Microwave-promoted Michael addition in neat water: A rapid, efficient and green method for the preparation of acyclic nucleosides

Article abstract of DOI:10.1055/s-2007-970772

Syntheses of acyclic nucleosides were achieved in water with the aid of microwave irradiation, providing a rapid, efficient and convenient method for the preparation of acyclic nucleosides in high yields. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-970772

LETTER
721
Microwave-Promoted Michael Addition in Neat Water: A Rapid, Efficient and
Green Method for the Preparation of Acyclic Nucleosides
Preparationof
A
cy
u
clic
N
ucleos
i
ide
s
-Rong Qu,* Zhi-Guang Zhang, Ming-Wei Geng, Ran Xia, Lin Zhao, Hai-Ming Guo*
College of Chemistry and Environmental Science, Henan Normal University, Xinxiang 453007, P. R. of China
Fax +86(373)3326336; E-mail: ghm@htu.cn
Received 10 December 2006
To initiate our study, we examined the ability of micro-
Abstract: Syntheses of acyclic nucleosides were achieved in water
with the aid of microwave irradiation, providing a rapid, efficient
and convenient method for the preparation of acyclic nucleosides in
high yields.
wave promoting the Michael addition of uracil and acrylo-
nitrile in water. The influence of various bases was
examined to evaluate their activities as well as their selec-
tivities. The results are summarized in Table 1. When
K2CO3 was used as a base, Michael addition reaction
proceeded smoothly to give the product 6a in 60% yield
(Table 1, entry 1). Uracil was exclusively alkylated at the
N-1 position. The site of N-alkylation was characterized
Key words: acyclic nucleoside, microwave irradiation (MWI),
Michael addition, green solvent
Strong interest in acyclic nucleosides started when
Acyclovir (ACV, 1) was firstly reported as a potent anti-
herpes drug in the mid-1970s.1 A great number of other
nucleoside analogues for antiviral chemotherapy belong-
ing to ‘open chain’ acyclic sugar analogues2 such as Gen-
cicovir (2), and 9-[2-(phosphonylmethoxy)ethyl]adenine
(3) were substantially found (Figure 1). Many methods
were reported to obtain these important nucleoside
analogues, including alkylation of the nucleobases with
various alkylating agents,3 Michael addition of purine and
pyrimidine to an activated multiple bond.4 Over the past
two decades, microwave irradiation has become a very
popular and useful technique in synthetic chemistry.5 To
the best of our knowledge, microwave-promoted Michael
addition in neat water has not yet been employed in the
synthesis of acyclic nucleosides. Water, a safe and envi-
ronmentally benign solvent, has attracted much attention
in synthetic chemistry recently.6 It is very cheap, clean,
non-toxic and non-flammable, which leads to easier work-
up and has great advantages over organic media. Herein,
we describe a rapid, efficient and green method for the
preparation of a series of acyclic nucleosides using trieth-
ylamine (Et3N) as the base under microwave irradiation in
water.
1
by H NMR and 13C NMR spectra analysis.4g Potassium
hydroxide or magnesium oxide could also be used as the
base and no significant changes in yield were observed
(entries 2 and 3). However, high yield was obtained by
employing 4-dimethylaminopyridine (DMAP) or Et3N as
the base instead of K2CO3 (entries 4 and 5). The base was
necessary because no reaction happened without base,
even prolonging the irradiation time to 15 minutes (entry
6). Finally, Et3N became the base of the choice because it
is very common and inexpensive. Using Et3N as the base
has an additional advantage in that it is unnecessary to
neutralize the reaction mixture by diluted HCl, for it can
be easily removed in vacuo because of its low boiling
point (88.8 °C). Changing irradiation time has some in-
fluence on the yield (entries 7–10). As shown in Table 1,
when the reaction time was shorter or longer than five
minutes, lower yield was obtained and N-3-alkylation
product was observed if reaction time was longer than five
minutes. Therefore five minutes was the optimized reac-
tion time.
After obtaining the optimized reaction conditions, we
applied this procedure to other uracil derivatives
(Table 2). Gratifyingly, all the substrates were exclusively
alkylated at N-1 position. When the reaction mixtures
were cooled to room temperature, 6b and 6c could pre-
NH2
O
O
N
N
N
N
N
NH
NH
HO
HO
O
N
N
N
N
NH2
N
NH2
P
O
HO
HO
O
O
OH
1
2
3
Figure 1 Structures of some acyclic nucleosides
SYNLETT 2007, No. 5, pp 0721–0724
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6.
