Highly regioselective 1,3-dipolar cycloaddition of 3′-O-propargyl guanosine with nitrile oxide: An efficient method for the synthesis of guanosine containing isoxazole moiety

Article abstract of DOI:10.1016/j.tetlet.2020.152464

The 1,3-dipolar cycloaddition reaction of 3′-O-propargyl guanosine with various in-situ generated nitrile oxides in the presence of DMF as a solvent is described. It is noteworthy that the reaction is highly regioselective that affords biologically import

Full text of DOI:10.1016/j.tetlet.2020.152464

Tetrahedron Letters 61 (2020) 152464
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Highly regioselective 1,3-dipolar cycloaddition of 30-O-propargyl
guanosine with nitrile oxide: An efficient method for the synthesis
of guanosine containing isoxazole moiety
⇑
Muthian Shanmugasundaram, Annamalai Senthilvelan, Anilkumar R. Kore
Life Sciences Solutions Group, Thermo Fisher Scientific, 2130 Woodward Street, Austin, TX 78744-1832, USA
a r t i c l e i n f o
a b s t r a c t
The 1,3-dipolar cycloaddition reaction of 30-O-propargyl guanosine with various in-situ generated nitrile
oxides in the presence of DMF as a solvent is described. It is noteworthy that the reaction is highly regios-
elective that affords biologically important guanosine containing isoxazole moiety in good yields with
high purities.
Article history:
Received 14 July 2020
Revised 10 September 2020
Accepted 13 September 2020
Available online 19 September 2020
Ó 2020 Elsevier Ltd. All rights reserved.
Keywords:
Guanosine
Nitrile oxide
Regioselective
1,3-Dipolar cycloaddition
The nucleic acid chemistry has been the subject of immense
interest in the area of medicinal chemistry as the nucleotides serve
as a versatile building blocks for both DNA and RNA synthesis as
well as involve in numerous critical biological process [1,2]. It
has been well documented in the literature that there are several
approved drugs on the market containing nucleoside/nucleotide
analogs have been used for the treatment of cancers, parasites, bac-
terial, viral and fungal infections [3,4]. Notably, these analogs rep-
resent the unique class of antiviral drugs for various viruses such as
human immunodeficiency virus (HIV), hepatitis C virus (HCV),
influenza virus, respiratory syncytial virus (RSV), hepatitis B virus
(HBV), human papillomavirus (HPV), human cytomegalovirus
(HCMV), herpes simplex virus (HSV), and varicella-zoster virus
(VZV) [5]. It is believed that nucleoside analog acts as a prodrug
which converts into the corresponding triphosphorylated analog
in the presence of kinase. The active form of the drug is the triphos-
phorylated analog that acts as an inhibitor for intracellular
enzymes and helps to inhibit the viral replication. The chemical
modifications in the sugar moiety has had a great impact in terms
of biological activity as well as degree of selective toxicity. Given
the global pandemic due to the outbreak of the novel severe acute
respiratory syndrome coronavirus (SARS-CoV-2) and also the
emerging infectious diseases worldwide from several other viruses
[6,7], the development of novel antiviral drugs containing nucle-
oside/nucleotide analogs with a broad spectrum activity against
different virus genotypes or viral strains has been warranted.
The 1,3-dipolar cycloaddition provides a powerful method for
the construction of novel heterocyclic compounds [8]. In particular,
the 1,3-dipolar cycloaddition of alkynes with nitrile oxide results
in the formation of biologically important isoxazole derivatives
[9]. It is to be noted that several approved drugs with isoxazole
derivatives display several therapeutic activities such as antibacte-
rial, antifungal, antitubercular, antipsychotic, antitumor, antide-
pressant, antirheumatic, anticonvulsant, and bronchodilatory
agent [10]. While the 1,3-dipolarcycloaddition reaction with nitrile
oxide involving pyrimidine nucleosides have been studied [11–15],
to the best of our knowledge, no example of 1,3-dipolarcycloaddi-
tion reaction with nitrile oxide involving guanosine nucleoside has
been reported in the literature. It has been reported that several 20-
deoxyuridine containing isoxazole derivatives display antiviral
activity against HSV and several RNA viruses [13]. In addition,
these analogs exhibit activities against several types of human can-
cer cell lines [14]. Furthermore, Guo et al. reported that several 5-
isoxazol-3-yl-pyrimidine nucleosides exhibited significant in vitro
antileishmanial activity [15]. We envisaged that the design of
new guanosine containing isoxazole moiety may possess poten-
tially interesting biological activity for various therapeutical appli-
cations such as antiviral, anticancer, and antileishmanial agents.
