Heterocyclic nucleoside analogues: Design and synthesis of antiviral, modified nucleosides containing isoxazole heterocycles

Article abstract of DOI:10.1016/S0960-894X(02)00182-8

We have designed and synthesized novel antiviral nucleoside analogues, which consist of isoxazole rings as modified sugars and nucleobases (thymine, uracil, and 5-fluorouracil) with a methylene linker between them. These compounds represent a new class of

Full text of DOI:10.1016/S0960-894X(02)00182-8

Bioorganic & Medicinal Chemistry Letters 12 (2002) 1395–1397
Heterocyclic Nucleoside Analogues: Design and Synthesis
of Antiviral, Modified Nucleosides Containing
Isoxazole Heterocycles
Yoon-Suk Lee and Byeang Hyean Kim*
National Research Laboratory, Department of Chemistry, Center for Integrated Molecular Systems,
Division of Molecular and Life Sciences, Pohang University of Science and Technology,
Pohang 790-784, South Korea
Received 31 October 2001; accepted 4 March 2002
Abstract—We have designed and synthesized novel antiviral nucleoside analogues, which consist of isoxazole rings as modified
sugars and nucleobases (thymine, uracil, and 5-fluorouracil) with a methylene linker between them. These compounds represent a
new class of modified nucleoside analogues and some of them show potent antiviral activities against Polio virus (Coxsackie B type
3 and Vesicular Stomatitis). # 2002 Elsevier Science Ltd. All rights reserved.
Modification of naturally occurring nucleosides is an
important area for the development of antiviral agents
such as anti-HIV drugs. Especially heterocyclic replace-
ment of the nucleoside ribose moiety has resulted in an
excellent anti-HIV drug, 3TC and is a promising field
for searching antiviral lead compounds.1 Adams and
coworkers reported on the preparation of heterocyclic
nucleoside analogues by using cycloaddition reactions
of 1-vinylthymine with 1,3-dipoles.2 Recently Zhao and
coworkers have successfully synthesized isoxazolidinyl
nucleosides 1 and dihydroisoxazole nucleosides 2 (Fig. 1),
and investigated their anti-HIV-1 activities.3 We
designed and synthesized novel isoxazole nucleosides 3
(Fig. 1), and report here their efficient syntheses by
using nitrile oxide cycloadditions with the propargylic
dipolarophiles. We also investigated antiviral activities
of these heterocyclic nucleoside analogues and reveal
some bioassay results in this letter.
Figure 1. Modified nucleoside analogues with heterocyclic rings.
allosteric binding site are exceptional antiviral com-
pounds having no 50-free hydroxyl group. In these cases,
50-hydroxyl groups are protected with bulky groups
such as t-butyldimethylsilyl group. We first synthesized
isoxazole and isoxazoline nucleosides with 50-hydroxyl
groups. Their antiviral activities are low. Then we
turned our attention to the design and synthesis of
isoxazole nucleosides 3 as antiviral compounds with
allosteric binding activities shown in TSAO and HEPT
analogues. The design of isoxazole nucleosides 3 as
novel heterocyclic nucleoside analogues were based on
the following considerations: (1) use of rigid isoxazole
heterocycles as surrogates for conformationally restricted
deoxyriboses, (2) introduction of the additional methy-
lene unit between isoxazole and nucleobase to render
conformational flexibility, (3) easy incorporation of
chiral amino functionality by using a-amino acids as
chiral starting materials.
Most of the nucleoside analogues possessing antiviral
activities rely upon specific phosphorylation of a virally
encoded kinase.4 Thus, it is well known that most anti-
viral nucleoside analogues should have 50-hydroxyl
groups for kinase phosphorylation. However, TSAO
analogues5 and HEPT analogues6 which bind to the
The N-protected amino aldoximes,7 the precursors of
nitrile oxides, were prepared from the corresponding amino
acids in a four-step procedure as reported by us before.7
*Corresponding author. Tel.: +82-54-279-2115; fax: +82-54-279-
3399; e-mail: bhkim@postech.ac.kr
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00182-8