0
3.
2
0
0
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Advanced online publication: 08.03.2007
DOI: 10.1055/s-2007-970772; Art ID: W25206ST
© Georg Thieme Verlag Stuttgart · New York

722
G.-R. Qu et al.
LETTER
Table 1 Optimization of Reaction Conditions under Microwave
Table 2 Michael Addition of Uracil Derivatives to Acrylonitrilea
Irradiationa
X
X
O
O
R
R
Et3N, H2O
NH
NH
+
CN
NH
NH
MWI, 5 min
MWI, base
H2O
N
H
O
N
O
+
CN
N
O
N
H
O
CH2CH2CN
CH2CH2CN
4a–e
5
6a–e
4a
5
6a
Entry
R
X
Product
Yield (%)b
Entry
Base
Irradiation time (min) Yield (%)b
1
2
3
4
5
H
Cl
I
O
O
O
O
S
6a
6b
6c
6d
6e
82
78
72
82
74
1
2
K2CO3
KOH
MgO
DMAP
Et3N
5
5
60
54
3
5
62
Me
H
4
5
83
5
5
82
a Reaction conditions: nucleobases (2 mmol), 5 (6 mmol), H2O
(5 mL), Et3N (6 mmol), MWI 250 W (100 °C).
b Isolated yields based on nucleobases.
6
Base-free
Et3N
15
4
No reaction
7
69
74
81
78
Table 3 Michael Addition of Uracil Derivatives to Acrylonitrilea
8
Et3N
4.5
NHR1
NHR1
N
9
Et3N
6
7
R2
R2
Et3N, H2O
N
10
Et3N
CN
+
MWI, 5 min
N
O
a Reaction conditions: uracil (2 mmol), acrylonitrile (6 mmol), base
(6 mmol), H2O (5 mL), MWI 250 W (100 °C).
b Isolated yields based on nucleobase.
N
H
O
CH2CH2CN
7a–d
5
8a–d
Entry
R1
R2
H
Product
Yield (%)b
cipitate from the reaction mixtures in high purities and
yields. But as far as 6a, 6d, and 6e were concerned, no
precipitate appeared, even if the reaction mixtures were
put into refrigerator for two hours. After volatilization of
the solvents in vacuo, the solid residues were subjected to
column chromatographic purification to provide 6a, 6d,
and 6e, as pure compounds in high yields. Interestingly,
when H-5 in 4a was replaced with Cl, CH3, or I, only a
negligible difference was observed in the yield7 (entries
2–4). No obvious change in yield was detected as 4-oxo
was substituted with 4-thio (entry 5).
1
2
3
4
Ac
H
8a
8b
8c
8d
83
83
84
80
H
H
F
H
Br
a Reaction conditions: nucleobases (2 mmol), 5 (6 mmol), H2O
(5 mL), Et3N (6 mmol), MWI 250 W (100 °C).
b Isolated yields based on nucleobase.
adenine derivatives that were substituted by 6-benzyl and
6-hydroxyethyl (9f–i), which behaved as Ca2+ antago-
nists, selective A1 agonists and pharmacological tools,9
were also alkylated exclusively at N-9 (entries 6–9). It is
also worthy of mention that the hydroxyl group in 9g–i
was not alkylated. Another method was employed to
prove that the substrates 9g–i were alkylated at N-9.10
HMBC spectra also ascertained that the site of alkylation
was N-9 and not N-7 or another position.
We have also extended this reaction to some cytosine de-
rivatives that provided compounds 8a–d in high yields
(Table 3). Firstly, cytosine derivative 7a was selected as a
nucleobase, and the exocyclic amino group of 7a was pro-
tected as an acetamide. The reaction proceeded smoothly
to provide the product 8a in 83% yield (entry 1). More in-
teresting is that cytosine with the free exocyclic amino
group could also give rise to the product 8b in 83% yield
(entry 2). This implied that our method has high regio-
selectivity. The reaction could also proceed with the H-5
position substituted by F or Br. High regioselectivity and
high yields were also obtained (entries 3 and 4).
