Synthesis of novel D-2′-Deoxy-2′-C-difluoromethylene-4′-thiocytidine as a potential antitumor agent

Article abstract of DOI:10.1021/ol017112v

(formula presented) 2′-Deoxy-2′-C-difluoromethylene-4′-thiocytidine (4) as a potential antitumor agent was synthesized starting from L-xylose via 2-deoxy-2-C-difluoromethylene-4-thiosugar as a key intermediate. An elimination product, 8, was always formed

Full text of DOI:10.1021/ol017112v

ORGANIC
LETTERS
2002
Vol. 4, No. 4
529-531
Synthesis of Novel D-2′-Deoxy-2′-
C-difluoromethylene-4′-thiocytidine as a
Potential Antitumor Agent
,†
Moo Hong Lim,† Hea Ok Kim,‡ Hyung Ryong Moon,§ Moon Woo Chun,* and
,§
Lak Shin Jeong*
College of Pharmacy and DiVision of Chemistry and Molecular Engineering, Seoul
National UniVersity, Seoul 151-742, Korea, and Laboratory of Medicinal Chemistry,
College of Pharmacy, Ewha Womans UniVersity, Seoul 120-750, Korea
lakjeong@mm.ewha.ac.kr
Received November 23, 2001
ABSTRACT
2′-Deoxy-2′-C-difluoromethylene-4′-thiocytidine (4) as a potential antitumor agent was synthesized starting from L-xylose via 2-deoxy-2-C-
difluoromethylene-4-thiosugar as a key intermediate. An elimination product, 8, was always formed as the major product during removal of the
protecting groups under acidic or basic conditions. However, utilizing neutral reaction conditions to remove the protecting groups afforded
the desired product 4 exclusively.
Promising biological activities such as antiviral and antitumor
are generally exhibited by 2′-modified nucleosides, among
which 2′-difluoro-modified gemcitabine (1)1 exhibits potent
antitumor activity and is being clinically used for the
treatment of various solid tumors. Recently, on the basis of
the antitumor activity of 1, Yoshimura and co-workers
synthesized two of its corresponding 4′-thio analogues 2 and
3 of which the former was found to be potent against human
T-cell leukemia CCRF-HSB-2 cells2 and the latter much
more potent than compound 2 and 1-â-D-arabinofuranosyl
cytosine (ara-C) in the same cells.2
Therefore, on the basis of the bioisosteric rationale, it was
of interest to design and synthesize compound 4 (Figure 1),
Figure 1. Rationale for the design of the target nucleoside 4.
† College of Pharmacy.
‡ Division of Chemistry and Molecular Engineering.
§ Laboratory of Medicinal Chemistry.
which combines the characteristics of compounds 2 and 3,
and evaluate its antitumor activity. During preparation of our
target compound 4 (2′-C-difluoromethylene), especially
during removal of the protecting groups, we have encoun-
tered the occurrence of unique chemistry in its sugar moiety
in sharp contrast to the preparation of 3 (2′-C-methylene).3
(1) (a) Hertel, L. W.; Kroin, J. S.; Misner, J. W.; Tustin, J. M. J. Org.
Chem. 1988, 53, 2406. (b) Hertel, L. W.; Boder, G. B.; Kroin, J. S.; Rinzel,
S. M.; Poore, G. A.; Todd, G. C.; Grindey, G. B. Cancer Res. 1990, 50,
4417.
(2) (a) Yoshimura, Y.; Kitano, K.; Satoh, H.; Watanabe, M.; Miura, S.;
Sakata, S.; Sasaki, T.; Matsuda, A. J. Org. Chem. 1996, 61, 822. (b)
Yoshimura, Y.; Kitano, K.; Yamada, K.; Satoh, H.; Watanabe, M.; Miura,
S.; Sakata, S.; Sasaki, T.; Matsuda, A. J. Org. Chem. 1997, 62, 3140.
10.1021/ol017112v CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/25/2002

