Synthesis of chiral hydroxylated enones as potential anti-tumor agents

Article abstract of DOI:10.1016/j.bmcl.2012.10.026

A series of chiral hydroxylated enones were synthesized as COTC ether analogues to investigate the structural features required for optimal and selective anti-tumor activity. The cytotoxicity of the seven COTC ether analogues against WRL-68 normal and Hep

Full text of DOI:10.1016/j.bmcl.2012.10.026

Bioorganic & Medicinal Chemistry Letters 22 (2012) 7562–7565
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Bioorganic & Medicinal Chemistry Letters
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Synthesis of chiral hydroxylated enones as potential anti-tumor agents
Tony K. M. Shing a, , Ho T. Wu a, H. F. Kwok b, Clara B. S. Lau b,
⇑
a Department of Chemistry and Center of Novel Functional Molecules, The Chinese University of Hong Kong, Shatin, Hong Kong, China
b Institute of Chinese Medicine and State Key Laboratory of Phytochemistry and Plant Resources in West China, The Chinese University of Hong Kong, Shatin, New Territories,
Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of chiral hydroxylated enones were synthesized as COTC ether analogues to investigate the struc-
tural features required for optimal and selective anti-tumor activity. The cytotoxicity of the seven COTC
ether analogues against WRL-68 normal and HepG2, HL-60 cancer cell lines were measured. C-4 ether
analogues with an aliphatic chain substituent were found to be more favorable than their aromatic coun-
terparts. Inversion of the configuration at C-4 in 5e to give 5f only resulted in reduced selectivity towards
cancer cells. These results show that 4-O-pentyl-gabosine D (5e) has optimum selectivity and cytotoxicity
towards two cancer cell lines.
Received 21 June 2012
Revised 17 September 2012
Accepted 4 October 2012
Available online 13 October 2012
Keywords:
Anti-tumor agents
COTC analogues
Hydroxylated enones
Synthesis
Ó 2012 Elsevier Ltd. All rights reserved.
Selective cytotoxicity
2-Crotonyloxymethyl-(4R,5R,6R)-4,5,6-trihydroxy-2-cyclohexe-
none (COTC, 1) was isolated from cultures of Streptomyces griseo-
sporeus in 1975 by Umezawa and co-workers.1 Studies have
demonstrated that COTC possesses potent activity against murine
and human cancer cell lines in cultures as well as in tumor-bearing
mice.1,2
The origin to the cytotoxicity of COTC aroused much interests
and extensive studies were conducted.3–11 Initially, the cytotoxic-
ity of COTC was believed to originate from its inhibition of the
detoxification enzyme glyoxalase I, thereby accumulating methyl-
glyoxal which contributed to the resultant cytotoxicity. In 2002,
however, an elegant investigation by Ganem and collaborators8
has demonstrated that the cytotoxicity of COTC did not stem from
the competitive inhibition of glyoxalase I; instead, its cytotoxicity
was attributable to the conjugate addition of glutathione (GSH)
to COTC under the catalysis of human glutathione transferase
(hGSTs) (Fig. 1).
hGSTs is a family of GSH-dependent enzymes in phase II detox-
ification system, responsible for drug resistance processes by cata-
lyzing the conjugation of GSH to a wide variety of anticancer
drugs.8,12 Under the catalysis of hGST, COTC undergoes an initial
Michael addition by one equivalent of GSH to give the enol inter-
mediate 2. Upon departure from the enzyme, enol 3 collapses to
expel crotonic acid and form the highly electrophilic exocyclic en-
one 4, which is responsible for the cytotoxicity of COTC.5,8 The exo-
cyclic enone moiety in 4 reacts with the nucleophilic groups in
nucleic acids and proteins inside cells, which directly accounts
for COTC’s cytotoxic activity; or with another equivalent of GSH.
It was suggested that by depleting the amount of GSH, the cyto-
toxic methylglyoxal would accumulate due to the lack of the vital
cofactor GSH for the detoxification glyoxalase system and there-
fore contributing indirectly to COTC’s cytotoxicity.11
Intrigued by the potent biological activity of COTC, different
strategies had been developed on the synthesis of COTC and
its analogues, namely starting from (À)-quinic acid,13–18 1,3-
cyclohexadiene,17,18 meso-1,3-dihydrodiol,19 sulfinylacrylates20
and carbohydrates.21,22 Recently, our group has prepared several
COTC analogues from D-glucose and found that they work synergis-
tically with cisplatin to achieve enhanced cytotoxicity towards
A549 lung cancer cell line.23 As a continuation of our studies,
several COTC ether analogues bearing different aromatic and alkyl
substituents C-4 were prepared in order to assess their selective
cytotoxicity towards cancer cells.
Seven COTC analogues disclosed (5a–g, Fig. 2) in this paper were
obtained from their own respective common intermediates:
a-allylic alcohol 7 and b-allylic alcohol 8 by standard transforma-
tion. Previously, our group already published a promising synthetic
route towards the two key allylic alcohols 7 and 8 using a
L
-proline mediated intramolecular direct aldol reaction of
a
D
-glucose-derived diketone as the key step.24 Conversion of 7 and
8 into the analogues were performed via a 5-step reaction
sequence. Thus, the free hydroxyl group in 7 and 8 was first alkylated
into their respective aliphatic or aromatic ethers. Deacetonation of
the ethers gave their respective diols, which were then
⇑
Corresponding author. Tel.: +852 39436344.
E-mail address: tonyshing@cuhk.edu.hk (T.K.M. Shing).
Tel.: +852 39436109.
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.10.026

