An entry to new isoxazoline analogues of dideoxynucleosides by bromonitrile oxide 1,3-dipolar cycloaddition

Article abstract of DOI:10.1016/S0040-4039(03)00752-4

A short and efficient synthesis of new isoxazolinyl nucleosides bearing the nucleobase at the 3-position is reported. The synthetic approach relies upon the 1,3-dipolar cycloaddition of bromonitrile oxide to allyl benzoate and subsequent substitution of the bromine by the nucleobase.

Full text of DOI:10.1016/S0040-4039(03)00752-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 3755–3758
An entry to new isoxazoline analogues of dideoxynucleosides by
bromonitrile oxide 1,3-dipolar cycloaddition
Evdoxia Coutouli-Argyropoulou* and Paraskevi Pilanidou
Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece
Received 5 February 2003; revised 14 March 2003; accepted 21 March 2003
Abstract—A short and efficient synthesis of new isoxazolinyl nucleosides bearing the nucleobase at the 3-position is reported. The
synthetic approach relies upon the 1,3-dipolar cycloaddition of bromonitrile oxide to allyl benzoate and subsequent substitution
of the bromine by the nucleobase. © 2003 Elsevier Science Ltd. All rights reserved.
Saturated and unsaturated 2%,3%-dideoxynucleosides
containing different functionalities and lacking the 3%-
hydroxyl group are expected to terminate DNA synthe-
sis after incorporation into the chains.1 Due to this
effect nucleoside analogues in which the furanose ring
has been replaced by a different carbo- or heterocyclic
ring have attracted special interest by virtue of their
biological action as antiviral and anticancer agents.2
Among them isoxazolidine and isoxazoline nucleosides
are emerging as an important class of dideoxynu-
cleoside analogues with potential pharmacological
activity. Thus, during the last decade several synthetic
approaches to isoxazolidine and isoxazoline nucleosides
have been reported.3 For the construction of the hetero-
cyclic ring the well known convenient 1,3-dipolar
cycloaddition approach has been applied in most cases.
Regarding the attachment of the nucleobase, two alter-
natives are followed, either before or after the forma-
tion of the heterocyclic ring. In the first case the
products are obtained from the reaction of nitrile
oxides or nitrones with appropriately designed vinyl
derivatives of pyrimidine and purine nucleobases. In the
second case, the cycloaddition takes place between the
dipole and an alkene bearing a suitable leaving group
which undergoes further nucleophilic substitution by
the base. In both cases the well established regioselec-
tivity of 1,3-dipolar cycloaddition reactions leads to the
formation of products of the general type 1, in which
the base is located at the 5-position of the ring next to
the oxygen atom. However, to the best of our knowl-
edge nothing is known about the synthesis of isoxazo-
lidine or isoxazoline nucleosides of types 2 and 3 in
which the base is attached to the 3- or 4-positions of the
ring (Scheme 1).
In this paper and in connection with our former studies
on cycloaddition reactions4 we wish to report the syn-
thesis of isoxazolin-3-yl nucleosides of type 3 applying a
new 1,3-dipolar cycloaddition approach. As appropri-
ate dipole and dipolarophile we chose bromonitrile
oxide and a protected allyl alcohol respectively. The
bromonitrile oxide introduces a bromine at the 3-posi-
tion of the ring, which can serve as a leaving group for
further substitution by the base. It should be mentioned
that in several synthetic schemes in which bromonitrile
oxide is used as a key intermediate, the bromine in the
3-bromoisoxazolines obtained is substituted readily by
oxygen nucleophiles, although there is no report about
nitrogen nucleophiles.5
The use of an allyl alcohol derivative as the dipolar-
ophile ensures the presence of a 5-hydroxymethyl group
in the final product, which potentially allows enzymatic
phosphorylation for antiviral expression or incorpora-
tion into automatic solid-phase synthesis.
Keywords: bromonitrile oxide; dipolar cycloaddition; isoxazolines;
dideoxynucleoside analogues.
* Corresponding author. Tel.: +2310997733; fax: +2310997679;
e-mail: evd@chem.auth.gr
Scheme 1.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00752-4

