticals.8
As proposed in a review by Tietze,9 natural product hy-
brids are an efficient and promising approach towards the
generation of lead compounds. The structural framework
of brazilin can be viewed as an indane motif merged with
a dihydrobenzopyranyl unit from the view point of struc-
ture-unit hybrid. We were inspired to establish a general
and flexible synthetic strategy leading to the synthesis of
brazilin-like compounds. We postulated that recombina-
tion of an indane structure unit with a dihydrocoumarin
motif, a dihydroquinolin-2-one ring, or a tetrahydroquin-
oline unit would result in the brazilin-like framework, and
finally lead to new types of biologically active agents.
Abstract: A general and flexible synthetic route, which leads to the
synthesis of brazilin-like compounds, was developed. The aza-
brazilin derivatives show strong anticancer activities in MTT assay
towards a number of human cancer cell lines including HT29,
A549, HL60, and K562.
Key words: synthetic strategy, bioactive derivatives, anticancer,
natural-product-like, brazilin
The dried heartwood of Caesalpinia sappan L. (Legumi-
nosae) has long been used in traditional Chinese medicine
as an analgesic, emmenagogue, hemostatic, and anti-
inflammatory agent. Brazilin (1), a tetracyclic homo-
isoflavanoid indicated in Figure 1, is the major compo-
nent isolated from the heartwood of Caesalpinia sappan.1
Previous biological studies showed that brazilin improved
rheological abnormalities in diabetes,2 demonstrated anti-
platelet activity,3 modulated immune function,4 exhibited
antiinflammatory activities,5 and protected oxidative inju-
ry.6 Recent reports have demonstrated that brazilin pos-
sesses antitumor activity and acts as a micromolar
telomerase inhibitor and produces DNA nicks.7
In seeking an efficient and flexible strategy towards the
synthesis of brazilin-like compounds, we drafted a new
synthetic route as indicated in Scheme 1. It was expected
that the indanone intermediate 7 could serve as a platform
for approaching molecular diversity.
Figure 1 Brazilin-related natural products and its structure units
SYNLETT 2011, No. 3, pp 0425–0429
x
x.
x
x.
2
0
1
1
Advanced online publication: 11.01.2011
DOI: 10.1055/s-0030-1259511; Art ID: W16610ST
© Georg Thieme Verlag Stuttgart · New York

426
C. Pan et al.
LETTER
R1
O
O
O
R1
R1
OR2
O
7
6
+
HO
X
flexible
synthetic
strategy
5
R3
8
X = O, N
Y = H/H, O
Z = H, OH
O
Z
Z
Y
O
Y
R1
R1
R1
X
X
X
H
H
HO
4
10
R3
9
R3
R3
brazilin-like compounds
Scheme 1 Synthetic route to brazilin-like compounds
Scheme 2 Synthesis of lactone analogue of brazilin
Synlett 2011, No. 3, 425–429 © Thieme Stuttgart · New York

LETTER
Design and Synthesis of Brazilin-Like Compounds
427
We began our research with commercially available 3- reduction of 9a and 9h followed by cyclization with a
(3,4-dimethoxyphenyl)propanoic acid (5a). Treatment of Friedel–Crafts reaction.
