Microwave-promoted "one-Pot" synthesis of 4- nitrobenzylthioinosine analogues using thiourea as a sulfur precursor

Article abstract of DOI:10.1002/asia.201100699

An efficient one-pot method for the synthesis of various C6-alkylthio-substituted purine nucleosides has been developed under microwave irradiation with good to excellent yields without any metal catalyst (see scheme). This process expands the scope of existing synthetic methodologies and was further successfully applied for synthesis of the biologically important compound 4-nitrobenzylthioinosine (NBTI).

Full text of DOI:10.1002/asia.201100699

DOI: 10.1002/asia.201100699
Microwave-Promoted “One-Pot” Synthesis of 4-Nitrobenzylthioinosine
Analogues Using Thiourea as a Sulfur Precursor
Hong-Ying Niu,[a, b] Chao Xia,[a] Gui-Rong Qu,*[a] Shan Wu,[a] Yi Jiang,[a] Xin Jin,[a] and
Hai-Ming Guo*[a]
C6-Alkylthio-substituted purines have potential therapeu-
tic applications such as in anticancer, antibacterial, and anti-
viral agents. For example, 4-nitrobenzylthioinosine (NBTI),
which has been extensively studied in recent years, is a well-
known inhibitor for the equilibrative nucleoside transport
protein ENT1 (Figure 1).[1]
sate the disadvantages, which result from the relatively high
cost of the starting material of 6-mercaptopurine, thiols, or
RSÀ anions.
In 2003, PeÇꢀÇory et al. have published “one-pot” two-
step photoinduced reactions of aryl halides with the thiourea
anion to afford aryl sulfur compounds.[8] Very recently,
Sekar and Prasad have reported a one-pot synthesis of un-
symmetrical diaryl thioethers from aryl halides and potassi-
Other C6-benzylthio-substitut-
ed purine nucleosides, which
were similar to NBTI, have
um ethyl xanthogenate catalyzed by CuACTHNGUTERNNUG
(OAc)2.[9] Also, Fir-
been reported to act as poten-
ouzabadi et al. have developed a new protocol for the thioe-
therification of aryl halides using thiourea and alkyl bro-
mides catalyzed by CuI in wet polyethylene glycol.[10] En-
couraged by the unique properties of thiourea and based on
the previous work of our studies on nucleoside analogues,[11]
herein, we report a green and efficient protocol for the one-
pot two-step synthesis of a series of the NBTI analogues
from various 6-halopurine nucleosides and thiourea without
any metal catalyst under microwave irradiated conditions. In
this protocol, 2-(9H-purin-6-yl)-isothiourea hydrochloride
tial ATP-competitive kinase in-
hibitors,[2]
antimycobacterial
agents,[3] anti-HBV (hepatitis B
virus) reagents,[4] and so on.[5]
The importance of these C6-al-
kylthio substituted purine nu-
cleosides in medicinal chemistry
Figure 1. 4-Nitrobenzylthioi-
noisine (NBTI).
has promoted continued methodological efforts.
The preparation of C6-alkylthio-substituted purines
mainly involves two different routes. One route is through
an alkylation reaction from 6-mercaptopurine and alkylating
agents such as alkyl halides,[6] and the other is through the
SNAr reaction of leaving groups from the heterocylic base
by sulfurated nucleophiles such as thiols or RSÀ anions.[7] As
far as we know, thiourea, a most attractive starting material
owing to its low cost and readily accessibility, is not used as
a sulfur precursor for the preparation of C6-sulfur-substitut-
ed purines. Obviously, using thiourea as a sulfur source for
synthesis of C6-sulfur-substituted purines would expand the
scope of existing synthetic methodologies and overcompen-
À
was firstly formed by the formation of C(aryl) S bond without
any catalyst. Next, a substitution reaction took place be-
tween 2-(9H-purin-6-yl)-isothiourea hydrochloride and an
alkyl halide under microwave irradiation in 10 minutes to
give C6-alkylthio-substituted purine nucleosides.
The first step was performed according the literature pro-
cedure[12]: thiourea (0.33 mmol) was added to a solution of
purine 1a (0.3 mmol) in ethanol (2 mL) and then the solu-
tion was refluxed at 958C for one hour to give the inter-
mediate compound, 2-(9H-purin-6-yl)-isothiourea hydro-
chloride (2a). We also tried the reaction under microwave
irradiation, however, unfortunately, the yield of 2a was very
low. Then, without the isolation of 2, we studied the nucleo-
philic substitution reaction between 2a and benzyl chloride
(3a) promoted by microwave irradiation. The results are
listed in Table 1.
