Synthesis and antiviral activities of N-9-oxypurine 1,3-dioxolane and 1,3-oxathiolane nucleosides

Article abstract of DOI:10.1016/S0960-894X(00)00452-2

Two series of 1,3-dioxolanes and 1,3-oxathiolane nucleosides containing N-9-oxypurine were synthesized as potential antiviral agents. These compounds were prepared by reacting the sugar moieties with iodo- or bromotrimethylsilane, followed by treatment with a mixture of sodium hydride and the desired N-hydroxy purine base. The preparation of these N-hydroxybases was also described. No significant antiviral activity was observed against HIV, HBV, HSV-1, HSV-2, or HCMV. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0960-894X(00)00452-2

Bioorganic & Medicinal Chemistry Letters 10 (2000) 2223±2226
Synthesis and Antiviral Activities of N-9-Oxypurine 1,3-Dioxolane
and 1,3-Oxathiolane Nucleosides
Nghe Nguyen-Ba, Nola Lee, Laval Chan and Boulos Zacharie*
Â
BioChem Pharma Inc., 275 Armand-Frappier Blvd., Laval, Quebec, Canada H7V 4A7
Received 18 May 2000; accepted 31 July 2000
AbstractÐTwo series of 1,3-dioxolanes and 1,3-oxathiolane nucleosides containing N-9-oxypurine were synthesized as potential
antiviral agents. These compounds were prepared by reacting the sugar moieties with iodo- or bromotrimethylsilane, followed by
treatment with a mixture of sodium hydride and the desired N-hydroxy purine base. The preparation of these N-hydroxybases was
also described. No signi®cant antiviral activity was observed against HIV, HBV, HSV-1, HSV-2, or HCMV. # 2000 Elsevier
Science Ltd. All rights reserved.
Nucleoside antimetabolites play an important role in
the ®eld of cancer chemotherapy and treatment of viral
diseases. The potent activity displayed by 30-azido-
30-deoxythymidine (AZT)1 against human immunode®-
ciency virus (HIV) provides impetus for the development
of novel nucleoside analogues.2 One approach is to
replace the carbohydrate moiety of 20,30-dideoxynucleo-
side analogues3a by other ®ve-membered rings.3b,4 It has
been demonstrated that hetero-substitution of these
rings has a profound e€ect on the biological activity of
the resulting nucleoside analogue5 as displayed by ( )-
20-deoxy-30-thiacytidine (3TC1, Epivir) 15c,6 and (+)-20-
deoxy-30-oxacytidine 2.7
which the acyclic substituent or the sugar moiety is
linked to the heterocyclic base through a nitrogen±
oxygen bond, possess useful biological properties.8 For
example, the guanine derivative 3 showed potent and
selective activity against HSV-1, HSV-2, and VZV and is
superior to that of acyclovir.8c,d Therefore, we replaced
the acyclic portion or the ribose moiety by 1,3-oxathio-
lane and 1,3-dioxolane rings. This is exempli®ed by the
general structure 5.
The synthetic route to (Æ)-1,3-dioxolane and 1,3-oxa-
thiolane nucleoside analogues 5 is based upon reaction
of N-hydroxyheterocycles 7 with a dioxolane or oxa-
thiolane moiety 6 bearing a suitable leaving group Y
(Scheme 1).
Scheme 1.
Our strategy was to build stepwise the N-hydroxypurine
base since its direct synthesis by oxidation of the base
has not yet been achieved. N-9-Hydroxy-6-chloropurine
12 was selected as the key base in this series since it
provides a versatile intermediate for the synthesis of
several purine derivatives. Scheme 2 describes the pre-
paration of this key compound 12 from 4,6-dichloro-2-
As part of an ongoing search for new antiviral leads, we
further explored this class of 30-heterosubstituted
nucleosides. It has been reported that compounds in
*Corresponding author. Tel.: +1-450-978-7801; fax: +1-450-978-7777;
e-mail: zacharieb@biochempharma.com
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00452-2

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N. Nguyen-Ba et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2223±2226
Scheme 2. (a) PhCH2ONH2, i-Pr2 EtN, diglyme; (b) (EtO)3CH, 12 M
HCl, DMF; (c) BCl3, CH2Cl2, 0 ꢀC; (d) NaOMe, MeOH; (e) NH3,
MeOH; (f) O-C6H4(COCl)2, DMAP, Et3N, THF, 0 ꢀC.