the acid with oxalyl chloride followed by aluminum chlo-
With the brazilin-like compounds in hand,12 we then initi-
ride provided indanone 6a. The indanone 6a was then re-
acted with dimethylcarbonate in the presence of sodium
ural product mimics. Considering the antitumor activities
ated studies towards the biological activities of these nat-
hydride and resulted in b-keto ester 7a (92%). Reduction
of keto ester 7a with sodium borohydride followed by de-
hydration with pyridinium p-toluenesulfonate gave an un-
saturated ester 11 (70%). Hydrolysis of the methyl ester in
the presence of potassium hydroxide in methanol afforded
acid 12 (66%). The ester 13 was obtained by a two-step
procedure as indicated in Scheme 2 (63%), treatment of
acid 12 with oxalyl chloride in the presence of DMF in
dichloromethane followed by reaction of the acyl chloride
with 3-methoxyphenol. After dihydroxylation with osmi-
um tetroxide, the resultant was then treated with boron tri-
fluoride diethyl etherate in dichloromethane to yield a
lactone analogue of brazilin (4a, Scheme 2). The structure
was unambiguously established by an X-ray crystallogra-
phy.10
of brazilin reported in the literature,7c we decided to focus
our initial biological study on the anticancer bioassay. The
Table 1 Anticancer Activity of Brazilin-Like Compounds (IC50,
mM/mL)
Compd K562 (mM)
A549 (mM)
>40
HT-29 (mM) HL60 (mM)
4a
4b
4c
>40
15.4 2.9
>40
>40
>40
>40
>40
>40
>40
13.2 3.4
9.5 3.9
2.4 0.8
3.6 1.2
11.5 2.8
30.0 6.5
2.8 1.1
6.2 2.2
2.8 1.3
3.9 1.1
6.2 1.9
3.1 0.5
>40
19.9 9.6
>40
0.4 0.6
4d
4e
>40
3.7 1.7
>40
2.0 0.8
4f
7.1 3.3
4.7 1.3
30.0 6.5
18.0 5.6
2.2 1.2
Next, we initiated the synthesis of lactam analogue of bra-
zilin by utilization of acid 12 as the starting material. We
successfully synthesized the lactam analogue 4c, and the
synthetic procedure is outlined in Scheme 3.
4g
4h
4i
26.3 8.
>40
26.7 6.0
>40
5.2 2.3
14.2 7.2
>40
5.4 2.9
6.1 2.3
Having successfully established the synthetic route to lac-
tone and lactam analogues of brazilin, we then focused
our attention on aza-brazilins, which were expected to be
better analogues from the viewpoint of solubility. The
keto esters 7a and 7b obtained by similar procedure indi-
cated in Scheme 2 were converted to b-ketoanilides 9a–h
by treatment with aniline derivatives in refluxing xylene.
b-Ketoanilides were then converted to compound 15a–h
by an oxidative hydroxylation with cerium(III) chloride in
the presence of oxygen.11 Reduction of compound 15a–h
led to a mixture of diols 10c–j, which upon treatment with
ethereal boron trifluoride in dichloromethane at 0 °C pro-
vided the desired aza-brazilins 4d–k. Brazilane (3)-like
compounds 4l and 4m (Scheme 4) were also obtained by
4j
18.6 4.7
12.0 4.4
9.9 2.8
4k
4l
>40
20.0 9.8
>40
2.3 0.7
> 40 mM
>40
4m
DDP
>40
8.2 1.1
5.8 0.6
a Cytotoxicity as IC50 values for each cell line, the concentration of
compound that caused 50% reduction in absorbance at l = 570 nm rel-
ative to untreated cells using the MTT assay.
b Human chronic myelogenous leukemia (K562), human lung adeno-
carcinoma (A-549), human colon carcinoma (HT29), human promy-
elocytic leukemia (HL-60).
MeO
OMe
MeO
OMe
MeO
OMe
1. (COCl)2, DMF
CH2Cl2, r.t.
OsO4, NMNO
OMe
OMe
2. m-methoxy-
aniline
acetone–H2O
(9:1)
OH
OH
N
O
HO
O
78%
81%
N
O
H
H
12
14
10b
CeCl3⋅7H2O
MeCN, NaI
76%
OH
O
OH
HO
HO
MeO
O
BBr3, CH2Cl2
68%
NH
NH
MeO
H
H
4c
4b
OH
OMe
Scheme 3 Synthesis of lactam analogue of brazilin
Synlett 2011, No. 3, 425–429 © Thieme Stuttgart · New York

428
C. Pan et al.
LETTER
Scheme 4 Synthesis of aza-brazilin analogues
cytotoxic properties of the synthesized brazilin-like com- exhibited potent activities against K562 cancer cell.
pounds were evaluated in vitro against four human tumor Among the 13 derivatives 4a–m, compound 4e and 4i
cell lines (including K562, A-549, HT-29, HL-60) showed promising antiproliferative activities against a
through MTT assay, and IC50 values (concentration re- broad spectrum of human tumor cell lines, including leu-
quired to produce 50% cell-growth inhibition) were calcu- kemia and solid tumors (lung, colon cancer cell lines).
lated. Cisplatin (DDP) was used as the reference drug.13
The results are summarized in Table 1.
