Synthesis of purine antiviral agents, hypoxanthine and 6-mercaptopurine

Article abstract of DOI:10.1023/A:1020822216986

Some potentially biologically active 6-substituted purine derivatives have been synthesized from simple organic reagents. The reaction of urea with ethyl cyanoacetate gave 6-aminopyrimidine-2,4-dione which was converted in two steps into purine derivative, xanthine. The latter was treated with formamide at 200°C to obtain hypoxanthine. The chlorination of hypoxanthine with POCl₃ gave 6-chloropurine which was converted into 6-mercaptopurine via reaction with thiourea in acetonitrile, followed by treatment with boiling ethanol.

Full text of DOI:10.1023/A:1020822216986

Russian Journal of Organic Chemistry, Vol. 38, No. 7, 2002, pp. 1053 1055. From Zhurnal Organicheskoi Khimii, Vol. 38, No. 7, 2002, pp. 1096 1098.
Original English Text Copyright 2002 by Sariri, Khalili.
Synthesis of Purine Antiviral Agents,
Hypoxanthine and 6-Mercaptopurine*
R. Sariri and G. Khalili
Departament of Chemistry, Guilan University, Rasht, Iran
Received June 27, 2001
Abstract Some potentially biologically active 6-substituted purine derivatives have been synthesized from
simple organic reagents. The reaction of urea with ethyl cyanoacetate gave 6-aminopyrimidine-2,4-dione
which was converted in two steps into purine derivative, xanthine. The latter was treated with formamide at
200 C to obtain hypoxanthine. The chlorination of hypoxanthine with POCl3 gave 6-chloropurine which was
converted into 6-mercaptopurine via reaction with thiourea in acetonitrile, followed by treatment with boiling
ethanol.
Despite intense efforts to discover drugs that may
be of value in the systematic treatment of human viral
infections, such infections have been resistant to
chemotherapy. Intracellular and intimate relationships
between viral and host functions and metabolism
make it difficult to destroy viruses without damage
to the host cell. Therefore, there are only a few agents
effective against viruses and having an acceptable
therapeutic index, i.e., the ratio of 50% cytotoxic dose
(IC50) to 50% antiviral dose (EC50).
Following the identification of retrovirus referred
to as human immunodeficiency virus (HIV), an etio-
logical agent of acquired immunodeficiency syndrome
(AIDS) [1 3], much efforts were made to search for
drugs capable of treating or preventing this lethal
disease. Several nucleosides were shown to exhibit
in vitro an anti-HIV activity commensurate with
development of clinical trials [4]. It was also found
that a variety of purine derivatives are potential anti-
viral agents. Hypoxanthine (V, 6-hydroxypurine) pos-
sesses a strong antitumor activity with minimal side
effects. Simultaneously, this compound is an inter-
mediate product in the synthesis of other 6-substituted
purines, e.g., 6-mercaptopurine.
6-Mercaptopurine (VIII) containing a water mole-
cule is an antiviral agent of the purine series, which
was synthesized for the first time in 1951. It is
a yellow solid with a molecular weight of 170.19,
which decomposes on heating above 308 C. Com-
pound VIII is insoluble in water, acetone, and ether
but readily soluble in dilute alkalies and poorly
soluble in dilute sulfuric acid [6]. 6-Mercaptopurine is
used as an oral medicine in the treatment of acute
lymphoblastic leukemia (ALL), acute myoblastic
leukemia (AML), and chronic myelocystic leukemia
(CML). Its oral absorption is relatively low, and about
50% of the drug appears in the plasma in 2 h [7].
However, due to minor side effects and the ability
to pass the blood brain barrier, 6-mercaptopurine
is now known as a potent anticancer agent with
a reasonable anti-HIV activity. Despite its good ability
to pass the blood brain barrier, its concentration in
central spinal fluid is insufficient for treatment of
leukemia [8].
We required a concise, efficient, and economic
synthetic procedure allowing large-scale preparation
of the target compounds. Starting from small mole-
cules, we targeted hypoxanthine as the first product
which may be starting compound for the synthesis of
6-mercaptopurine. The synthesis is based on a new
approach involving closure of pyrimidine ring and
subsequent formation of imidazole ring in situ, which
should lead to purine system already containing
appropriate substituents [9 12]. The total synthesis
of hypoxanthine (V) is shown in Scheme 1, and
Scheme 2 illustrates its subsequent transformation into
6-mercaptopurine (VIII).
Two main methods for building up purine ring
system are known. The first way includes synthesis
of pyrimidine nuclei with subsequent introduction of
appropriate substituents for closure of the second
purine ring [5, 6]. The second route (it is not con-
sidered in the present article) begins with the syn-
thesis of imidazole ring, followed by closure of
pyrimidine ring.
____________
*
The original article was submitted in English.
1070-4280/02/3807-1053$27.00 2002 MAIK Nauka/Interperiodica