formamidopyrimidine.9 The latter was treated with
benzyloxyamine to give 8, followed by cyclization to the
imidazole using triethylorthoformate.
Attempts to remove the benzyl group of 8 using TMSI
or Me2BBr resulted in degradation of the purine ring.
However, hydroxypurine 12 has been successfully pre-
pared in almost quantitative yield by treatment of chloro-
derivative 8 with BCl3 in CH2Cl2 at 0 ꢀC. This procedure is
amenable to scale-up. This chloro compound was also
converted to other 6-substituted purine derivatives. For
example, methoxypurine 13 was prepared by reaction of
chloro 8 with methanolic sodium methoxide followed by
catalytic hydrogenation.
Scheme 3. (a) Et2O, BF3/DME, 60 ꢀC; (b) HC(OEt)3, re¯ux; (c) HCl/
MeOH; Pd/C, H2; (d) PhCONCS, acetone; (e) K2CO3/aq MeOH±
acetone; (f) Cu(OCOCH3)2, aq NaOH; (g) Pd/C, EtOH, cyclohexane,
re¯ux; (h) Ac2O, pyridine; (i) C6H5COCl, pyridine.
Adenine 10 was also prepared from chloro 8.9 The
amino function was then protected as phthalimido
(Phth) 11 before hydrogenation to the free N-hydroxyl
compound. Removal of the benzyl group a€orded the
N-hydroxy adenine derivative 14. The synthesis of
9-hydroxy hypoxanthine 18 was performed through the
key intermediate 5-amino-4-aminocarbonyl-1-benzyloxy-
imidazole 1610 (Scheme 3). This compound is the pre-
cursor for a number of antiviral purine derivatives and
its preparation was based upon cyclization of the form-
amidine 15.11 Closure of the aminoimidazole ring 16 by
heating with triethylorthoformate followed by hydrogen-
ation gave N-9-hydroxyhypoxanthine 18 in a good yield.
Two approaches were considered for the preparation of
purine nucleosides. The ®rst route was based upon cou-
pling of a suitably functionalized N-hydroxy purine
base with 1,3-dioxolane or oxathiolane sugars under
Mitsunobu conditions. Unfortunately, this reaction
appeared to be ine€ective and resulted in low yield. In
fact, the Mitsunobu reaction between acylated furanose
and 1-hydroxybenzotriazole gave similar results.13
The second approach o€ers a more general route for the
synthesis of these nucleosides. Scheme 4 illustrates a
representative example where the sugar moiety of 1,3-
E€orts were then directed to the synthesis of N-9-ben-
zyloxy-2-amino-6-chloropurine, in particular guanine.
Our synthetic strategy was based on the cyclodesulfuri-
zation of the amino carbonyl 20 (Scheme 3). Thus,
treatment of 16 with benzoyl isothiocyanate in acetone
a€orded the thiocarbamoyl 19, which was hydrolyzed to
the thiourea 20.12 Cyclodesulfurization of the latter
under alkaline conditions gave protected guanine 21 in
high yield. This was hydrogenated to give hydroxy-
guanine 24. Due to the high polarity of the latter, we
decided to protect the 2-amino function of 24 as the
acetate or benzoate. This was achieved by treating 21
with acetic anhydride or benzoyl chloride in pyridine to
give 22 and 23, respectively. Unlike the 1-(benzyloxy)
imidazole which is unstable under alkaline conditions,8c
the N-benzyloxy purines 8±11, 17 and 21±23 are stable
under a variety of conditions. These include acidic and
basic conditions, as well as temperatures (<90 ꢀC) and
catalytic hydrogenation. In addition, the ®nal free
N-hydroxy compounds 12±14, 18 and 24±26 can be
stored for months at 0 ꢀC without decomposition.