[a] Dr. H.-Y. Niu, Dr. C. Xia, Prof. G.-R. Qu, S. Wu, Y. Jiang, X. Jin,
Prof. Dr. H.-M. Guo
College of Chemistry and Environmental Science
Key Laboratory of Green Chemical Media and Reactions of Ministry
of Education
As shown in Table 1, an excellent yield was obtained
when two equivalents of benzyl chloride and two equiva-
lents of K2CO3 were used (Table 1, entry 1). When the
amount of benzyl chloride or K2CO3 was reduced, 4a was
afforded in lower yield (Table 1, entries 2–4). An exhilarat-
ing yield was obtained after the microwave power was in-
creased to 400 W (Table 1, entry 5). When the reaction was
performed at lower temperature or within a shorter time
period, 4a was obtained in lower yield (Table 1, entries 6–8).
The reaction was also carried out in a convectional heating
Henan Normal University
No 46 Jianshe Road, Xinxiang, 453007 (China)
Fax : (+86)373-332-9276
E-mail: quguir@sina.com
guohm518@hotmail.com
[b] Dr. H.-Y. Niu
School of Chemistry and Chemical Engineering
Henan Institute of Science and Technology
No 1 Hualan Avenue, Xinxiang 453003 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100699.
Chem. Asian J. 2012, 7, 45 – 49
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
45

COMMUNICATION
Table 1. Optimization of the Reaction Conditions[a]
Table 2. (Continued)
Entry
R
Product
Yield [%][b]
2
o-Cl-Bn (1b)
4b
98
Entry Substrate
K2CO3 [equiv] Power [W] t [min] Yield [%][b]
3a [equiv]
1
2
3
4
5
6
7
2
2
2
1.5
1.5
1.2
1.5
1.5
1.5
2
1.5
–
2
2
2
2
2
2
300
300
300
300
400
400
400
400
–
10
10
10
10
10
10
8
95
88
0
85
96
72
83
74
84
3
4
5
Me (1c)
4c
4d
4e
95
97
88
Et (1d)
8[c]
9[d]
10
60
[a] Reaction conditions: 1a (0.3 mmol), EtOH (2 mL), MWI, reflux at
1008C. [b] Yield of isolated product based on purine 1a. [c] The reaction
temperature was 508C. [d] The reaction was carried out in a convectional
heating bath at 1008C. MWI=microwave irradiation.
n-Propyl (1e)
bath at 1008C, and a similar result was obtained albeit with
a longer reaction time (Table 1, entry 9).
6
4 f
80
83
Under the optimized reaction conditions, the substrate
scope was explored to various 6-chloropurine derivatives
with different substituents at the N9 position. The results
are summarized in Table 2. The substrates with various alkyl
substituents at the N9 position gave high to excellent yields
(Table 2, entries 1-5). To our delight, acyclic nucleoside
acetic acid 2-(6-chloro-purin-9-ylmethoxy)-ethyl ester (1 f)
gave hydrolysis product (4 f) in 80% yield (Table 2, entry 6).
To avoid the hydrolysis process, we simply replaced K2CO3
with Na2CO3, and the corresponding acetyl protected prod-
uct (4g) was obtained in 83% yield (Table 2, entry 7).
2ꢂ,3ꢂ,5ꢂ-Triacyl-6-chloropurine nucleoside (1g) gave the same
results (Table 2, entries 8–9).
7[c]
4g
8[d]
4h
84
93
Using Na2CO3 as the base, various haloalkanes were sub-
jected to the reactions with 6-chloropurine nucleoside (1g),
and the results are shown in Table 3. It was found that all
haloalkanes (Cl, Br, I) could smoothly produce the corre-
9[c]
4i
Table 2. Substrate Variation of 6-Chloropurines[a]
[a] Reaction conditions of step one: purines
1 (0.3 mmol), thiourea
(0.33 mmol), EtOH (2 mL), reflux at 1008C for 1 h. Step two: benzyl
chloride 3a (0.45 mmol), K2CO3 (0.6 mmol), MWI 400 W, reflux at 1008C
for 10 min. [b] Yield of isolated product based on purines. [c] 2 equiva-
lents of Na2CO3 were employed. [d] 3 equivalents of K2CO3 were em-
ployed.