T. K. M. Shing et al. / Bioorg. Med. Chem. Lett. 22 (2012) 7562–7565
7563
O
H
O
O
O
O
SG
OH
O
O
O
SG
OH
- GST
GSH
OH
OH
HO
OH
HO
OH
HO
COTC ( )
HO
1
enol intermediate 2
enol 3
GST
protein/DNA
SG
OH
cell death
O
OH
HO
exocyclic enone 4
Figure 1. Mechanism for generation of exocyclic enone 4.
AcO
O
AcO
O
AcO
4
1
O
O
O
OMe
OH
2
3
HO
OH
HO
OH
HO
5c
5b
5a
5 steps
overall yield 37% from 7
5 steps
overall yield 53% from 7
5 steps
overall yield 70% from 7
AcO
AcO
O
AcO
O
O
O
O
O
HO
OH
HO
OH
HO
OH
5f
5e
5d
5 steps
5 steps
overall yield 34% from 7
5 steps
overall yield 38% from 8
overall yield 46% from 7
AcO
O
O
HO
OH
5g
5 steps
overall yield 22% from 7
Figure 2. Synthesized COTC analogues 5a–g.
regioselectively acetylated with the use of 2,4,6-collidine to give
their respective primary acetates. Pyridinium dichromate (PDC)
oxidation, followed by acid hydrolysis of the trans-diacetal group
then yielded the desired COTC analogues (Scheme 1).
With the COTC analogues successfully prepared, their biological
activities against a normal human liver cell line (WRL-68) and two
cancer cell lines (hepatoma HepG2 and leukemia HL-60) were
examined (Table 1). The optimal selective cytotoxic activity should
demonstrate low IC50 values for the cancer cells but a high IC50 va-
lue for normal cells.
Disappointingly, biphenylmethyl ether 5b was non-selective and
toxic against normal cell line, therefore it was abandoned from fur-
ther investigations. These results suggest that arylmethyl substitu-
ents are not desirable structural features for a selective anticancer
agent.
Prompted by our findings, several alkyl ether analogues were
then prepared with a 1-, 3-, 5- and 7-carbon unit in their respective
ether chains. For enone 5c (R = Me), its cytotoxicity against both
normal and cancer cell lines were low. When the carbon chain
length was changed to 3 (5d), it was found that although a minor
drop in the IC50 value in normal cell line was observed, its cytotox-
icity against both cancer cell lines was improved. Further increas-
ing the ether chain length to a 5-carbon unit (5e) resulted in an
Initially, the cytotoxicity of diols 5a and 5b against normal cell
line was measured. For the 2-naphthalenylmethyl ether 5a, it
showed low cytotoxicity against both normal and cancer cell lines.

7564
T. K. M. Shing et al. / Bioorg. Med. Chem. Lett. 22 (2012) 7562–7565
O
O
O
8 steps
D-Glucose
25% yield
O
O
MeO
OMe
6
AcO
AcO
O
O
O
OR
aq. TFA
OR
OH
O
OR
1) 80% AcOH
2) AcCl,Collidine
3) PDC
O
O
OMe
HO
O
MeO
O
OMe
MeO
5a
5b
5d
R = 2-naphthylmethyl;
5c
10 R = 2-naphthylmethyl;
7 R = H
9 R = alkyl
R = biphenylmethyl;
5e
R = n-propyl;
R = methyl;
11 R = biphenylmethyl; 12 R = methyl;
NaH, R'Br
THF
13
15
14
R = n-pentyl;
R = n-propyl;
R = n-heptyl
R = n-pentyl;
5g R = n-heptyl
AcO
O
O
AcO
O
OR
OR
aq. TFA
1) 80% AcOH
O
OR
2) AcCl,Collidine
O
O
O
O
OMe
HO
OH
3) PDC
MeO
OMe
MeO
5f R = n-pentyl
17 R = n-pentyl
8 R = H
NaH, R'Br
THF
16 R = n-pentyl
Scheme 1. Synthesis of COTC analogues.
Table 1
determined. Since 18 possess an allylic alcohol functionality at
C-1 rather than a ketone functionality, it showed no cytotoxicity
against the normal and cancer cell lines, which gives support to
the aforesaid mechanism.
IC50 values for COTC analogue 5a–g and 18
Compd.
R
IC50
(WRL-68)
(
l
M)
IC50
(HepG2)
(
l
M)
IC50 (lM)
(HL-60)
5a
5b
5c
5d
5e
5f
2-Naphthylmethyl
Biphenylmethyl
Methyl
n-Propyl
n-Pentyl
n-Pentyl
n-Heptyl
—
>200
71.3
>200
191
169
48.3
152
>50
—
>50
—
>50
10.1
3.1
2.6
39.8
>50
In summary, seven COTC analogues were synthesized from
D-glucose and their direct cytotoxic activities towards WRL-68,
HepG2 and HL-60 cell lines were assessed. Aliphatic ether ana-
logues were found to display more selective cytotoxicity towards
cancer cells and greater potency than the arylmethyl ether ana-
logues. Configurational inversion of the stereochemistry at C-4
from alpha to beta drastically decreased the cytotoxic selectivity
towards cancer cells. Finally, alpha n-pentyl ether 5e was found
to be the most cancer cell-selective cytotoxic agent amongst the
seven analogues tested.
>50
33.6
16.1
10.1
>50
>50
5g
18
>200
HO
HO
HO
OH
OH
Acknowledgements
Streptol (18)
This work was supported by a Strategic Investments Scheme
administrated by the Center of Novel Functional Molecules, and
by a Direct Grant, The Chinese University of Hong Kong.
Figure 3. Structure of Streptol 18.
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