Scheme 4.

N. Nguyen-Ba et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2223±2226
2225
dioxolane was reacted with iodo- or bromotrimethylsilane
then the solution was treated with a mixture of sodium
hydride and protected N-9-hydroxy guanine 26 in
DMF. This gave the desired nucleoside 27 as a 1:1 mix-
ture of cis and trans isomers in high yields. Replacement
of halotrimethylsilane with trimethylsilyltri¯ate or the base
sodium hydride with triethylamine did not alter the ratio of
isomers but reduced the yield. Separation of the isomers of
27 was achieved by ¯ash chromatography on silica gel or
by reverse chromatography HPLC after deprotection.
The protecting groups where then removed by treat-
ment with methanolic ammonia to give the expected
nucleosides 28 and 29 in good yields.14
100 mg/mL and compared with 3TC1 (Epivir) and AZT.
In this assay, none of the nucleosides displayed any
inhibitory activity or cytotoxicity up to 100 mg/mL
except hypoxanthine 45, which showed cytotoxicity at
CD50 of 10 mg/mL. The anti-HBV activity of these
nucleosides was assessed in hepatoma cell line 2.2.15
transfected with human HBV. None of these com-
pounds showed activity against extracellular HBV. The
anti-herpetic activities and cytotoxicities of these com-
pounds were determined in plaque reduction assays
in vero cells and Flow 2002 (human ®broblast) cells
infected with HSV-1 (KOS strain), HSV-2 (186 strain)
and HCMV (WFI strain), respectively. The cis and trans
guanine derivatives 28 and 29 were weakly inhibitory to
HSV-1 and HSV-2 replication with no cytotoxicity up
to 100 mg/mL.
Similarly, 6-methoxy purine 39±42 and hypoxanthine
43±46 derivatives were produced using the same con-
ditions. However, in the case of 6-chloro 35±38 and
adenine 47±50 nucleosides, the yield was low with
undesirable byproducts. We therefore decided to
investigate a number of other synthetic routes to pre-
pare these compounds. For example, the syntheses of
chloro 35±38 were achieved by treating the hypo-
xanthine 32 or 33 with either phosphorous oxychloride
in DMF or with carbon tetrachloride±triphenyl-
phosphine (3:1) in acetonitrile. In the case of adenine
47±50, two approaches were considered for their prep-
aration. The ®rst route was to react the 6-methoxy-
purine derivative 34 with ethanolic ammonia in a bomb
for 48 h. This method resulted in low yield. An alter-
native approach was by a halogen amino group inter-
change of the appropriate 6-chloropurine precursor.
For example, treatment of the chloro derivative 30 or 31
with ethanolic ammonia in a bomb gave high yields of
the expected adenine 47±50.
Described herein is a novel class of (Æ)-1,3-dioxolane
and 1,3-oxathiolane nucleoside analogues. The biologi-
cal results demonstrate that linking the sugar to the
heterocyclic base through an oxygen causes a dramatic
reduction in antiviral activity.
Acknowledgements
We thank Drs. R. Storer and T. Bowlin for reading the
manuscript, Mrs. L. Bernier and J. Dugas for technical
assistance with HPLC puri®cation and Mrs. L. Marcil
for secretarial and technical assistance.
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The anti-HIV activity of (Æ)-1,3-dioxolane and 1,3-
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in MT-4 (human T helper) cells at concentrations up to

2226
N. Nguyen-Ba et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2223±2226
8. (a) Wyatt, P. G. US patent 4,910,307. (b) Harnden, M. R.