In order to check the generality of our method, other
Michael acceptors were investigated. Treatment of 6-
chloropurine with methyl acrylate, ethyl acrylate, tert-bu-
tyl acrylate under the similar conditions as mentioned
above successfully gave rise to the corresponding prod-
ucts 12a–c in high yields and regioselectivity. The results
are summarized in Table 5. It could be concluded that the
length of alkoxy group (OR) in acrylate has little in-
fluence on the result of reaction.
Applying this procedure to a series of purine derivatives,
the desired products were also obtained in moderate to
high yields (Table 4). To our delight, it is also unneces-
sary to protect the exocyclic amino group in purine. The
most probable reason was their relatively weak nucleo-
philic character according to previous observations.8 The
Synlett 2007, No. 5, 721–724 © Thieme Stuttgart · New York

LETTER
Preparation of Acyclic Nucleosides
723
Table 4 Michael Addition of Various Purine Derivatives to
Extension of this strategy to other Michael acceptors such
as a,b-unsaturated esters and diethyl vinylphosphonate
for the synthesis of acyclic nucleoside analogues, which
may represent a new class of drugs and tools for chemical
biology, is currently in progress in our laboratories.
Acrylonitrilea,11
R1
R1
N
N
N
Et3N, H2O
N
N
+
CN
MWI, 5 min
N
R2
N
R2
N
H
CH2CH2CN
Acknowledgment
9a–i
5
10a–i
We are grateful for financial support from the National Natural
Science Foundation of China (grants 20372018).
Entry R1
R2
H
Product12 Yield (%)b
1
2
3
4
5
6
7
8
9
Cl
10a
82
84
86
77
80
74
78
73
76
References and Notes
NH2
Cl
H
10b
10c
10d
10e
10f
10g
10h
10i
(1) (a) Schaeffer, H. J.; Beauchamp, L.; De Miranda, P.; Elion,
G. B.; Bauer, D. J.; Collins, P. Nature (London) 1978, 272,
583. (b) Elion, G. B.; Furman, P. A.; Fyfe, J. A.; De
Miranda, P.; Beauchamp, L.; Schaeffer, H. Proc. Natl. Acad.
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(2) (a) Barrio, J. R.; Bryant, J. D.; Keyser, G. E. J. Med. Chem.
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(d) Chu, C. K.; Culter, S. J. J. Heterocycl. Chem. 1986, 23,
289. (e) Jeffery, A. L.; Kim, J.-H.; Wiemer, D. E.
NH2
Cl
F
NH2
NH2
Benzyl amino
HOCH2CH2NH
HOCH2CH2NH
HOCH2CH2NH
H
H
Cl
NH2
Tetrahedron 2005, 56, 5077. (f) Kumar, R.; Nath, M.;
Tyrrell, D. L. J. J. Med. Chem. 2002, 45, 2032.
a Reaction conditions: nucleobase (2 mmol), 5 (6 mmol), H2O (5 mL),
Et3N (6 mmol), MWI 250 W (100 °C).
(3) (a) Suwiꢀski, J.; Walczak, K. Synthesis 2001, 225. (b) Qu,
G.-R.; Li, Y.; Han, S.-H. J. Chem. Res. 2005, 167.
(c) Alahiane, A.; Rochdi, A.; Taourirte, M.; Redwane, N.;
Sebti, S.; Lazrek, H. B. Tetrahedron Lett. 2001, 41, 3579.
(d) Khalafi-Nezhad, A.; Soltani Rad, M. N.; Khoshnood, A.
Synthesis 2004, 583. (e) Singh, D.; Wani, M. J.; Kumar, A.
J. Org. Chem. 1999, 64, 4665. (f) Qu, G.-R.; Han, S.-H.;
Zhang, Z.-G.; Geng, M.-W.; Xue, F. Can. J. Chem. 2006, 84,
819.
(4) (a) Cai, Y.; Sun, X.-F.; Wang, N.; Lin, X.-F. Synthesis 2004,
671. (b) Guillarme, S.; Legoupy, S.; Aubertin, A.-M.;
Olicard, C.; Bourgougnonc, N.; Huet, F. Tetrahedron 2003,
59, 2177. (c) Geen, G. R.; Kincey, P. M.; Choudary, B. M.