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E. Coutouli-Argyropoulou, P. Pilanidou / Tetrahedron Letters 44 (2003) 3755–3758
Scheme 2. Reagents and conditions: (i) K2CO3, EtOAc, rt, 24 h, 92%; (ii) B, NaH, DMF, 90°C, 3 days, 60–70%; (iii) KOH,
MeOH/H2O, rt, 10 h, 85–90%.
well as the N3 positions although alkylation at N1
predominates.9 The structure of the isomers can be
differentiated on the basis of the coupling constants
between H-C5 and H-N3 of 1-substituted uracils and
between H-C6 and H-N1 of 3-substituted thymines and
Thus bromonitrile oxide 5, derived in situ from the
aldoxime 4, was added regioselectively to the ester 6 to
give the bromoisoxazoline 7. In accordance with
analogous cycloadditions of 5 referred to in the litera-
ture,5 the reaction was regioselective and the 5-isomer
was obtained, after purification, as the sole product in
1
uracils in their H NMR spectra. In low concentrations
1
of very pure samples the couplings between H-C6 and
H-N1 and between H-C5 and H-N3 can be observed
and have been measured to be 6 Hz and 2 Hz, respec-
tively.10 In our case it was possible to see a coupling of
2 Hz between H-C5 and H-N3 of compound 8b. Thus,
high yield (92%), although the H NMR of the crude
reaction mixture showed contamination by traces of the
4-isomer. For the next step, the nucleophilic substitu-
tion, the nucleobases adenine, uracil and thymine were
tested under several reaction conditions usually
employed in glycosylation procedures. The use of silyl-
ated bases or phase-transfer catalysts such as tris[2-(2-
methoxyethoxy)ethyl]amine, applying several reaction
times and temperatures was unsuccessful. Finally, cou-
pling of the bases was achieved using the sodium salts
of the bases prepared with sodium hydride in DMF and
the isoxazoline nucleosides 8 were obtained in satisfac-
tory yields (60–70%). These were further hydrolyzed to
the hydroxy derivatives 9 following standard alkaline
hydrolysis procedures (Scheme 2).
1
in the H NMR spectra of an analytically pure sample
of compound 8b, H-C5 and H-C6 appear at l 5.8 as a
doublet of doublets (J=7.8, 2.0 Hz) and at 7.87 as a
doublet (J=7.8 Hz), respectively. The presence of a
coupling constant between H-C5 and NH and the
absence of such a coupling for H-C6 supports the N1
substitution.
In order to confirm the site of attachment for the
thymine derivatives we prepared the methyl derivative
10 and performed NOE measurements as depicted in
Scheme 3. The lack of any essential increase of H-C6
upon saturation of N-CH3 is in accordance with the 3
N-CH3 and 1N isoxazoline positions. If the isoxazoline
ring was on the 3-position methylation should occur at
All the product structures were supported by their
spectral and analytical data. From the spectral data the
attachment of the base to the isoxazoline ring was clear.
Mass spectrometry gave molecular ions corresponding
1
to 1:1 adducts of the base with the isoxazoline ring. H
and 13C NMR spectra contained all the expected char-
acteristic chemical shifts corresponding to both moi-
eties.6 The only point, which probably needed further
clarification was the site of attachment. The adenine
anion is known to react primarily with electrophiles at
N9.7 Previously, the 13C assigned chemical shifts of
several substituted methyladenines have been used as
model compounds for the identification of the site of
attachment of other adenine derivatives.8 The resem-
blance of the 13C chemical shifts of compounds 8a and
9a with those of 9-methyladenine supported N9 substi-
tution in accordance with the expected reaction path-
way for the adenine anion.
Pyrimidine anions are less regioselective in their reac-
tions with several electrophiles. Thus, alkylation of
uracil and thymine can generally occur at the N1 as
Scheme 3. Reagents and conditions: (i) CH3I, NaH, DMF,
50°C, 48 h, 70%.