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J=6.0 Hz), 4.23 (dd, 1H, J=4.3, 9.9Hz), 4.08 (dd, 1H, J=1.3,
10.0Hz), 3.46 (dd, 2H, J=3.4, 5.9 Hz); LRMS (FAB) m/z 270
(MH+). For 41: mp 162±164 ꢀC; dH (DMSO-d6) 8.65 (s, 1H), 8.56
(s, 1H), 6.20 (dd, 1H, J=2.8, 4.4 Hz), 5.60 (t, 1H, J=6.0 Hz),
5.45 (t, 1H, J=4.9 Hz), 3.84 (m, 1H), 3.70 (m, 1H), 3.57 (m,
2H); dC (DMSO-d6) 160.89, 152.66, 147.90, 140.78, 112.21,
106.61, 89.43; HRMS (FAB) M+ calcd for C10H12N4O4S
285.065752, found 285.068300. For 42: mp 178 ꢀC, dH (DMSO-
d6) 8.66 (s, 1H), 8.56 (s, 1H), 6.30 (dd, 1H, J=2.9, 4.2 Hz), 5.86
(t, 1H), 5.17 (t, 1H, J=5.0 Hz), 3.55 (m, 1H), 3.48 (m, 1H),
3.47 (m, 2H); dC (DMSO-d6) 160.86, 152.74, 148.29, 140.69,
117.12, 111.13, 87.09, 64.07, 54.54, 36.16; HRMS (FAB) M+
calcd for C10H12N4O4S 285.065752, found 285.068600. For 45:
mp 238ꢀC (d); dH (DMSO-d6) 12.21 (s, 1H), 8.36 (s, 1H), 8.09 (s,
1H), 6.12 (dd, 1H, J=2.8, 4.1 Hz), 5.52 (t, 1H, J=5.8 Hz), 5.46
(t, 1H, J=4.9Hz), 3.88 (m, 1H), 3.68 (m, 1H), 3.53 (m, 2H); dC
(DMSO-d6) 156.87, 146.99, 144.61, 137.30, 120.25, 112.28, 89.26,
65.22, 35.98; HRMS (FAB) M+ calcd for C9H10N4O4S
271.050102, found 271.052970. For 46: mp 147 ꢀC; dH (DMSO-
d6) 12.22 (b, 1H), 8.35 (s, 1H), 8.08 (s, 1H), 6.26 (dd, 1H, J=2.8,
4.3 Hz), 5.83 (t, 1H, J=5.2 Hz), 5.17 (t, 1H, J=5.0 Hz), 3.64 (m,
1H), 3.56 (m, 1H), 3.45 (m, 2H); HRMS (FAB) M+ calcd for
C9H10N4O4S 271.050102, found 271.052970.
11. Harnden, M. R.; Jennings, L. J.; Mckie, C. M. D.; Parkin,
A. Synthesis 1990, 893.
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guanines has been reported by (a) Alhede, B.; Clausen, F. P.;
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Org. Chem. 1991, 56, 2139. (b) Harnden, M. R.; Parkin, A.;
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14. Selected data for 28: mp >220 ꢀC; dH (DMSO-d6) 11.68
(bs, 1H), 7.86 (s, 1H), 6.57 (bs, 2H), 5.87 (d, 1H, J=3.8 Hz),
5.23 (t, 1H, J=6.0 Hz), 5.13 (t, 1H, J=3.9 Hz), 4.38 (d, 1H,
J=10.4 Hz), 4.04 (dd, 1H, J=4.1, 10.4 Hz), 3.57 (m, 2H).
LRMS (FAB) m/z 270 (MH+). For 29: mp >240 ꢀC (d); dH
(DMSO-d6) 11.71 (bs, 1H), 7.87 (s, 1H), 6.62 (bs, 2H), 5.96 (dd,
1H, J=2.7, 4.1 Hz), 5.42 (t, 1H, J=3.3 Hz), 4.98 (t, 1H,