Synthesis and anti-HIV activity of 1,3-dithiolane nucleosides

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Synthesis and anti-HIV activity of 1,3-dithiolane nucleosides
Nghe Nguyen-Ba, William L. Brown, Laval Chan, Nola Lee, Livio Brasili, Dominique Lafleur and Boulos
Zacharie*
BioChem Pharma Inc., 275 Armand-Frappier Blvd., Laval, Québec, Canada H7V 4A7
Received (in Corvallis, OR, USA) 3rd March 1999, Accepted 20th May 1999
HS
HS
RO
RO
The potent activity displayed by 3A-azido-3A-deoxythymidine
(AZT)1 against human immunodeficiency virus (HIV) provides
impetus for the development of novel nucleoside analogues.2
Unfortunately, those compounds with the natural stereochem-
istry possess undesirable pharmacological properties3 and are
susceptible to the development of resistant stains of HIV.3,4 In
an attempt to overcome some of these detrimental side effects,
the carbohydrate moiety of 2A,3A-dideoxynucleoside analogs has
been replaced by other five membered rings.3d,4 It has been
demonstrated that hetero-substitution of these rings has a
profound effect on the biological activity of the resulting
nucleoside analogue5 as displayed by (2)-2A-deoxy-3A-thiacyti-
dine (3TC, Epivir) 1.5c,6
S
O
i
ii
O
Cl
O
S
S
Cl
O
7
9
iii
RO
S
S
Ac
OH
iv
RO
H
S
8
11
10
a
R = TBDPS, b R = Bz, c R = H, d R = Ac
Scheme 2 Reagents and conditions: i, NaSH, absolute EtOH, 210 °C; ii, 8,
Znl2, CH2Cl2; iii, DIBAL-H, PhMe or BH3·THF, B(MeO)3; iv, AcOCl,
Py.
immediately treated with aldehyde 8 in the presence of ZnI2 as
catalyst in CH2Cl211 to give the desired (±)-1,3-dithiolan-4-one
9 in moderate yield (50–60%). The initial synthetic procedure
was based upon using the TBDPS protecting group for the
hydroxy function of 9. Reduction of 9a with DIBAL-H (1.1
equiv.) in toluene gave thiolactol 10a which was subsequently
acylated to give the key intermediate 11a in high yield. The silyl
protecting group was later replaced by a benzoate group in order
to facilitate the separation of the cis and trans nucleoside
isomers. Applying the same conditions to reduce the benzoate
9b did not result in any reduction product. However, using
excess of DIBAL-H (3 equiv.) was successful and both the
thiolactone and the benzoate function were reduced to give diol
10c in 40% yield. Compound 10c was then bis-acylated giving
intermediate 11d in high yield. Efforts were then directed to
scale-up this procedure. Unfortunately, the DIBAL-H reduction
proved particularly intractable. We therefore investigated other
reducing agents that are selective and require little work-up.
Only BH3·THF (1.2 equiv.) catalyzed by B(OMe)3 (1 equiv.)
gave satisfactory results. The reduction was completed in 16 h
and the product 10b was obtained in 95% yield. This compound
was then acylated to give the expected product 11b. Following
the same procedure, a number of different leaving groups (Bz,
m-ClC6H4CH2 and p-O2NC6H4CH2) were successfully in-
troduced at C4 of the sugar moiety 11.
