Synthesis and potent anti-HIV activity of L-2',3'-didehydro-2',3'-dideoxy-2'-fluoro-4'-thiocytidine.

Article abstract of DOI:10.1021/ol0171665

[reaction: see text] L-2'-Fluoro-4'-thio-2',3'-unsaturated cytidine 11 was synthesized from (R)-2-fluorobutenolide 2, which was prepared from 2,3-O-isopropylidene-L-glyceraldehyde 1. The synthesized compound 11 shows potent antiviral activity against HIV-

Full text of DOI:10.1021/ol0171665

ORGANIC
LETTERS
2002
Vol. 4, No. 2
305-307
Synthesis and Potent Anti-HIV Activity
of L-2′,3′-Didehydro-2′,3′-
dideoxy-2′-fluoro-4′-thiocytidine
Yongseok Choi,† Hyunah Choo,† Youhoon Chong,† Sookwang Lee,†
,†
Sureyya Olgen,† Raymond F. Schinazi,‡ and Chung K. Chu*
Department of Pharmaceutical and Biomedical Sciences, College of Pharmacy,
The UniVersity of Georgia, Athens, Georgia 30602, and Emory UniVersity,
School of Medicine/Veterans Affairs Medical Center, Decatur, Georgia 30033
dchu@mail.rx.uga.edu
Received December 3, 2001
ABSTRACT
L-2′-Fluoro-4′-thio-2′,3′-unsaturated cytidine 11 was synthesized from (R)-2-fluorobutenolide 2, which was prepared from 2,3-O-isopropylidene-
L-glyceraldehyde 1. The synthesized compound 11 shows potent antiviral activity against HIV-1.
Synthetic nucleosides, which have no hydroxyl groups at 2′-
and 3′-positons, can be categorized as (a) 2′,3′-dideoxy-
nucleosides such as ddC1 and ddI,1 (b) 2′,3′-dideoxy-3′-oxa-
or -3′-thia-nucleosides (dioxolane or oxathiolane nucleosides)
such as DAPD2 and 3TC,3 and (c) 2′,3′-didehydro-2′,3′-
dideoxynucleosides such as d4T.4 Generally, nucleosides in
these categories show unique antiviral activities and toxicity
profiles. Often both nucleosides with natural and unnatural
configurations (D and L) are active against viruses, but
cytotoxicity resides, in some cases, mainly in one of the
isomers. For example, 3TC3a (L-2′,3′-dideoxy-3′-thiacytidine)
and FTC5 (L-2′,3′-dideoxy-3′-thia-5-fluorocytidine) show
antiviral activity against human immunodeficiency virus type
1 (HIV-1) with EC50 values of 0.07 and 0.009 µM in CEM
cells, respectively, and both exhibit no cytotoxicity up to
100 µM in CEM cells. In contrast, the D-isomers, (+)-BCH-
189 and D-FTC, are less active against HIV-1 (EC50 0.2 and
0.84 µM in CEM cells, respectively) than the L-isomers. (+)-
BCH-189, however, does show cytotoxicity (IC50 2.7 µM
in CEM cells). Therefore, it is of interest to investigate the
dideoxynucleosides in search of potent antiviral agents with
favorable toxicity profiles.
Although various modifications on the carbohydrate
moiety and heterocyclic bases of natural nucleosides have
been made, 2′,3′-unsaturated 4′-thionucleosides have not been
well investigated as a result of the synthetic difficulties. The
first synthesis of 4′-thio congeners of natural 2′-deoxy-
nucleosides, 2′-deoxy-4′-thionucleosides, was reported in
1991 by Walker et al.6 and Secrist et al.;7 these exhibited
† The University of Georgia.
‡ Emory University.
(1) Mitsuya, H.; Broder, S. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 1911-
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Cannon, D. L.; Alves, A. J.; Jeong, L. S.; Beach, J. W.; Chu, C. K. J. Med.
Chem. 1993, 36, 30-37.
(3) (a) Schinazi, R. F.; Chu, C. K.; Peck, A.; McMillan, A.; Mathis, R.;
Cannon, D.; Jeong, L. S.; Beach, J. W.; Choi, W. B.; Yeola, S.; Liotta, D.
C. Antimicrob. Agents Chemother. 1992, 36, 672-676. (b) Coates, J. A.
V.; Cammack, N.; Jenkinson, H. J.; Mutton, I. M.; Pearson, B. A.; Storer,
R.; Cameron, J. M.; Penn, C. R. Antimicrob. Agents Chemother. 1992, 36,
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M.; Peck, A.; Sommadossi, J. P.; St. Clair, M.; Wilson, J.; Furman, P. A.;
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10.1021/ol0171665 CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/21/2001