Entry
1
R
Product
Yield [%][b]
sponding products in good to excellent yields. Therefore,
this one-pot two-step synthetic method proved to be general
for the synthesis of NBTI analogues.
Bn (1a)
4a
96
This method also applies to other electron-deficient aryl
halides, and the results are shown in Table 4. Several typical
46
www.chemasianj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 45 – 49

Microwave-Promoted “One-Pot” Synthesis of 4-Nitrobenzylthioinosine Analogues
Table 3. Substrate variation of haloalkanes[a]
Table 3. (Continued)
Entry Alkyl-X
Product
Yield [%][b]
6
4o
80
Entry
1
Alkyl-X
Et-I
Product
Yield [%][b]
7
4p
85
4j
4k
4l
87
2
3
n-Propyl-I
93
95
8
4q
99
n-Pentyl-Br
9
4r
81
[a] Reaction conditions of step one: purines
1 (0.3 mmol), thiourea
4
4m
99
(0.33 mmol), EtOH (2 mL), reflux at 958C for 1 h. Step two: haloalkane
3 (0.45 mmol), Na2CO3 (0.6 mmol), MWI 400 W, reflux at 908C for
10 min. [b] Yield of isolated product based on purines.
In conclusion, we have developed a rapid and operation-
ally simple method for the preparation of various 6-thio-sub-
stituted purine nucleosides, which are important candidates
for biologically active compounds. Thiourea, an easily avail-
able and cheap starting material, was first used as a sulfur
precursor for synthesis of 6-thio-substituted purine nucleo-
sides. In this protocol, isothiourea hydrochloride was firstly
5
4n
80
À
formed by formation of the C(aryl) S bond without any cata-
lyst, and subsequently, a substitution reaction took place be-
tween an alkyl halide and the intermediate product under
microwave irradiation in 10 minutes to give the 6-thio-sub-
stituted purine nucleosides. One of the important applica-
tions to showcase such a mild and efficient protocol is the
synthesis of the biologically important compound, NBTI.
This methodology offers an important complement to the
reported methods for synthesis of 6-thio-substituted purine
nucleosides from thiourea without any metal catalyst.
aryl and heteroaryl chlorides underwent the reactions
smoothly with good yields (Table 4, entries 1–4). Iodoben-
zene and some electron-rich aromatics, such as para-methyl-
phenyl bromide and meta-bromoanisole were not successful
in the reaction, and the corresponding products were not
obtained.
Finally, we used this protocol to synthesize 4-nitroben-
zylthioinosine (NBTI, 7), a well-known inhibitor for the nu-
cleoside transport protein ENT1. To our delight, NBTI was
efficiently achieved in an isolated yield of 87% by using this
one-pot two-step method (Scheme 1).
Chem. Asian J. 2012, 7, 45 – 49
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
47

H.-M. Guo, G.-R. Qu et al.
COMMUNICATION
Table 4. Substrate variation of heteroaromatics[a]
Compound 4a
White powder, m.p. 101–1038C. 1H NMR (CDCl3, 400 MHz): d=8.79 (s,
1H), 8.04 (s, 1H), 7.48–7.46 (m, 2H), 7.34–7.21 (m, 8H), 5.41 (s, 2H),
4.68 ppm (s, 2H). 13C NMR (CDCl3, 100 MHz): d=160.5, 151.9, 148.5,
142.5, 137.2, 134.9, 130.4, 129.1, 129.0, 128.5, 128.4, 127.7, 127.2, 47.3,
32.8 ppm.
Entry
1
Product
Yield [%][b]
80
Acknowledgements
6a
We are grateful for financial support from the National Nature Science
Foundation of China (Grant Nos 20802016, and 21072047), the Program
for New Century Excellent Talents in University of Ministry of Educa-
tion (No. NCET-09-0122), Excellent Youth Foundation of Henan Scien-
tific Committee (No. 114100510012), the Program for Changjiang Schol-
ars and Innovative Research Team in University (IRT1061), the National
Students Innovation Experiment Program, and the Excellent Youth Pro-
gram of Henan Normal University.
2
3
6b
6c
85
75
4
6d
88
[a] Reaction conditions of step one: aryl chloride 5 (0.3 mmol), thiourea
(0.33 mmol), EtOH (2 mL), reflux at 958C for 1 h. Step two: haloalkane
Keywords: 4-nitrobenzylthioinosine · microwave chemistry ·
nucleosides · synthetic methods · thiourea
3
(0.45 mmol), K2CO3 (0.6 mmol), MWI 400 W, reflux at 908C for
10 min. [b] Yield of isolated product based on aryl halides.