1396
Y.-S. Lee, B. H. Kim / Bioorg. Med. Chem. Lett. 12 (2002) 1395–1397
protection group (Boc or benzoyl) of the amino func-
tionality played a vital role in the antiviral activities and
complete deprotecition gave much poorer antiviral
activities as shown in compounds 11a–11b. Among 17
isoxazole nucleoside analogues, compounds 8b and 9c
showed most potent anti-polio activities (Table 1).
Especially compound 8b is better than the reference
drug, ribavirin in terms of EC50 value but have poorer
selectivity index (SI) values. Although we do not know
Figure 2. Amino aldoximes prepared.
Starting from l-alanine, l-phenylalanine, l-valine,
l-leucine and l-isoleucine, we obtained the correspond-
ing N-Boc amino aldoximes 4a–4e as mixtures of E and
Z isomers (Fig. 2). The dipolarophilic N1-propargyl
N3-benzoyl pyrimidines 7a–7c were synthesized from
corresponding N3-benzoyl pyrimidines 6a–6c by treating
with propargyl alcohol in the presence of diethyl azodi-
carboxylate and triphenylphosphine.8 The required
N3-benzoyl pyrimidines 6a–6c were prepared by dibenz-
oylation of nucleobases 5a–5c at N1 and N3 positions
followed by selective deprotection at the N1 position
with weak base (Scheme 1).9
With both N-Boc amino aldoximes and N1-propargyl
N3-benzoyl pyrimidines in hand, we next carried out
[3+2] nitrile oxide cycloadditions by using a commer-
cial bleaching agent (typically contains 4% NaOCl) as a
practical reagent for nitrile oxide generation (Scheme 2).
We obtained ten isoxazole nucleosides having thymine
and uracil bases in the range of 41–78% yield.
5-Fluorouracil has been widely used for the develop-
ment of pharmaceutically active compounds.10 Thus we
designed and synthesized isoxazole nucleoside analo-
gues having a 5-fluorouracil base and a bulky groupat
the chiral carbon. Scheme 3 shows the preparation of
compounds 10a and 10b and fully deprotected com-
pounds 11a and 11b. In order to investigate the effect of
protecting groups at N3 of pyrimidine base and amino
site in the antiviral activity, we also prepared several
compounds with different protection patterns at two
nitrogen sites (Fig. 3).
Scheme 2.
With 17 modified nucleosides (8a–8e, 9a–9e, 10a–10b,
11a–11b, 12–14) in hand, we next investigated their
antiviral (anti-HIV, anti-HSV, anti-Polio) activities.
From the bioassay results, we recognized that the bulky
Scheme 1.
Scheme 3.

Y.-S. Lee, B. H. Kim / Bioorg. Med. Chem. Lett. 12 (2002) 1395–1397
1397
given by a referee. Antiviral bioassay was carried out at
the laboratory of Dr. J. Lee (KRICT).
References and Notes
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Figure 3. Prepared compounds with different protection patterns.
Table 1. Antiviral activities in terms of EC50 values
Compd
Anti-polio activity
(EC50, mg/mL)
Selectivity index
(CC50/EC50
)
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VSVb
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about the exact mechanism of action for antiviral
activity of our heterocyclic nucleoside analogues, we
speculate that these heterocyclic nucleoside analogues
may bind at allosteric sites rather than the catalytic
sites.
We are carrying out further modification of these com-
pounds to improve antiviral activity and selectivity
index and will report the results in due course.
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Acknowledgements
We are grateful to KISTEP for the financial support
through NRL (Laboratory for Modified Nucleic Acid
Systems) program. We also thank Korea Health 21
R&D project, CIMS and BK21 program for partial
support. We acknowledge the valuable suggestions