antiviral activities along with cytotoxicity. Several 4′-
thionucleosides such as 2′,3′-dideoxy,8 2′,3′-dideoxy-3′-C-
hydroxymethyl,9 and 2′,3′-didehydro-2′,3′-dideoxy-4′-thio-
nucleosides10 have been synthesized. Among these, L-2′,3′-
didehydro-2′,3′-dideoxy-4′-thionucleosides drew our attention
because of their anti-HIV activity, as well as the possibility
to improve their chemical stabilities by isosteric replacement
of 2′-hydrogen with 2′-fluorine, which has been demonstrated
by our previous work.11 It is well-established that 2′,3′-
dideoxy and 2′,3′-didehydro-2′,3′-dideoxy purine nucleosides
are quite unstable under acidic conditions, resulting in the
cleavage of a glycosidic bond. When a hydrogen atom is
replaced by a fluorine atom at the 2′ position, the stability
of the glycosidic bond in acidic media is greatly increased.
Herein we describe the preliminary enantiomeric synthesis
and anti-HIV activity of L-2′,3′-didehydro-2′,3′-dihydroxy-
2′-fluoro-4′-thiocytidine (11).
gave a hydroxy methylester, which was treated with iodine,
triphenylphospine, and imidazole in toluene at 60 °C for 4
h to give the iodoester 4 in 83% overall yield. During the
iodination, however, high temperature and longer reaction
time resulted in partial epimerization at C4. Currently, we
are optimizing the conditions, which prevent the epimeriza-
tion. A thiolacetate group was introduced by nucleophilic
displacement of the iodide group with potassium thiolacetate
in DMF to give the corresponding thiolacetate 5 in 91%
yield. DIBAL-H induced cyclization of the thiolacetate 5
followed by the Moffatt-type oxidation of the resulting
thiolactol gave a thiolactone 6 in 54% yield. The thiolactone
6 was deprotonated by LiHMDS and trapped as a TMS enol
ether, which was phenylselenylated using PhSeBr to intro-
duce the 2-phenylselenyl group exclusively at R position of
lactone 7 in 74% yield. 2-â-Fluoro-2-R-phenylselenyl thio-
lactone 7 showed no contamination by its â-isomer, 2-R-
fluoro-2-â-phenylselenyl thiolactone, because the sterically
demanding phenylselenyl group selectively occupied the R
position instead of the sterically crowded â position during
phenylselenylation of the TMS enol ether. The thiolactone
7 was reduced by DIBAL-H to give the corresponding lactol,
which was acetylated to afford the acetate 8 in 90% yield.
The target compound 11 was synthesized from (R)-2-
fluorobutenolide 2, which was prepared from 2,3-O-iso-
propylidene-L-glyceradehyde 1 in three steps by a known
method12 (Scheme 1). (R)-2-Fluorobutenolide 2 was hydro-
Condensation of the acetate 8 with N4-benzoylcytosine in
Vorbru¨ggen conditions gave the corresponding cytidine
analogue 9 in 51% yield, which underwent mCPBA oxida-
tion followed by elimination to give the N4,5′-protected 2′,3′-
unsaturated 2′-fluoro-4′-thiocytidine 10 in 80% yield. Under
the carefully controlled oxidation conditions, no significant
sulfoxide formation was obtained. After deprotection of
TBDPS and benzoyl groups, the target compound 1113 was
obtained in 88% yield (Scheme 2).
Scheme 1a
Scheme 2a
a Reagents and conditions: (a) H2, Pd(0), EtOAc; (b) (i) NaOH,
aq. EtOH, (ii) dimethyl sulfate, DMSO, (iii) I2, Ph3P, imidazole,
toluene, 60 °C; (c) KSAc, DMF; (d) (i) DIBAL-H, toluene, -78
°C, (ii) Ac2O, DMSO; (e) LiHMDS, TMSCl, PhSeBr, THF, -78
°C; (f) (i) DIBAL-H, toluene, -78 °C, (ii) Ac2O, TEA, CH2Cl2.
a Reagents and conditions: (a) silylated N4-benzoylcytosine,
TMSOTf, CH3CN; (b) mCPBA, CH2Cl2, -78 °C; pyridine, rt; (c)
(i) TBAF, THF, (ii) NH3, MeOH.
genated by treatment with 5% Pd-C under H2 to allow
complete conversion to â-2-fluorolactone 3 in a quantitative
yield. 1H NMR of compound 3 showed only a single isomer.
The lactone 3 was converted to an iodoester 4 in three
consecutive steps. Hydrolysis using NaOH in aqueous EtOH
followed by methylation of corresponding carboxylic acid
The anti-HIV activity of the cytidine analogue 11 was
evaluated in vitro in human peripheral blood mononuclear
306
Org. Lett., Vol. 4, No. 2, 2002