[1] a) R. A. Tromp, R. F. Spanjersberg, J. K. V. F. D. Kꢃnzel, A. P. IJzer-
man, J. Med. Chem. 2005, 48, 321; b) R. A. Tromp, S. Ameijde, C.
Pꢃtz, C. Sundermann, B. Sundermann, J. K. F. D. Kꢃnzel, A. P. IJzer-
man, J. Med. Chem. 2004, 47, 5441; c) M. J. Robins, Y. Peng, V. L.
Damaraju, D. Mowles, G. Barron, T. Tackaberry, J. D. Young, C. E.
Cass, J. Med. Chem. 2010, 53, 6040; d) E. F. Williams, Life Sci. 1996,
59, 2051; e) Y. A. Kim, A. Sharon, C. K. Chu, R. H. Rais, O. N. A.
Safarjalani, F. N. M. Naguib, M. H. Kouni, J. Med. Chem. 2008, 51,
3934; f) C. R. Boyer, P. L. Karjian, G. M. Wahl, M. Pegram, S. T. C.
Neuteboom, Anti-Cancer Drugs 2002, 13, 29; g) W. W. Lin, J. K.
Buolamwini, J. Med. Chem. 2007, 50, 3906; h) P. Y. F. Deghati, A.
Borghini, A. M. C. H. Nieuwendijk, M. D. Groote, A. P. IJzerman,
Bioorg. Med. Chem. 2003, 11, 899; i) N. J. Curtin, H. C. Barlow, K. J.
Bowman, A. H. Calvert, R. Davison, B. T. Golding, B. Huang, P. J.
Loughlin, D. R. Newell, P. G. Smith, R. J. Griffin, J. Med. Chem.
2004, 47, 4905; j) J. K. Addo, J. K. Buolamwini, Bioconjugate Chem.
2004, 15, 536; k) Z. X. Zhu, J. Furr, J. K. Buolamwini, J. Med. Chem.
2003, 46, 831; l) A. Gupte, J. K. Buolamwini, Bioorg. Med. Chem.
Lett. 2004, 14, 2257.
Scheme 1. Convenient Synthesis of 4-Nitrobenzylthioinosine (NBTI).
[2] S. A. Laufer, D. M. Domeyer, T. R. F. Scior, W. Albrecht, D. R. J.
Hauser, J. Med. Chem. 2005, 48, 710.
[3] A. K. Pathak, V. Pathak, L. E. Seitz, W. J. Suling, R. C. Reynolds, J.
Med. Chem. 2004, 47, 273.
[4] a) K. Sekiya, H. Takashima, N. Ueda, N. Kamiya, S. Yuasa, Y. Fuji-
mura, M. Ubasawa, J. Med. Chem. 2002, 45, 3138; b) X. C. Chen,
E. R. Kern, J. C. Drach, E. Gullen, Y. C. Cheng, J. Zemlicka, J. Med.
Chem. 2003, 46, 1531.
Experimental Section
Typical Experimental Procedure
Purine 1 (0.5 mmol) was placed in a 10 mL glass vial equipped with a
small magnetic stirring bar. Then ethanol (2 mL) and thiourea (1.5 equiv)
were added and the solution was refluxed for 1 h. S-(purin-6-yl)isothiour-
ea hydrochloride 2 was formed as a precipitate. Then the haloalkane
(1.5 equiv) and K2CO3 (2 equiv) were added (for 4a–4 f and 6a–6d). For
[5] a) V. Yadav, C. K. Chu, R. H. Rais, O. N. A. Safarjalani, V. Guarcel-
lo, F. N. M. Naguib, M. H. Kouni, J. Med. Chem. 2004, 47, 1987;
b) J. Q. Wang, X. S. Fei, T. A. Gardner, G. D. Hutchinsa, Q. H.
Zheng, Bioorg. Med. Chem. 2005, 13, 549.
[6] a) P. Thomson, M. A. Naylor, M. R. L. Stratford, G. Lewis, S. Hill,
K. B. Patel, P. Wardmana, P. D. Davis, Bioorg. Med. Chem. Lett.
2007, 17, 4320; b) A. Gupte, J. K. Buolamwini, Bioorg. Med. Chem.