1054
SARIRI, KHALILI
Scheme 1.
Scheme 2.
Although various methods of synthesis of purine
acid, and the mixture was stirred for 2 h to complete
derivatives are known, the procedure proposed by us
is quite efficient. In most steps, the yield was greater
than 80%. The initial compounds were of reagent
grade and had low costs. We are now continuing
studies in the field of synthesis of possible antagonists
for the purine and pyrimidine moieties of nucleic
acids. In the framework of these studies, some other
purine derivatives have been prepared, which may
be used as intermediate products in other synthetic
methods [9].
crystallization of the product. Yield of pyrimidine I
1
214 g (95%), mp >300 C. IR spectrum, , cm :
3408, 3150, 1711, 1631, 1544, 1400, 631, 552, 526.
6-Amino-5-nitrosopyrimidine-2,4(1H,3H)-dione
(II). A solution of 150 g of sodium nitrite in 400 ml
of water was added to a mixture of 241 g of 5-amino-
2,4-dihydroxypyrimidine (I) and 120 ml of water.
Glacial acetic acid, 170 g, was then added dropwise
under vigorous stirring. A red solid precipitated, the
mixture was stirred for 6 h at room temperature, and
the precipitate was filtered off and washed with
1
EXPERIMENTAL
ethanol and water. Yield 86%. IR spectrum, , cm :
3255, 1693, 1551, 1288, 1146, 1093, 787, 717, 562.
The IR spectra of compounds pelleted with KBr
(0.2% dispersions) were obtained on a Perkin Elmer
457 spectrometer.
5,6-Diaminopyrimidine-2,4(1H,3H)-dione sulfate
(III). A 3-l flask was charged with 250 g of 6-amino-
2,4-dihydroxy-5-nitrosopyrimidine (II), and 1300 ml
of hot (90 C) distilled water was added. The colored
suspension was heated on a boiling water bath, and
100 g of sodium dithionite was added over a period
of 15 min. The mixure lost its color, and the viscous
solution was stirred with a mechanical stirrer for
20 min on heating. The precipitate of 5,6-diamino-
2,4-dihydroxypyrimidine dihydrogen sulfate was fil-
tered off, dried, and transferred into a 3-l flask.
6-Aminopyrimidine-2,4(1H,3H)-dione (I). Ethyl
cyanoacetate, 212 ml, was added from a dropping
funnel under vigorous stirring to a solution of sodium
ethoxide prepared from 92 g of metallic sodium and
2000 ml of ethanol. A white solid separated from the
solution. After the addition of ethyl cyanoacetate was
complete, the mixture was stirred for 20 min at room
temperature. Urea, 120 g, was added, and the mixture
was heated for 3 h under reflux on a water bath. A solution of 150 ml of concentrated sulfuric acid in
The white precipitate was filtered off, washed with
ethanol, and dissolved in 1000 ml of water. The solu-
tion was adjusted to pH 7 by adding glacial acetic
1500 ml of water was added, and the mixture was
heated for 1 h on a boiling water bath and cooled.
The precipitate was filtered off, washed with acetone,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 7 2002

SYNTHESIS OF PURINE ANTIVIRAL AGENTS
1055
1
and dried. Yield 200 g (70%). IR spectrum, , cm :
3384, 3156, 1715, 1649, 1533, 1404, 1107, 1051, 539.
6-Mercaptopurine (VIII). A 500-ml flask was
charged with 10 g of S-(purin-6-yl)thiuronium
chloride (VII) and 130 ml of anhydrous ethanol, and
the mixture was heated for 2 h under reflux. The
mixture was cooled, and the precipitate of 6-mercapto-
purine was filtered off and reprecipitated from water.
Xanthine (IV). A suspension of 150 g of 5,6-di-
amino-2,4-dihydroxypyrimidine sulfate in 750 ml of
formamide in a 3-l flask was heated for 90 min at
180 C. The mixture was cooled, and the precipitate
was filtered off and thoroughly washed with water
and cold ethanol. Yield 80 g (90%), mp >300 C. IR
1
Yield 6 g (81%), mp >300 C. IR spectrum, , cm :
3000, 2679, 1615, 1529, 1409, 1346, 1224, 1015, 877.
1
spectrum, , cm : 3065, 1572, 1491, 1392, 1320,
1233, 989, 635, 605.
REFERENCES
Hypoxanthine (V). A mixture of 100 g of xanthine
in 1000 ml of formamide in a high-pressure reactor
was heated for 30 min at 200 C under stirring with
a magnetic stirrer. The mixture was cooled, and the
precipitate was filtered off and dried at 80 C. Yield
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1
65 g (70%), mp >300 C. IR spectrum, , cm : 3050,
1670, 1580, 1421, 1214, 1137, 965, 892, 547.
6-Chloropurine (VI). A flask equipped with
a reflux condenser was charged with a mixture of
22.6 g of hypoxanthine (V), 65 g of acetonitrile,
and 64 g of ethylbenzene, and 82 ml of phosphoryl
chloride was added from a dropping funnel. When the
entire amount of POCl3 was added, the mixture was
stirred for 6 h at 65 C. It was then cooled, and the
yellow precipitate was filtered off and dissolved in
5 N NaOH. Activated charcoal, 3 g, was added, and
the mixture was stirred for 30 min and filtered.
Acetone, 72 ml, was added to the yellow solution,
and the mixture was adjusted to pH 7 by adding dilute
hydrochloric acid. The precipitate was filtered off,
washed with 100 ml of aqueous acetone and 50 ml
of acetone, and dried. Yield 14 g (53%), mp >300 C.
1
IR spectrum, , cm : 3065, 1573, 1491, 1392, 1330,
vol. 114, no. 163876.
1233, 990, 640, 605.
9. Khalili, Gh.H., Production of Anti-Cancer Drugs,
Master of Sci. Thesis, Guilan University, 2000.
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vol. 58, p. 3443b.
12. US Patent no. 2830053, 1958; Chem. Abstr., 1958,
S-(Purin-6-yl)thiuronium chloride (VII). A mix-
ture of 10 g of 6-chloropurine, 4.6 g of thiourea, and
150 ml of acetonitrile was refluxed for 90 min. After
cooling, the yellow precipitate was filtered off and
washed with cold acetonitrile. Yield 14.2 g. The prod-
uct was brought into the next step without additional
purification. mp 246 C.
vol. 52, p. 14711.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 38 No. 7 2002