Compound 11b is suitable for coupling with silylated
cytosine or 5-fluorocytosine under refluxing conditions in
CH2Cl2 and in the presence of SnCl4 (Scheme 3). This gave the
desired nucleoside analogue 12 or 13 as a 1+2 mixture of cis and
trans isomers in moderate yields. Replacement of SnCl4 with
TMSI altered the ratio of the isomers. For example, compound
6 reacted with silylated N-acetylcytosine to give a mixture of
the cis and trans nucleosides 12 in 62% yield with a slight
predominance of the cis isomer.9 Similar results were obtained
using other leaving groups at C4. This did not improve the yield
O
O
S
HO
BASE
N
N
HO
S
O
1
3TCTM
NH2
NH2
2
N
S
N
BASE
HO
S
N
S
N
HO
3
4
O
As part of an ongoing search for new anti-AIDS leads, we
further explored this class of thioribonucleosides. These
compounds possess improved metabolic stability to phosphor-
ylases which cleave the glycosidic bond in nucleosides.7
Recently, we reported the anti-HIV activity of 2,4-disubstituted
1,3-oxathiolane nucleosides 2 by transposing the sulfur and
oxygen atoms of 1.4a In this series, the (2)-adenine derivative
3 with the natural configuration was found to be twice as active
as ddI in MT-4 cells. We further modified the oxathiolane ring
by replacement of the oxygen atom of 2 with sulfur. This is
exemplified by the general structure 4. Here we report the
synthesis and anti-HIV activity of this class of compounds.
The synthetic route to (±)-1,3-dithiolane compounds is based
upon coupling a persilylated heterocyclic base with a dithiolane
moiety 5 bearing a suitable leaving group Y at the 4-position
under Vorbruggen’s conditions.8 Two approaches were con-
sidered for the preparation of the ring 5. The first route is the
introduction of a leaving group at position 4 using peroxide
derivatives. For example, dithiolane 6 was prepared by treating
5a with benzoyl peroxide in refluxing benzene (Scheme 1).9
The second approach offers a more general route for the
synthesis of the key intermediate 1,3-dithiolane 11† with a
variety of displaceable leaving groups at C-4. Our synthetic
strategy was based on the preparation of 1,3-dithiolan-4-one 9,
followed by reduction and acylation to give 4-acyloxy-
1,3-dithiolane 11 (Scheme 2). Thus, reaction of freshly distilled
ClCH2COCl with excess NaSH (3 equiv.) in absolute ethanol at
210 °C gave 710 in quantitative yield. The crude product was
NH2
NH2
NH2
X
X
X
N
N
N
+
BzO
N
O
HO
N
O
HO
N
O
i
ii
HS
S
11b
S
S
S
Y
i
RO
BzOCH2CHO
+
S
S
S
S
HS
5a R = Bz, Y = H
R = Bz, Y = OBz
12 X = H
13 X = F
15 X = H
17 X = F
14 X = H
16 X = F
ii
6
Scheme 1 Reagents and conditions: i, C6H6, TSA; ii, BzOOH, C6H6,
Scheme 3 Reagents and conditions: i, 2,4-Bis(trimethylsilyloxy)pyr-
heat.
imidine, ClCH2CH2Cl, SnCl4, reflux, 16 h; ii, NH3, MeOH.
Chem. Commun., 1999, 1245–1246
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Cl
N
Cl
N
for technical assistance with HPLC separation of enantiomers,
and Ms L. Marcil for secretarial and technical assistance.
N
N
N
N
N
N
HO
HO
X
X
S
S
Notes and references
18 X = H
19 X = NH2
20 X = H
21 X = NH2
S
S
† Selected data for 10b: colorless oil; dH(CDCl3): 8.03 (m, 2H), 7.57 (dt,
1H, J 7, 1), 7.44 (t, 2H, J 7), 5.85 (m, 1H), 4.84 and 4.76 (t’s, 1H, J 7), 4.68
and 4.39 (m’s, 1H), 4.48 and 4.27 (m’s, 1H), 3.30 (m, 2H), 2.85 (m, 1H). For
16: dH(DMSO-d6) 8.34 (d, 1H, J 7.5), 7.83 (br s, 1H), 7.59 (br s, 1H), 6.39
(m, 1H), 5.57 (t, 1H, J 5.5), 4.62 (t, 1H, J 6), 3.75 (t, 2H, J 6), 3.59 (m, 2H).
