Discovery of a new template for anticancer agents: 2′-deoxy-2′- fluoro-4′-selenoarabinofuranosyl-cytosine (2′-F-4′-seleno-ara- C)

Article abstract of DOI:10.1021/jm900852b

The first synthesis of 2′-deoxy-2′-fluoro-4′- selenoarabinofuranosyl pyrimidines as potent anticancer agents was accomplished using the DAST fluorination as a key step. It was first revealed that selenium atom participated in the DAST fluorination of 4′-s

Full text of DOI:10.1021/jm900852b

J. Med. Chem. 2009, 52, 5303–5306 5303
DOI: 10.1021/jm900852b
Recently, we have reported the synthesis of 40-selenoribo-
furanosyl pyrimidines and their unusual conformations.8
X-ray single crystallographic analysis indicated that 40-seleno-
uridine adopted unusual 20-endo/30-exo (south) conformation
unlike uridine, which showed a 20-exo/30-endo (north) con-
formation. Very recently, we have also reported the synthesis
of novel 20,30-didehydro-20,30-dideoxy-40-selenopyrimidine
nucleosides as potential antiviral agents.9 Therefore, it would
be of great interest to synthesize the 40-selenoarabinofurano-
syl analogues 4 (X=OH or F), combining the properties of 40-
selenonucleosides and arabinofuranosyl nucleosides and to
compare their anticancer activity with that of 40-oxonucleo-
side 1. It would be also very interesting to compare the
behavior of selenium in DAST fluorination with that of sulfur
because selenium has a larger empty d orbital than sulfur and
is more nucleophilic. Herein, we report the first asymmetric
synthesis of 40-selenoarabinofuranosyl pyrimidines (4) as
highly potent anticancer agents as well as the unusual beha-
vior of selenium in DAST fluorination.
For the introduction of the fluorine at the 20-position,
substrate 11 for DAST fluorination was first synthesized from
40-selenouridine (7),8 as shown in Scheme 2. Treatment of 40-
selenouridine (7) with trityl chloride afforded the 50-O-trityl
derivative 8. Refluxing compound 8 with 1,10-thiocarbonyl
diimidazole in toluene gave the 2,20-anhydro nucleoside 9 in
good yield. The remaining hydroxyl group in 9 was protected
with a tetrahydropyran (THP) group by treating with dihy-
dropyran (DHP) in the presence of p-TsOH to produce
compound 10. Compound 10 was treated with 1 N NaOH
to give the 20-hydroxyl derivative 11 with an arabino config-
uration.
Discovery of a New Template for Anticancer
Agents: 20-deoxy-20-fluoro-40-selenoarabino-
furanosyl-cytosine (20-F-40-Seleno-ara-C)
Lak Shin Jeong,* Dilip K. Tosh, Won Jun Choi,
Sang Kook Lee, You-Jin Kang, Sun Choi, Jin Hee Lee,
Hankil Lee, Hyuk Woo Lee, and Hea Ok Kim
Department of Bioinspired Science and Division of Life and
Pharmaceutical Sciences, Laboratory of Medicinal Chemistry,
College of Pharmacy, Ewha Womans University,
Seoul 120-750, Korea
Received June 11, 2009
Abstract: The first synthesis of 20-deoxy-20-fluoro-40-selenoarabi-
nofuranosyl pyrimidines as potent anticancer agents was accom-
plished using the DAST fluorination as a key step. It was first
revealed that selenium atom participated in the DAST fluorination
of 40-selenonucleosides and that conformational bias induced by
bulky selenium acted as a decisive factor in the DAST fluorination.
Among compounds tested, 20-F-40-seleno-ara-C (4a) exhibited
highly potent anticancer activity in all cancer cell lines tested and
was more potent than ara-C (1).
The carbohydrate moieties of nucleosides have been used