Tetrahedron Lett. 1992, 33, 4609. (d) Scheiner, P.; Geer,
A.; Bucknor, A.-M.; Imbach, J.-L.; Schinazi, R. F. J. Med.
Chem. 1989, 32, 73. (e) Baker, B. R.; Tanna, P. M. J. Org.
Chem. 1965, 30, 2857. (f) Lira, E. P.; Huffman, C. W. J.
Org. Chem. 1966, 31, 2188. (g) Khalafi-Nezhad, A.; Zarea,
A.; Soltani Rad, M. N.; Mokhtari, B.; Parhami, A. Synthesis
2005, 419.
b Isolated yields based on nucleobase.
Table 5 Michael Addition of 6-Chloropurine to Acrylatea
Cl
N
Cl
N
N
N
Et3N, H2O
N
N
OR
+
MWI, 5 min
N
H
N
O
CH2CH2CO2R
9a
11a–c
12a–c
Entry
R
Product
12a
Yield (%)b
1
2
3
Me
Et
79
83
76
12b
(5) (a) Loupy, A. Microwaves in Organic Synthesis; Wiley-
VCH: Weinheim, 2002, 147. (b) Varma, R. S. Green Chem.
1999, 1, 43. (c) Kappe, C. O. Angew. Chem. Int. Ed. 2004,
43, 6250. (d) Lidström, P.; Tierney, J.; Wathey, B.;
Westman, J. Tetrahedron 2001, 57, 9225.
t-Bu
12c
a Reaction conditions: 6-chloropurine (2 mmol), 11 (6 mmol), H2O
(5 mL), Et3N (6 mmol), MWI 250 W (100 °C).
b Isolated yields based on 9a.
(6) (a) Surendra, K.; Krishnaveni, N. S.; Sridhar, R.; Rao, K. R.
J. Org. Chem. 2006, 71, 5819. (b) Li, C.-J. Chem. Rev.
2005, 105, 3095. (c) Organic Synthesis in Water; Grieco, P.
A., Ed.; Blackie Academic and Professional: London, 1998.
(d) Eissen, M.; Metzger, J. O.; Schmidt, E.; Schneidewind,
U. Angew. Chem. Int. Ed. 2002, 41, 414. (e) Li, C.-J.; Chan,
T.-H. Organic Reactions in Aqueous Media; Wiley: New
York, 1997. (f) Paul, S.; Gupta, M.; Singh, P. P.; Gupta, R.;
Loupy, A. Synth. Commun. 2005, 35, 325. (g) Kormos, C.
M.; Leadbeater, N. E. Synlett 2006, 1663.
(7) Khalafi-Nezhad, A.; Zare, A.; Parhami, A.; Soltani Rad, M.
N.; Nejabat, G. R. Can. J. Chem. 2006, 84, 979.
(8) Dueholm, K. L.; Egholm, M.; Behrens, C.; Christensen, L.;
Hansen, H. F.; Vulpius, T.; Petersen, K. H.; Berg, R. H.;
Nielsen, P. E.; Buchardt, O. J. Org. Chem. 1994, 59, 5767.
In conclusion, we have developed a green, rapid, and
operationally simple method for regioselective Michael
additions for preparation of acyclic nucleosides in neat
water under microwave irradiation. All of the products
were obtained in moderate to high yields. Pyrimidine and
purine derivatives were exclusively alkylated at N-1 and
N-9, respectively. This method also has the practical
advantage that the protection of hydroxyl and amino
groups is not necessary.
Synlett 2007, No. 5, 721–724 © Thieme Stuttgart · New York

724
G.-R. Qu et al.
LETTER
(9) (a) Kikugawa, K.; Iizuka, K.; Ichino, M. J. Med. Chem.
1973, 16, 358. (b) Lin, Q.; Jiang, F.; Schultz, P. G.; Gray, N.
S. J. Am. Chem. Soc. 2001, 123, 11608. (c) Thompson, R.
D.; Secunda, S.; Daly, J. W.; Olsson, R. A. J. Med. Chem.
1991, 34, 2811. (d) Holy, A.; Votruba, I.; Tloustova, E.;
Masojidkova, M. Collect. Czech. Chem. Commun. 2001, 66,
1545.