E. Coutouli-Argyropoulou, P. Pilanidou / Tetrahedron Letters 44 (2003) 3755–3758
3757
N1 and saturation of the N-CH3 should cause a strong
NOE increase of H-C6 almost equal with that observed
upon saturation of the 6-CH3.
Coutouli-Argyropoulou, E.; Cardin, C. J.; Teixeira, S.;
Kavounis, C. A. J. Chem. Soc., Perkin Trans. 1 1997,
949–957; (d) Coutouli-Argyropoulou, E.; Argyropoulos,
N. G. J. Organomet. Chem. 2002, 654, 117–122.
In conclusion, new isoxazoline dideoxynucleoside ana-
logues have been prepared applying a simple and short
procedure. It is also worth noting that the established
replacement of the bromine by nitrogen nucleophiles in
the bromonitrile oxide cycloadducts extends the scope
of its use as a key intermediate in synthesis. Further
work on the synthesis of other isoxazoline analogues is
in progress.
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6. Compound 8a: 1H NMR (CDCl3/DMSO-d6) l 3.86
(dd, 1H, J=17.7, 7.3 Hz), 3.92 (dd, 1H, J=17.7, 11.0
Hz), 4.52 (dd, 1H, J=12.5, 5.2 Hz), 4.60 (dd, 1H, J=
12.5, 3.7 Hz), 5.28 (m, 1H), 7.14 (br s, 2H), 7.42 (t,
2H, J=7.4 Hz), 7.58 (t, 1H, J=7.4 Hz), 7.98 (d, 2H,
J=7.4 Hz), 8.27 (s, 1H), 8.34 (s, 1H); 13C NMR
(CDCl3/DMSO-d6)
l 34.9, 64.6, 78.9, 118.8, 127.9,
128.9, 132.8, 136.5, 148.6, 149.9, 153.4, 155.9, 165.2.
Compound 9a: 1H NMR (CDCl3/DMSO-d6) l 3.69–
3.90 (m, 4H), 4.91 (m, 1H), 4.98 (br s, 1H), 7.12 (br s,
2H), 8.23 (s, 1H), 8.31 (s, 1H); 13C NMR (CDCl3/
DMSO-d6) l 34.6, 62.2, 82.7, 119.2, 136.9, 148.9, 150.2,
153.6, 156.2. Compound 8b: 1H NMR (CDCl3) l 3.58
(dd, 1H, J=18.0, 7.7 Hz), 3.82 (dd, 1H, J=18.0, 10.9
Hz), 4.47 (dd, 1H, J=12.2, 5.1 Hz), 4.55 (dd, 1H, J=
12.2, 3.9 Hz), 5.18 (dddd, 1H, J=10.9, 7.7, 5.1, 3.9
Hz), 5.87 (dd, 1H, J=7.8, 2.0 Hz), 7.44 (t, 2H, J=7.7
Hz), 7.58 (t, 1H, J=7.7 Hz), 7.87 (d, 1H, J=7.8), 8.02
(d, 2H, J=7.7 Hz), 8.75 (br, 1H); 13C NMR (CDCl3) l
36.5, 64.8, 80.6, 104.6, 128.5, 129.4, 134.4, 140.4, 148.3,
153.5, 162.0, 166.1. Compound 9b: 1H NMR (CDCl3/
DMSO-d6) l 3.41–3.61 (m, 4H), 4.77 (br s, 1H), 4.99
(m, 1H), 5.73 (d, 1H, J=7.7 Hz), 7.78 (d, 1H, J=7.7),
8.2 (br s, 1H); 13C NMR (CDCl3/CD3OD) l 38.3, 62.5,
83.5, 103.9, 140.7, 148.7, 154.0, 163.5. Compound 8c:
1H NMR (CDCl3) l 1.96 (d, 3H, J=1.3 Hz), 3.57 (dd,
1H, J=18.1, 7.4 Hz), 3.83 (dd, 1H, J=18.1, 10.3 Hz),
4.46 (dd, 1H, J=12.0, 5.4 Hz), 4.54 (dd, 1H, J=12.0, 4
Hz), 5.16 (dddd, 1H, J=10.3, 7.4, 5.4, 4.0 Hz), 7.44 (t,
2H, J=7.7 Hz), 7.57 (t, 1H, J=7.7 Hz), 7.73 (q, 1H,
J=1.3 Hz), 8.03 (d, 2H, J=7.7 Hz), 8.38 (br s, 1H);
13C NMR (CDCl3) l 12.3, 36.6, 64.8, 80.4, 113.2,
128.5, 129.4, 133.4, 136.0, 148.3, 153.5, 162.8, 166.1.
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3H, J=1.4 Hz), 3.44 (dd, 1H, J=17.5, 8.4 Hz), 3.55
(dd, 1H, J=17.5, 10.5 Hz), 3.57 (dd, 1H, J=12.2, 4.9
Hz), 3.64 (dd, 1H, J=12.2, 3.9 Hz), 4.78 (dddd, 1H,
J=10.5, 8.4, 4.9, 3.9 Hz), 4.93 (br s, 1H), 7.61 (q, 1H,
J=1.4 Hz), 11.2 (br s, 1H); 13C NMR (CDCl3/CD3OD)
l 12.4, 35.8, 62.3, 83.2, 111.9, 135.9, 150.3, 154.5,
165.8.
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