as good modification sites for the development of therapeu-
tically useful antiviral and antitumor agents. Among those,
nucleosides with an arabino configuration have been shown to
be a good template for antiviral and antitumor agents. For
example, 1-β-D-arabinofuranosylcytosine (ara-C,a 1)1 isbeing
used clinically as an anticancer agent and its 20-fluoro-analo-
gue 22 was also reported to have antiherpetic activity
(Figure 1).
As in the case of 40-thionucleosides,4-6 it was expected that
treatmentofcompound11withDAST would yieldthedesired
20-fluoro compound 12 as the sole product with the retention
of stereochemistry, which would proceed via the episelenium
intermediate (Scheme 3).
However, the 2,20-anhydro nucleoside 10, which was
formed from the abstraction of the amide hydrogen by
the fluoride anion was unexpectedly produced as the
major product. This finding has never been observed
in the 40-thionucleosides.4-6 To confirm the participation
of the C2 oxygen of the uracil, the N-3 position of
compound 11 was protected with an electron withdraw-
ing benzoyl group. As expected, DAST fluorination of
N3-benzoyl compound 13 afforded the 20-fluoro com-
pound 14 as the major product with concomitant forma-
tion of 10.
The protecting groups of compound12 were removed using
80% acetic acid to give the 20-fluoro-40-seleno-ara-U (15),
which wasconverted tothe final20-fluoro-40-seleno-ara-C(4a)
using the conventional method (Scheme 4).10
The stereochemistry of the fluorine in compound 15 was
confirmed by the typical long-range couplings shown in 20-
“UP”-fluoronucleosides and 2D NOESY experiments. The
long-range coupling constant between 20-fluorine and H-6 in
compound 15 was observed as 1.2 Hz and the same coupling
constant between 20-fluorine and 50-H was also measured,
confirming that the fluorine possessed the arabino configura-
tion. 2D NOESY experiments between H-6 and 20-H also
confirmed the stereochemistry of the fluorine atom (see
On the basis of a bioisosteric rationale, the 40-thio analogue
3
3,4 was also reported to be a potent anticancer agent. While
synthesizing this compound, the fluorination reaction pro-
ceeded via an episulfonium intermediate due to the participa-
tion of the sulfur in diethylaminosulfur trifluoride (DAST)
fluorination.4
Jeong et al.5-7 first reported that sulfur behaved differ-
ently during the DAST fluorination of a 20-hydroxyl group
with an arabino or ribo configuration, as shown in Scheme 1.
In the case of 20-hydroxyl with arabino configuration,5,7
DAST fluorination of compound 5 proceeded via an epi-
sulfonium intermediate, giving the 20-fluoro compound 5a
with an arabino configuration as the sole product without
intramolecular participation of the C2 oxygen of the uracil.
However, in case of 20-hydroxyl with the ribo configura-
tion,6 DAST fluorination of compound 6 produced 2,20-
anhydro nucleoside 6b directly from an attack of the C2
oxygen as an exclusive product instead of forming an
episulfonium intermediate.
*To whom correspondence should be addressed. Phone: 82 2 3277
3466. Fax: 82 2 3277 2851. E-mail: lakjeong@ewha.ac.kr.
a Abbreviations: ara-C, 1-β-D-arabinofuranosyl cytosine; DAST,
diethylaminosulfur trifluoride; THP, tetrahydropyran; DHP, dihydro-
pyran; NOESY, nuclear Overhauser effect spectroscopy; SRB, sulfo-
rhodamine B; TCI, 1,10-thiocarbonyl diimidazole.
r
2009 American Chemical Society
Published on Web 08/19/2009
pubs.acs.org/jmc