(10) Qu, G.-R.; Geng, M.-W.; Han, S.-H.; Zhang, Z.-G.; Xue, F.
Chin. Chem. Lett. 2006, 17, 1149.
(11) General Procedure for the Michael Addition of Uracil to
Acrylonitrile.
2246, 1631, 1568, 1520, 1481, 1418, 1363, 1326, 1172, 905,
867, 781, 636 cm–1. HRMS: m/z calcd for C8H7ClN6:
222.0421; found: 222.1410. Anal. Calcd for C8H7ClN6: C,
43.16; H, 3.17; N, 37.75. Found: C, 43.14; H, 3.14; N, 37.69.
Compound 10d: yield 77%; pale yellow powder; mp
>300 °C. 1H NMR (400 MHz, DMSO-d6): d = 3.141 (t, 2 H,
J = 6.4 Hz), 4.412 (t, 2 H, J = 6.4 Hz), 7.821 (s, 2 H), 8.211
(s, 1 H) ppm. 13C NMR (100 MHz, DMSO-d6): d = 18.58,
39.79, 118.19, 118.65, 141.61, 150.94, 153.53, 157.25 ppm.
IR (KBr): 3494, 3418, 3308, 3148, 3087, 3070, 3021, 2979,
2936, 2930, 2860, 2257, 1654, 1597, 1570, 1515, 1479,
1449, 1423, 1409, 1360, 1311, 1257, 644 cm–1. HRMS: m/z
calcd for C8H7ClN6: 222.0421; found: 222.0411. Anal.
Calcd for C8H7ClN6: C, 43.16; H, 3.17; N, 37.75. Found: C,
43.13; H, 3.11; N, 37.70.
Compound 10g: yield 78%; white powder; mp 144–145 °C.
1H NMR (400 MHz, DMSO-d6): d = 3.166 (t, 2 H, J = 6.4
Hz), 3.605 (s, 4 H), 4.455 (t, 2 H, J = 6.4 Hz), 4.712 (s, 1 H),
7.514 (s, 1 H), 8.184 (s, 1 H), 8.230 (s, 1 H) ppm. 13C NMR
(100 MHz, DMSO-d6): d = 18.18, 42.84, 59.99, 118.24,
119.07, 140.44, 149.18, 152.58, 154.83 ppm. HRMS: m/z
calcd for C10H12N6O: 232.1072; found: 232.1081. Anal.
Calcd for C10H12N6O: C, 51.72; H, 5.21; N, 36.19. Found: C,
51.70; H, 5.15; N, 36.11.
To a mixture of uracil (2 mmol, 0.224 g) and Et3N (0.85 mL,
6 mmol) in neat H2O (5 mL), acrylonitrile (6 mmol, 0.4 mL)
was added. Then the mixture was put into the cavity of a
commercially available single-mode microwave synthesis
apparatus equipped with a high sensitivity infrared sensor
for temperature control and measurement (MAS-I, Sineo
Microwave Chemical Technology Co. Ltd., Shanghai, P. R.
of China) and irradiated at 250 W (internal temperature:
100 °C) for 5 min. After completion of the reaction, the
mixture was concentrated to dryness under reduced pressure
and the residue was purified by column chromatography
using EtOAc–cyclohexane (9:1) as the eluent to afford 6a in
82% yield.
(12) Compound 6e: yield 74%; yellow crystals; mp 164–165 °C.
1H NMR (400 MHz, DMSO-d6): d = 2.940 (t, 2 H, J = 6.4
Hz), 3.982 (t, 2 H, J = 6.4 Hz), 6.318 (d, 1 H, J = 7.2 Hz),
7.606 (d, 1 H, J = 7.2 Hz) 12.766 (s, 1 H) ppm. 13C NMR
(100 MHz, DMSO-d6): d = 17.07, 44.46, 112.86, 118.65,
141.39, 148.63, 191.05 ppm. IR (KBr): 3188, 3102, 3073,
3042, 2965, 2926, 2252, 1681, 1622, 1451, 1355, 1317,
1255, 1149, 1094, 866, 855, 638 cm–1. HRMS: m/z calcd for
C7H7N3OS: 181.0310; found: 181.0320. Anal. Calcd for
C7H7N3OS: C, 46.40; H, 3.89; N, 23.19. Found: C, 46.37; H,
3.85; N, 23.11.