Herein, we report the synthesis of a novel D-2′-deoxy-2′-C-
difluoromethylene-4′-thiocytidine and its related chemistry
combined with biological activity.
Scheme 2a
Our strategy toward the target nucleoside 4 involved
preparation of a glycosyl donor, 2-deoxy-2-C-difluorometh-
ylene-4-thiosugar, and then condensation with silylated
cytosine. The key step for the synthesis of the glycosyl donor
6 (Scheme 1) was to introduce the difluoromethylene group
Scheme 1
a Reagents: (a) i. mCPBA, CH2Cl2, -78 °C; ii. silylated N4-
acetylcytosine, TMSOTf, ClCH2CH2Cl, 0-50 °C, 1 h.
To avoid the formation of the elimination product 8
obtained under acidic conditions, we decided to change the
protecting group to a benzoyl group (Scheme 3). Treatment
at the C2 position. Thus, L-xylose was converted to the
known ketone 53 according to an efficient procedure
developed in our laboratory. Initial attempts to synthesize
the difluoromethylene compound 6 from ketone 5, utilizing
dibromodifluoromethane/Zn-dust/triphenylphosphine in vari-
ous solvents such as CH3CN, DMF, or CH2Cl2 either failed
to produce the desired compound 6 or resulted in disap-
pointing yields (2%).4 However, using HMPT5 instead of
triphenylphosphine under refluxing conditions gave the key
intermediate 6 in 47% yield.
Scheme 3a
The key intermediate 6 obtained was oxidized to the
sulfoxide, which was condensed with silylated N4-acetyl-
cytosine to give the protected nucleosides 7a and 7b (Scheme
2). The anomeric configurations of 7a and 7b were readily
assigned by 2D NOESY experiments. Treatment of 7a with
boron tribromide in methylene chloride followed by quench-
ing with methanol did not give the desired nucleoside 4 but
afforded the elimination product 8 exclusively. Employing
boron trichloride in methylene chloride followed by quench-
ing with methanol also gave the same result. Since this
phenomenon never occurred in the case of 2′-deoxy-2′-C-
methylene-4′-thiocytidine,3 it is believed that strong elec-
tronegative difluoro substituents played a major role in
forming the elimination product under strongly acidic condi-
tions.
a Reagents (a) BBr3, CH2Cl2, -78 °C, 3 h; (b) BzCl, pyridine,
rt, 1 h; (c) i. mCPBA, CH2Cl2, -78 °C; ii. silylated N4-benzoyl
cytosine, TMSOTf, ClCH2CH2Cl, 0-50 °C, 1 h.
of 6 with boron tribromide at -78 °C gave only the diol 9
in 92% yield. Unlike the case of 7a, the elimination product
was not detected, probably due to the absence of an electron-
withdrawing nucleobase. The diol 9 was reacted with benzoyl
chloride in pyridine to give dibenzoate 10, which was
converted to the protected nucleoside 11 according to a
procedure similar to that used in Scheme 2. However, it is
interesting to note that condensation with dibenzoate 10 only
afforded the â-isomer 11, unlike condensation with the
(3) Jeong, L. S.; Moon, H. R.; Choi, Y. J.; Chun, M. W.; Kim, H. O. J.
Org. Chem. 1998, 63, 4821.
(4) (a) Hayashi, S.-I.; Nakai, T.; Ishikawa, N.; Burton, D. J.; Naae, D.
G.; Kesling, H. S. Chem. Lett. 1979, 983. (b) Burton, D. J.; Kesling, H. S.;
Naae, D. G. J. Fluorine Chem. 1981, 18, 293. (c) Burton, D. J.; Yang,
Z.-Y.; Qiu, W. Chem. ReV. 1996, 96, 1641.
(5) (a) Motherwell, W. B.; Tozer, M. J.; Ross, B. C. Chem. Commun.
1989, 1437. (b) Houlton, J. S.; Motherwell, W. B.; Ross, B. C.; Tozer, M.
J.; Williams, D. J.; Slawin, A. M. Z. Tetrahedron 1993, 49, 8087.
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Org. Lett., Vol. 4, No. 4, 2002

dibenzyl derivative 6. It is likely that these results are related
to the stability of condensation products; the R-isomer might
be thermodynamically unstable under the reaction conditions,
presumably due to the electron-withdrawing benzoyl groups.
To remove the benzoyl protecting groups of 11, we first used
more than 3 equiv of sodium methoxide, in which only
elimination product 8 was produced in 63% yield after silica
gel column chromatography. However, using a catalytic
amount of diluted 0.01 M sodium methoxide gave the desired
nucleoside 4, but elimination product 8 was still obtained as
a major product. As removal of the protecting groups under
basic conditions failed to give the desired nucleoside 4 as a
major product, we decided to remove the protecting groups
under neutral or almost neutral conditions (Scheme 4).
Scheme 4a
The diol 9 was converted to tert-butyldimethylsilyl (TBS)
protected sugar 12. After mCPBA oxidation of 12, the
resulting sulfoxide was condensed with N4-benzoylcytosine
to give 13 (31%) as the main product as in the case of
dibenzoate sugar 10. Treatment of 13 with sodium methoxide
gave 14 in 95% yield without forming any elimination
product. Since n-tetrabutylammonium fluoride as desilylating
agent was too basic, we used the neutralized (acetic acid
treated) desilylating agent for the removal of the TBS groups
in 14, resulting in the exclusive formation of the desired
nucleoside 4 in 99% yield. Similarly, using neutral triethy-
lamine-trihydrofluoride6 as desilylating agent also produced
the same results. However, use of a weak acid, 70% acetic
acid, as desilylating agent yielded the elimination product 8
as the main product (85%).
a Reagents: (a) TBSOTf, NEt3, CH2Cl2, 0 °C, 30 min; (b) i.
mCPBA, CH2Cl2, -78 °C; ii. silylated N4-benzoylcytosine, TM-
SOTf, ClCH2CH2Cl, 0-50 °C, 1 h; (c) 30% NaOMe, MeOH, rt, 3
h.
lines. These studies not only helped generate novel 2′-
modified nucleosides for biological evaluation but also show
useful pointers for designing potential anticancer agents.
The final nucleosides 4 and 8 were tested against solid
tumor cell lines such as lung cancer (A549) and colon cancer
(Col2), but they did not exhibit any cytotoxicity in this
system. Biological evaluation against leukemia cells is in
progress in our laboratory and it will be reported elsewhere.
Acknowledgment. This work was supported by a grant
from the Korea Health R & D Project, Ministry of Health &
Welfare, Korea (HMP-00-CH15-0014). The authors thank
Dr. Satyam Nampalli (Amersham Pharmacia Biotech) for
his help in preparing the manuscript.
In summary, on the basis of the bioisosteric rationale we
have synthesized a novel 2′-deoxy-2′-C-difluoromethylene-
4′-thiocytidine (4) along with the unexpected elimination
product 8 and evaluated their cytotoxicity against tumor cell
Supporting Information Available: Complete experi-
mental procedures and characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
(6) Pirrung, M. C.; Shuey, S. W.; Lever, D. C.; Fallon, L. Bioorg. Med.
Chem. Lett. 1994, 4, 1345.
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