For 17; dH(DMSO-d6) 8.10 (d, 1H, J 7 Hz), 7.78 (br s, H), 7.54 (br s, 1H),
6.41 (t, 1H, J 2), 5.36 (t, 1H, J 5.5), 4.81 (t, 1H, J 7 Hz), 3.45 (m, 4H). For
19: dH(DMSO-d6) 8.45 (s, 1H), 7.03 (br s, 2H), 6.40 (t, 1H, J 4), 5.53 (t, 1H,
J 6), 4.73 (t, 1H, J 7), 3.80 (dd, 1H, J 13, 4.5), 3.74 (t, 2H, J 6), 3.63 (dd,
1H, J 13, 5); dC(DMSO-d6) 159.80, 153.49, 150.01, 141.95, 123.61, 65.57,
64.52, 56.23, 42.93; HRMS (FAB): M+ calc. for C9H11ClN5OS2 304.00937,
found 304.00880. For 21: dH(DMSO-d6) 8.34 (s, 1H), 7.02 (br s, 2H), 6.44
(t, 1H, J 3), 5.41 (t, 1H, J 6), 4.86 (t, 1H, J 7), 3.58 (m, 3H), 3.49 (m, 1H);
HRMS (FAB): M+ calc. for C9H11ClN5OS2 304.00937, found 304.00840.
For 26: mp 108–110 °C; dH(DMSO-d6) 8.06 (d, 1H, H-6A, J 7.63), 7.20 (br
d, 2H, NH2), 6.47 (t, 1H, J 4.4), 5.74 (d, 1H, J 7.42) 5.50 (t, 1H), 4.61 (t, 1H,
J 6.5 Hz), 3.74 (t, 2H, J 6.00 Hz), 3.46 (dd, 1H, J 4.30, 12.9), and 3.36 (dd,
1H, J 4.1, 10.4); HRMS (FAB): M+ calc. for C8H12N3O2S2 246.03709,
found 246.03610. For 27: mp 200–202 °C (decomp.); dH(DMSO-d6) 7.90
(d, 1H, J 7.35), 7.17 (br d, 2H), 6.48 (d, 1H, J 3.72), 5.69 (d, 1H, J 7.47),
5.38 (t, 1H), 4.74 (t, 1H, J 6.83), 3.43 (m, 4H); HRMS (FAB): M+ calc. for
C8H12N3O2S2 246.03709, found 246.034640.
i
O
O
N
N
N
N
NH
NH
HO
HO
N
X
N
X
S
S
22 X = H
23 X = NH2
24 X = H
25 X = NH2
S
S
Scheme 4 Reagents and conditions: i, 25% aq. Me3N–H2O, 55–65%.
or the cis+trans ratio. The next step was the separation of the
isomers 12 or 13. This was achieved by flash chromatography
on silica gel by prior acetylation of the amino group (cytosine)
or by reverse chromatography HPLC after deprotection
(5-fluorocytosine). The protecting groups were then removed
by treatment with methanolic ammonia to give the desired
nucleosides 14–17 in high yields. The relative stereochemistry
of these products was assigned by difference NOE spectra.
Similarly, uracil, thymine, adenine and guanine derivatives
were produced from 11b using the same conditions. However,
in the case of hypoxanthine and guanine analogs 22–25,
synthesis was undertaken by treating the corresponding
6-chloropurine or 2-amino-6-chloropurine derivatives 18–21
with 20 equiv. of an aqueous Me3N solution in water (Scheme
4).
The anti-HIV activity of (±)-1,3-dithiolane nucleoside ana-
logues was evaluated in MT-4 (human T helper) cells at
concentrations up to 100 mg ml21 and compared with 3TC™
(Epivir)12 and the 5-fluoro derivative (FTC).13 In this assay,
only cis cytosine and 5-fluorocytosine derivatives 14 and 16
displayed inhibitory activity at ID50 of 9.3 and 4.8 mg ml21 and
were not cytotoxic at 100 mg ml21, whereas 3TC™ and FTC
showed anti-HIV activity at 0.3 and 0.14 mg ml21, respectively.
All the other nucleosides did not exhibit antiviral activity with
no cytotoxicity up to 100 mg ml21. In contrast, cis and trans
6-chloropurine derivatives 18 and 20 showed cytotoxicity at
1 H. Mitsuya, J. K. Weinhold, P. A. Furman, M. H. St-Clair, S. Nusinoff-
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J. Branalt and I. Kvarnstrom, J. Org. Chem., 1996, 61, 3604 and
references cited therein.