2007, 15, 7726; c) U. Mꢃnch, L. Chen, S. F. Bayly, P. F. Torrence,
Bioorg. Med. Chem. 2003, 11, 2041; d) A. M. L. Carmo, F. G. Braga,
M. L. De Paula, A. P. Ferreira, H. C. Teixeira, A. D. da Silva, E. S.
Coimbra, Lett. Drug Des. Discovery 2008, 5, 122; e) R. N. Salvatore,
R. A. Smith, A. K. Nischwitz, T. Gavin, Tetrahedron Lett. 2005, 46,
8931; f) S. Majumdar, K. B. Sloan, Bioorg. Med. Chem. Lett. 2006,
16, 3590.
4h and
7 K2CO3 (3 equiv) was added. For 4g and 4i–4r Na2CO3
(2 equiv) was added. Then the mixture was put into the cavity of the mi-
crowave and irradiated at 400 W at 908C for 10 min. After evaporation
of the solvent, the crude product was purified by column chromatography
over silica gel using EtOAc/Petroleum Ether (v/v=1:4) as the eluent to
give desired products 4a–4e, 4g, and 6a–6d, EtOAc/Petroleum Ether (v/
v=1:2) was used as the eluent to give desired products 4 f and 4i–4r, and
EtOAc/Methanol (v/v=50:1) was used as the eluent to give desired prod-
ucts 4h and 7.
48
www.chemasianj.org
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 45 – 49

Microwave-Promoted “One-Pot” Synthesis of 4-Nitrobenzylthioinosine Analogues
[7] a) E. A. Vꢀliz, P. A. Beal, J. Org. Chem. 2001, 66, 8592; b) L. A. Pa-
quette, A. L. Kahane, C. K. Seekamp, J. Org. Chem. 2004, 69, 5555;
c) K. Lee, C. Cass, K. A. Jacobson, Org. Lett. 2001, 3, 597; d) S. Bae,
M. K. Lakshman, J. Org. Chem. 2008, 73, 3707; e) X. Lin, M. J.
Robins, Org. Lett. 2000, 2, 3497; f) S. Bae, M. K. Lakshman, J. Am.
Chem. Soc. 2007, 129, 782; g) A. Conejo-Garcꢄa, M. C. NfflÇez, J. A.
Marchal, F. Rodrꢄguez-Serrano, A. Arµnega, M. A. Gallo, A. Espi-
nosa, J. M. Campos, Eur. J. Med. Chem. 2008, 43, 1742; h) M. Ikejiri,
T. Ohshima, K. Kato, M. Toyama, T. Murata, K. Shimotohno, T.
Maruyama, Bioorg. Med. Chem. 2007, 15, 6882; i) L. K. Huang,
Y. C. Cherng, Y. R. Cheng, J. P. Jang, Y. L. Chao, Y. J. Cherng, Tetra-
hedron 2007, 63, 5323.
[10] H. Firouzabadi, N. Iranpoor, M. Gholinejad, Adv. Synth. Catal.
2010, 352, 119.
[11] a) H. M. Guo, T. F. Yuan, H. Y. Niu, J. Y. Liu, R. Z. Mao, D. Y. Li,
G. R. Qu, Chem. Eur. J. 2011, 17, 4095; b) H. M. Guo, L. L. Jiang,
H. Y. Niu, W. H. Rao, L. Liang, R. Z. Mao, D. Y. Li, G. R. Qu, Org.
Lett. 2011, 13, 2008; c) H. M. Guo, W. H. Rao, H. Y. Niu, L. L.
Jiang, G. Meng, J. J. Jin, X. N. Yang, G. R. Qu, Chem. Commun.
2011, 47, 5608; d) H. M. Guo, C. Xia, H. Y. Niu, X. T. Zhang, S. N.
Kong, D. C. Wang, G. R. Qu, Adv. Synth. Catal. 2011, 353, 53;
e) G. R. Qu, Z. J. Mao, H. Y. Niu, D. C. Wang, C. Xia, H. M. Guo,
Org. Lett. 2009, 11, 1745.
[12] F. Seyama, K. Akahori, Y. Sakata, S. Misumi, M. Aida, C. Nagata, J.
Am. Chem. Soc. 1988, 110, 2192.
[8] J. E. Argꢃello, L. C. Schmidt, A. B. PeÇꢀÇory, Org. Lett. 2003, 5,
4133.
Received: August 18, 2011
[9] D. J. C. Prasad, G. Sekar, Org. Lett. 2011, 13, 1008.
Published online: October 14, 2011
Chem. Asian J. 2012, 7, 45 – 49
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
49