Our continuous interest in the area of nucleic acid chemistry
[16], prompted us to synthesize guanosine containing isoxazole
⇑
Corresponding author.
E-mail address: anil.kore@thermofisher.com (A.R. Kore).
https://doi.org/10.1016/j.tetlet.2020.152464
0040-4039/Ó 2020 Elsevier Ltd. All rights reserved.

M. Shanmugasundaram et al.
Tetrahedron Letters 61 (2020) 152464
Scheme 1. 1,3-Dipolar cycloaddition of 30-O-propargyl guanosine 4 with nitrile oxide.
moiety and study its biolgical evaluation. Herein, we report the
first example of the 1,3-dipolar cycloaddition of 30-O-propargyl
guanosine with nitrile oxides, leading to the formation of guano-
sine containing isoxazole moiety in good yields with high purities.
Treatment of benzaldehyde oxime (1a) with N-chlorosuccin-
imide (2) in the presence of DMF as a solvent, followed by the reac-
tion with 30-O-propargyl guanosine (4) using triethylamine as a
base gave guanosine containing isoxazole moiety 5a in 84% yield
(Scheme 1). The overall reaction involves the generation of N-ben-
zhydroxyiminoyl chloride (3a), followed by the in-situ formation of
the corresponding nitrile oxide and subsequent 1,3-dipolar
cycloaddition to afford the final product, guanosine containing
isoxazole moiety 5a. The 1,3-dipolar cycloaddition is highly regios-
elective affording exclusive 3,5-disubstituted isoxazole 5a. The
structure of 5a was ascertained by 1H and 13C NMR and mass spec-
tral data [17]. The regiochemistry of 5a was unequivocally estab-
lished by typical NOESY NMR techniques. The data show that
there is no nuclear overhauser effect (NOE) signal between one
of the C5 adjacent methylene protons and ortho proton of the phe-
nyl ring present in the isoxazole moiety (Fig. 1). The other possible
3,4-disubstituted regioisomer 6a was not detected in the 1H NMR
spectrum of the crude reaction mixture (Fig. 2).
thermore, the cycloaddition was successfully extended to hetero-
cyclic oximes. Under similar conditions, the cycloaddition
reaction of
4 with 3-pyridinealdoxime (1f), 4-bromo-2-thio-
phenecarbaldehyde oxime (1g), and 1H-indole-3-carboxaldehyde
oxime (1h) afforded the corresponding products 5f-h in 77%,
71%, and 79% yields, respectively (entries 6–8).
There are several interesting features that merit comments for
the present 1,3-dipolar cycloaddition reaction. First, unlike con-
ventional 1,3-dipolar cycloaddition reaction [8], the present reac-
tion involves the in-situ generation of nitrile oxide from aldoxime
without isolation and handling of potentially harmful and unstable
hydroxyiminoyl chloride. Second, in all cases, the 1,3-dipolar
cycloaddition is highly regioselective affording exclusive 3,5-dis-
ubstituted regioisomer 5 in good yields. Third, the purification pro-
cedure is simple and straightforward that involves pass through
silica gel column furnishing final pure product 5 with extremely
high purities based on HPLC (>99.5%). It is to be noted that high
purity product is necessary for the successful biological applica-
tions. Fourth, several approved drugs on the market with nucle-
oside analogs have been used for the treatment of cancer,
Table 1 delineates the results obtained for the reaction of 30-O-
propargyl guanosine (4) with various oximes 1a-h. Aryl oximes
with an electron-withdrawing or an electron-donating group on
the aromatic ring react efficiently with 4 under optimized condi-
tions afforded the corresponding guanosine containing isoxazole
moieties 5b-d in good yields indicating that this cycloaddition
reaction is insensitive to electronic effects (entries 2–4). Similarly,
the addition is also insensitive to steric substituent. The reaction of
4 with 9-anthraldehyde oxime (1e) under similar conditions
afforded the corresponding product 5e in 72% yield (entry 5). Fur-
Fig. 2. Structure of 3,4-disubstituted regioisomer 6a.