5304 Journal of Medicinal Chemistry, 2009, Vol. 52, No. 17
Jeong et al.
Scheme 3. DAST fluorination through the Participation of the
Selenium
Figure 1. Rationale for the design of the target nucleosides.
Scheme 1. Participation of the Sulfur in DAST Fluorination
Scheme 4. Synthesis of 20-Fluoro-40-selenoarabinofuranosyl-
nucleosides (4a and 15)
Scheme 2. Synthesis of the Substrate 11 for DAST Fluorina-
tion.
mechanism was examined using ab initio calculation as shown
in Figure 2. Using the Gaussian 03 program,12 the geometry
optimizations and vibration frequency calculations for 40-
thionucleoside and 40-selenonucleoside intermediates were
performed at B3LYP/6-311þG(d,p) level.
When compound 11, which adopts the 20-endo/30-exo
(south) conformation, was treated with DAST, selenium first
attacked the C2 position via a SN2 reaction, forming an
episelenium ion intermediate 11a, whose optimized geometry
is shown in Figure 2a. In this model, nucleophiles (fluoride or
the oxygen atom of the uracil) can only attack axially from the
convex side of 11a. The intramolecular SN2 reaction product
10 was obtained as a major product because the oxygen atom
˚
Supporting Information). Next, the conformation of com-
pound 15 was comparedwiththatof the corresponding40-thio
analogue,11 which has the 20-exo/30-endo (north) conforma-
of the uracil is quite close (with the distance of 2.724 A) to the
C2 position of the episelenium ion intermediate and is ori-
ented to the axial position, while axial attack of the fluoride
anion to the C2 position of the episelenium ion is hindered by
the axial 50-substituent, giving the intermolecular SN2 reac-
tion product 12 as a minor product. On the other hand,
the corresponding 40-thio analogue 110 adopts the 20-exo/30-
endo (north) conformation.11 The same DAST reaction of
the model compound 110 would form an episulfonium ion
(11a0)4-7 similarly to 11a, whose optimized geometry is shown
in Figure 2b. In this model, nucleophiles (fluoride or the
oxygen atom of the uracil) could attack from the convex side
but only in the equatorial direction, making an intramolecular
reaction from the convex side by the oxygen atom of the uracil
impossible. Hence, no 2,20-anhydronucleoside could be
formed. Instead, an intermolecular SN2 reaction by the fluo-
ride anion occurs exclusively to form 20-arabino-fluoronu-
cleoside 120 as the sole product. From this study, it was
revealed that the steric effect by bulky selenium overhelmed
the electronic effect (gauche effect) by sulfur, forcing the
conformation of 40-seleno-arabinonucleosides to adopt the
0
0
tion. The coupling constants (J1 H,2 F) of both compounds
were compared because the coupling constants in the 20-endo/
30-exo (south) conformerofanarbinosugar was reported tobe
larger than that in the 20-exo/30-endo (North) conformer.11
The coupling constant (3J1 H,2 F=11.2 Hz) in compound 15
0
0
was larger than that (3J1 H,2 F=7.9 Hz) in the corresponding
40-thio analogue, which was reported to adopt the northern
conformation,11 indicating that compound 15 has the unusual
20-endo/30-exo (south) conformation. The stereochemitry
of the fluorine and the conformation of compound 15
were finally confirmed by the X-ray crystal structure (data
not shown). This conformational bias was induced by the
steric effects of the selenium atom in the 4-selenofuranose,
which overwhelmed the electronic effects (gauche effects) of
sulfur or oxygen in the corresponding 4-thiofuranose or
furanose.
0
0
To determine why 40-thionucleosides and 40-selenonucleo-
sides gave different results on DAST fluorination, the reaction