(PBM) cells, in which the compound 11 showed potent anti-
HIV activity (EC50 0.12 µM) without cytotoxicity up to 100
µM in PBM, CEM, and Vero cells.
In summary, we have developed the enantiomeric synthesis
of 2′,3′-unsaturated L-2′-fluoro-4′-thiocytidine 11 from L-
glyceraldehyde derivative 1, which exhibits potent anti-HIV
activity in vitro. This interesting biological result prompted
us to synthesize other purine and pyrimidine nucleoside
derivatives and optimize the procedures.
Acknowledgment. This research was supported by the
U.S. Public Health Service Research Grant AI 32351 and
AI25899 from the National Institute of Allergy and Infectious
Diseases, NIH.
(6) Dyson, M. R.; Coe, P. L.; Walker, R. T. J. Med. Chem. 1991, 34,
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OL0171665
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Young, R. J. J. Chem. Soc., Perkin Trans. 1 1995, 677-681.
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(13) Compound 11: mp 89-91 °C (dec); [R]26D 205.3° (c 0.14, MeOH);
UV(H2O) λmax 279.5 nm (ꢀ 19,900, pH 2), 272.0 nm (ꢀ 15,900, pH 7),
1
272.5 nm (ꢀ 16,200, pH 11); H NMR (DMSO-d6) δ 7.83 (d, J ) 7.4 Hz,
1H), 7.33, 7.37 (2 br s, 2H), 6.85 (s, 1H), 5.94 (s, 1H), 5.84 (d, J ) 7.5 Hz,
1H), 5.28 (t, J ) 5.4 Hz, 1H, D2O exchangeable), 4.02 (m, 1H), 3.59-
3.62 (m, 2H); 13C NMR (MeOH-d4) δ 167.5, 158.4, 156.6 (d, 2JC-F ) 276
Hz), 143.0, 112.6 (d, 3JC-F ) 17 Hz), 97.4, 65.7, 62.7 (d, 3JC-F ) 24 Hz),
48.9. Anal. Calcd for C9H10FN3O2S‚0.32CH2Cl2: C, 41.39; H, 3.97; N,
15.54; S, 11.86. Found: C, 41.15; H, 4.10; N, 15.55; S, 11.82.
(11) Lee, K.; Choi, Y.; Gullen, E.; Schlueter-Wirtz, S. Schinazi, R. F.;
Cheng, Y. C.; Chu, C. K. J. Med. Chem. 1999, 42, 1320-1328.
(12) Gumina, G.; Chong, Y.; Choi, Y.; Chu, C. K. Org. Lett. 2000, 2,
1229-1231.
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