Table 1
Results of the 1,3-dipolar cycloaddition of 30-O-propargyl guanosine with nitrile
oxide.
Entry Chloroxime Reaction
time (h)
Product
R
Yielda
(%)
1
2
3
4
5
6
7
3a
3b
3c
3d
3e
3f
24
36
48
36
60
48
60
5a
5b
5c
5d
5e
5f
phenyl
4-methoy-phenyl 74
84
2-chloro-phenyl
4-nitro-phenyl
9-anthracyl
3-pyridyl
4-bromo-2-
thiophenyl
3-indolyl
70
77
72
77
71
3g
5g
8
3h
48
5h
79
a
Isolated yield of pure product.
Fig. 1. Structure of 3,5-disubstituted regioisomer 5a.
2

M. Shanmugasundaram et al.
Tetrahedron Letters 61 (2020) 152464
(c) J.C. Lainson, S.M. Daly, K. Triplett, S.A. Johnston, P.R. Hall, C.W. Diehnelt,
ACS Med. Chem. Lett. 8 (2017) 853–857;
(d) G. Schoretesanitis, E. Spina, C. Hiemke, J. Leon, Expert Rev. Clin. Pharmacol.
10 (2017) 965–981.
bacterial and viral infections. Given the precedent in the literature,
it is likely that the present guanosine containing isoxazole moiety
can be a potential lead candidate in the field of medicinal
chemistry.
In summary, we have developed the 1,3-dipolarcycloaddition
reaction of 30-O-propargyl guanosine with various nitrile oxides
that allows an efficient synthesis of guanosine containing isoxazole
moiety in good yields with high purities. The reaction is highly
regioselective affording exclusive 3,5-disubstituted isoxazole. Fur-
ther work is in progress to extend the scope of this reaction to
triphosphate analogs and other purine bases and to study the bio-
logical applications of these new guanosine containing isoxazole
moiety.
[11] Y.-S. Lee, B.H. Kim, Bioorg. Med. Chem. Lett. 12 (2002) 1395–1397.
[12] E. Coutouli-Argyropoulou, P. Lianis, M. Mitakou, A. Giannoulis, J. Nowak,
Tetrahedron 62 (2006) 1494–1501.
[13] Y.-S. Lee, M. Park, B.H. Kim, Bioorg. Med. Chem. Lett. 19 (2009) 1126–1128.
[14] Y.-S. Lee, S.M. Park, H.M. Kim, S.-K. Park, K. Lee, C.L. Lee, B.H.B. Kim, Med.
Chem. Lett. 19 (2009) 4688–4691.
[15] S. Guo, J. Wang, X. Zhang, S. Cojean, P.M. Loiseau, X.B. Fan, Med. Chem. Lett. 25
(2015) 2617–2620.
[16] (a) A.R. Kore, M. Shanmugasundaram, I. Charles, A.V. Vlassov, T.J. Barta, J. Am.