Letter
Journal of Medicinal Chemistry, 2009, Vol. 52, No. 17 5305
Scheme 5. Synthesis of 40-Selenoarabinofuranosyl-nucleosides
(4b and 16)
Table 1. Anticancer Activity of the Final 40-Selenoarabinofuranosyl-
nucleosides, 4a, 4b, 15, and 16, in Several Human Cancer Cell Lines
IC50 (μM)a
compd
HCT116b A549c SNU638d T47De PC-3f K562g
4a
4b
15
16
1
1.1
7.13
0.47
8.83
0.14
4.72 ND
0.79
0.58
ND
0.63
86.6
>100
>100
5.3
0.005
>100
>100
1.9
0.09 ND
>100
>100 >100 >100
>100 >100 >100
>100
0.15
2.7
ND
55.9
0.05
gemcitabineh
0.04 ND
a Measured using SRB method. b Human colon cancer cell lines.
c Human lung cancer cell lines. d Human stomach cancer cell lines.
e Human breast cancer cell lines. f Human prostate cancer cell lines.
g Human leukemia cell lines. h Approved clinically for the treatment of
lung cancer; ND, not determined.
(K562) cells, using sulforhodamine B (SRB) protein staining
method.13 As shown in Table 1, 20-fluoro-40-seleno-ara-C (4a)
exhibited the most potent anticancer activity in all human
cancer cell lines among compounds tested. In general, the
cytosinederivatives4aand4bexhibitedvery potent anticancer
activity, while the uracil derivatives 15 and 16 did not show
any anticancer activity up to 100 μM. It was also found that
the 20-fluoro analogue 4b was more potent than the 20-hydro-
xyl derivative 4a. 20-Fluoro-40-seleno-ara-C (4a) was more
potent than the control, ara-C (1), in all tumor cells lines
except leukemia cell lines. 40-Seleno-ara-C (4b) is slightly
less potent than ara-C (1). The anticancer activity of the 20-
fluoro analogue 4b was compared with that of gemcitabine
approved clinically for the treatment of lung cancer. As shown
in Table 1, the 20-fluoro analogue 4b was found to be less
potent than gemcitabine. The comformation of 4b was also
compared with that of gemcitabine. Unlike compound 4b,
which adopts the 30-exo (south) conformation, gemcitabine
adopts the 30-endo (north) conformation,14 indicating that
they might have different mechanisms of action for anticancer
activity, but mechanism of action of 4b should be further
studied.
In summary, this paper reports the first synthesis of 20-
deoxy-20-fluoro-40-selenoarabinofuranosyl cytosine as a po-
tent anticancer agent, starting from 40-selenouridine using
DASTfluorination as a key step. It was revealed thatselenium
participates in the DAST fluorination like sulfur, but unlike
sulfur or oxygen, bulky selenium induced conformational bias
because steric effects by selenium overwhelmed electronic
effects (gauche effects) by sulfur or oxygen. 20-Fluoro-40-
seleno-ara-C (4a) exhibited very potent anticancer activity in
all human cancer cell lines tested. We are sure that this novel
template as well as unusual chemistry discovered in this study
will contribute greatly to the development of novel biologi-
cally active nucleosides.
Figure 2. Geometry optimized structures of (a) 40-selenonucleoside
intermediate and (b) 40-thionucleoside intermediate using the ab
initio calculations and their proposed reaction mechanisms.
20-endo/30-exo (south) conformation. This is opposite to the
corresponding 40-thio analogue which adopted the opposite
North conformation. This conformational bias induced
by selenium acted as a decisive factor in the DAST fluorina-
tion of 40-selenonucleosides. To the best of our knowledge,
this is the first example that selenium participates during
the DAST fluorination and DAST fluorination of the 4-thio-
or 4-selenosugar is absolutely controlled by the sugar con-
formation.
In addition to 20-fluoro-40-seleno-ara-U (15) and 20-fluoro-
40-seleno-ara-C (4a), 40-seleno-ara-U (16) and 40-seleno-ara-C
(4b) were also synthesized from 11 as shown in Scheme 5.
Treatment of 11 with 80% AcOH at 60 °C afforded the
40-seleno-ara-U (16), which was smoothly converted to the
40-seleno-ara-C (4b) using the same procedure used in
Scheme 4.
Anticancer activity of all synthesized 1-β-D-arabinofurano-
syl-40-selenonucleosides (4a, 4b, 15, and 16) were measured in
several human cancer cell lines such as colon cancer
(HCT116), lung cancer (A549), stomach cancer (SNU638),
breast cancer (T47D), prostate cancer (PC-3), and leukemia
Acknowledgment. This work was supported by the grant
(R31-2008-000-10010-0) from the World Class University
(WCU) project and the National Core Research Center

5306 Journal of Medicinal Chemistry, 2009, Vol. 52, No. 17
Jeong et al.
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procedures and characterization data for all compounds des-
1
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NOESY spectrum of compound 15. This material is available
free of charge via the Internet at http://pubs.acs.org.
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