5 (a) T. S. Mansour, C. A. Evans, M. A. Siddiqui, M. Charron, B.
Zacharie, N. Nguyen-Ba, N. Lee and B. Korba, Nucleosides Nucleo-
tides, 1997, 16, 993; (b) H. Soudeyns, X. J. Yao, Q. Gao, B. Belleau,
J.-L. Kraus, N. Nguyen-Ba, B. Spira and M. A. Wainberg, Antimicrob.
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Bioorg. Med. Chem. Lett., 1997, 7, 1475.
CD50 of 10 mg ml21
.
Described herein is a novel class of anti-HIV (±)-1,3-dithio-
lane nucleoside analogues. The biological results demonstrate
that replacement of an oxygen atom of the oxathiolane with
sulfur causes reduction in antiviral activity. It should be noted
that compounds 14 and 16 are racemic. Resolution of the
enantiomers may improve the activity. Therefore, enantiomeric
separations of the racemic cis 14 was undertaken by chiral
HPLC.14 This gave the two enantiomers 26 and 27 in a
6 U.S. Pat. 05047407, BioChem Pharma Inc.; M. A. Nowak, S. Bon
hoeffer, A. M. Hill, R. Bochme, H. C. Thomas and H. McDade, Proc.
Natl. Acad. Sci. U.S.A., 1996, 93, 4398.
NH2
NH2
N
N
7 J. A. Secrist III, K. N. Tiwari, A. T. Shortnacy-Fowler, L. Messini, J. M.
Riordan and J. A. Montgomery, J. Med. Chem., 1998, 41, 3865; M. R.
Dyson, P. L. Coe and R. T. Walker, J. Med. Chem., 1991, 34, 2782.
8 H. Vorbruggen, K. Krolikiewicz and B. Bennua, Chem. Ber., 1981, 114,
1234.
N
O
N
O
S
S
HO
HO
S
S
26 [α]2D5 +75 (c 0.10, H2O)
27 [α]2D5 –70 (c 0.10, H2O)
9 N. Nguyen-Ba, W. Brown, N. Lee and B. Zacharie, Synthesis, 1998,
759.
10 M. Therien, J. Y. Gauthier and R. N. Young, Tetrahedron Lett., 1988,
29, 6733.
11 J. Y. Gauthier, T. Henien, L. Lo, M. Therien and R. N. Young,
Tetrahedron Lett., 1988, 29, 6729.
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Future, 1993, 18, 319 and references cited therein.
13 L. W. Frick, L. St-John, L. C. Taylor, G. R. Painter, P. A. Furman, D. C.
Liotta, E. S. Furfine and D. J. Nelson, Antimicrob. Agents Chemother.,
1993, 37, 2285.
reasonable yield. It was found that isomer 27 possesses the
natural configuration as evidenced by enzymatic resolution of
the racemic mixture. Thus treatment of the mixture of the two
enantiomers with cytidine deaminase converted only 27 to its
corresponding uracil derivative. However, the unnatural en-
antiomer 26 was recovered and characterized by comparison of
HPLC retention time and optical rotation with the previously
isolated isomer. Both enantiomers were submitted for anti-HIV
evaluation. Neither of the two compounds displayed improved
antiviral activity.
14 Chiral Column: Cyclobond I 2000 Beta-RSP 4.6 mm ID 3 250 mm;
mobile phase 10% MeCN–0.05% (AcOH–Et3N, pH 6.74); pressure 965
psi and flow rate 0.50 ml min21. For 26: tR = 19.880 min; for 27: tR
=
We thank Drs T. Bowlin and R. Storer for reading the
manuscript, Drs P. Hopewell and N. Cammack of Glaxo Group
Research for testing the compounds, Ms L. Bernier and J. Dugas
22.201 min.
Communication 9/01927H
1246
Chem. Commun., 1999, 1245–1246