In conclusion, we have developed a flexible strategy for
the synthesis of brazilin-like compounds. A series of new
Lactone 4a showed no activity (>40 mM for four cancer brazilin-like compounds have been synthesized. Prelimi-
cell lines), lactam 4c displayed mild activity against K562 nary screening of a number of representative derivatives
and HL-60 cancer cell lines. To our delight, compound 4e has already resulted in some significant cytotoxic proper-
and 4i displayed potent cytotoxic activity in vitro with ties. Interpretation of natural lead structure as a combina-
IC50 values at the range of 0.4–5.4 mM towards all cancer tion of easily accessible common building block, in
cell lines tested. Compound 4e (IC50 = 0.4 mM), 4f particular as indane and dihydrobenzopyranyl unit in this
(IC50 = 4.7 mM), 4i (IC50 = 2.2 mM), 4j (IC50 = 18.6 mM) research, provided an alternative way for design and syn-
and 4l (IC50 = 9.9 mM) showed potential activities against thesis of biologically active natural product-like com-
myeloid leukaemia (HL-60); compounds 4d,e,f,i,j,l also
Synlett 2011, No. 3, 425–429 © Thieme Stuttgart · New York

LETTER
Design and Synthesis of Brazilin-Like Compounds
429
(7) (a) Chin, R. L.; Tolman, A. C. WO 0,193,864, 2001.
pounds. Further analogues are now being prepared for
bioassay, and the results will be reported in due course.
(b) Mar, W.; Lee, H.-T.; Je, K.-H.; Choi, H.-Y.; Seo, E.-K.
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Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
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Acknowledgment
Int. Ed. 2003, 42, 3996.
This work was supported by grants (20832005, 20925205) from
Natural Science Foundation of China, National Basic Research Pro-
gram of China (973 Program 2009CB522300), and Natural Science
Foundation of Yunnan Provincial Science & Technology Depart-
ment (2007B0006Z).
(10) CCDC804781 (for compound 4a) and CCDC804782 (for
compound 4d) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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References and Notes
(12) Representative Brazilin-Like Compounds
Lactone analogue 4e: pale red syrup. 1H NMR (300 MHz,
CDCl3): d = 7.02 (1 H, d, J = 7.5 Hz), 6.70–6.86 (4 H, m),
4.15 (1 H, s), 3.84 (3 H, s), 3.81 (3 H, s), 3.77 (3 H, s), 3.34
(1 H, d, J = 15.3 Hz), 3.04–3.13 (2 H, m), 2.84 (1 H, d,
J = 15.6 Hz) ppm. 13C NMR (75 MHz, CDCl3): d = 148.41,
148.12, 146.88, 137.37, 133.69, 130.87, 122.96, 121.67,
117.82, 108.72, 77.83, 56.10, 55.48, 52.51, 48.07, 42.13
ppm. MS (EI): m/z (%) = 328 (25) [M+ + 1], 327 (100) [M+],
309 (23), 308 (86), 294 (32), 278 (9), 250 (4), 239 (4), 220
(4), 208 (4), 191 (4), 176 (58), 151 (65), 133 (13), 107 (12).
HRMS: m/z calcd for C19H21NO4 [M]+: 327.1471; found:
327.1479.
(13) The cytotoxicity assay was carried out on four cell lines
(K562, A549, HT-29, and HL60). Cells were cultured at 37
°C under a humidified atmosphere of 5% CO2 in RPMI 1640
medium supplemented with 10% fetal serum and dispersed
in replicate 96-well plates. Compounds were then added.
After 48 h exposure to the compounds, cells viability were
determined by the [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-
tetrazolium bromide] (MTT) cytotoxicity assay by measur-
ing the absorbance at l = 570 nm with a microplate
spectrophotometer. Each test was performed in triplicate.
Cisplatin (DDP) was used as the reference drug.
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Synlett 2011, No. 3, 425–429 © Thieme Stuttgart · New York