Compound 10h: yield 73%; white powder; mp 152–154 °C.
1H NMR (400 MHz, DMSO-d6): d = 3.134 (t, 2 H, J = 6.4
Hz), 3.498–3.597 (m, 4 H), 4.414 (t, 2 H, J = 6.4 Hz), 4.771
(t, 1 H, J = 5.2 Hz), 8.169 (s, 1 H), 8.208 (s, 1 H) ppm. 13
C
NMR (100 MHz, DMSO-d6): d = 18.39, 42.90, 45.28, 59.46,
60.67, 118.37, 141.14, 149.78, 153.45, 155.38 ppm. HRMS:
m/z calcd for C10H11ClN6O: 266.0683; found: 266.0692.
Anal. Calcd for C10H11ClN6O: C, 45.04; H, 4.16; N, 31.51.
Found: C, 45.02; H, 4.15; N, 31.48.
Compound 10i: yield 76%; white powder; mp 163–164 °C.
1H NMR (400 MHz, DMSO-d6): d = 3.087 (t, 2 H, J = 6.4
Hz), 3.572 (s, 4 H), 4.254 (t, 2 H, J = 6.4 Hz), 4.707 (s, 1 H),
5.852 (s, 2 H), 6.958 (s, 1 H), 7.747 (s, 1 H) ppm. 13C NMR
(100 MHz, DMSO-d6): d = 17.97, 38.47, 42.66, 60.35,
113.39, 118.38, 136.95, 151.28, 155.20, 160.35 ppm.
HRMS: m/z calcd for C10H13N7O: 247.1182; found:
247.1174. Anal. Calcd for C10H13N7O: C, 48.58; H, 5.30; N,
39.65. Found: C, 48.51; H, 5.21; N, 39.55.
Compound 12b: yield 83%; pale yellow crystals; mp 72–
74 °C. 1H NMR (400 MHz, DMSO-d6): d = 1.094 (t, J = 6.8
Hz, 3 H), 3.014 (t, J = 6.8 Hz, 2 H), 4.013 (q, J = 7.2 Hz, 2
H), 4.532 (t, J = 6.8 Hz, 2 H), 8.689 (s, 1 H), 8.783 (s, 1 H)
ppm. 13C NMR (100 MHz, DMSO-d6): d = 14.04, 33.36,
60.47, 130.98, 147.78, 149.09, 151.58, 152.08, 170.54 ppm.
HRMS: m/z calcd for C10H11ClN4O2: 254.0571; found:
254.0562. Anal. Calcd for C10H11ClN4O2: C, 47.16; H, 4.35;
N, 22.00. Found: C, 47.08; H, 4.29; N, 21.91.
Compound 8d: yield 80%; white powder; mp 226–228 °C.
1H NMR (400 MHz, DMSO-d6): d = 2.920 (t, 2 H, J = 6.4
Hz), 3.928 (t, 2 H, J = 6.4 Hz), 6.993 (s, 1 H), 7.842 (s, 1 H),
8.147 (s, 1 H) ppm. 13C NMR (100 MHz, DMSO-d6): d =
17.14, 45.07, 85.98, 118.82, 147.05, 154.52, 162.81 ppm. IR
(KBr): 3364, 3107, 2963, 2945, 2247, 1674, 1650, 1500,
1438, 1410, 1377, 1333, 1280, 1207, 1099, 1028, 988, 824,
778, 653, 628 cm–1. HRMS: m/z calcd for C7H7BrN4O:
241.9803; found: 241.9810. Anal. Calcd for C7H7BrN4O: C,
34.59; H, 2.90; N, 23.05. Found: C, 34.54; H, 2.85; N, 22.97.
Compound 10c: yield 86%; white powder; mp 220 °C (dec.).
1H NMR (400 MHz, DMSO-d6): d = 3.145 (t, 2 H, J = 6.4
Hz), 4.354 (t, 2 H, J = 6.4 Hz), 6.980 (s, 2 H), 8.181 (s, 1 H)
ppm. 13C NMR (100 MHz, DMSO-d6): d = 18.17, 39.26,
118.62, 123.65, 143.23, 150.01, 154.37, 160.20 ppm. IR
(KBr): 3494, 3291, 3168, 3155, 3113, 2961, 2947, 2927,
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