Chem. Soc. 131 (2009) 6364–6365;
(b) A.R. Kore, M. Shanmugasundaram, A. Senthilvelan, B. Srinivasan,
Nucleoside Nucleotides Nucl. Acids 31 (2012) 423–431;
(c) A.R. Kore, A. Senthilvelan, M. Shanmugasundaram, Tetrahedron Lett. 53
(2012) 3070–3072;
(d) A.R. Kore, M. Shanmugasundaram, Tetrahedron Lett. 53 (2012) 2530–
2532;
Declaration of Competing Interest
(e) A.R. Kore, A. Senthilvelan, M. Shanmugasundaram, Nucleosides
Nucleotides and Nucl. Acids 34 (2015) 92–102;
(f) A. Senthilvelan, M. Shanmugasundaram, A.R. Kore, Tetrahedron Lett. 57
(2016) 2006–2008;
(g) M. Shanmugasundaram, A. Senthilvelan, Z. Xiao, A.R. Kore, Nucleosides
Nucleotides and Nucl. Acids 35 (2016) 356–362;
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
(h) M. Shanmugasundaram, I. Charles, A.R. Kore, Bioorg. Med. Chem. 24 (2016)
1204–1208;
References
(i) M. Shanmugasundaram, A. Senthilvelan, A.R. Kore, Tetrahedron Lett. 60
(2019) 157–160.
[1] K.A. Watanabe, in: Chemistry of Nucleosides and Nucleotides, Springer US,
Boston, MA, 1994, pp. 421–535.
[17] General procedure to make 5: To a stirred solution of aldoxime (2.0 mmol) in
DMF (10.0 mL) at room temp., N-chlorosuccinimide (2.0 mmol) was added and
the mixture was stirred for 15 h. After 15 h, triethylamine (2.0 mmol) and 30-
O-propargyl guanosine (1.0 mmol) were added to the reaction mixture and
stirred for the time specified in Table 1. The reaction mixture was evaporated
under rotary evaporator to give crude reaction mixture. The crude reaction
mixture was purified by silica gel column chromatography using Biotage
instrument (DCM/MeOH, 9:1). The fractions containing the pure product were
evaporated and dried to give a solid 5. Data for 5a: 1H NMR (DMSO-d6, 400
MHz) d 10.65 (bs, 1H), 7.96 (s, 1H), 7.89 (m, 2H), 7.53 (m, 3H), 7.10 (s, 1H), 6.48
(bs, 2H), 5.75 (d, J = 6.8 Hz, 1H), 4.96 (d, J = 13.6 Hz, 1H), 4.85 (d, J = 13.6 Hz,
1H), 4.66 (m, 1H), 4.15 (m, 1H), 4.05 (m, 1H), 3.60 (m, 2H); 13C NMR (DMSO-
d6, 162 MHz) d 170.59, 162.29, 157.16, 154.21, 151.89, 135.91, 130.75, 129.63,
128.95, 127.09, 117.08, 102.03, 86.43, 83.65, 79.24, 73.62, 63.06, 61.77; MS
(m/z): 439 [M-H]-.
[2] L.T.C. Franca, E. Carrilho, T.B.L. Kist, Q. Rev. Biophys. 35 (2002) 169–200.
[3] S. Mahmoud, S. Hasabelnaby, S.F. Hammad, T.M. Sakr, J. Adv. Pharm. Res. 2
(2018) 73–88.
[4] K. Seley-Radtke, M.K. Yates, Antiviral Res. 154 (2018) 66–86.
[5] E. De-Clercq, G. Li, Clin. Microbiol. 29 (2016) 695–746.
[6] W. Chen, P.W. Horby, F.G. Hayden, G.F. Gao, Lancet 395 (2020) 470–473.
[7] K. Yuki, M. Fujiogi, S. Koutsogiannaki, Clin. Immunol. 215 (2020) 108427.
[8] A. Padwa, (Ed.), 1,3-Dipolar cycladdition chemistry, Wiley-Interscience, New
York, 10984, Vols. 1 and 2.
[9] A. Padwa, Intermolecular 1,3-Dipolar cycloaddition, in: B.M. Trost, I. Fleming
(Eds.), Comprehensive Organic Synthesis, Vol. 4, Pergamon Press, Oxford, 1991,
p. 1069.
[10] (a) N. Agrawal, P. Mishra, Med. Chem. Res. 27 (2018) 1309–1344;
(b) M. Buoli, S. Grassi, V. Ciappolino, M. Serati, A.C. Altamura, Clin.
Neuropharmacol. 40 (2017) 85–92;
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