High yielding, base catalyzed C6 regioselective amination and N9 alkylation in purine nucleotide

Article abstract of DOI:10.14233/ajchem.2019.22235

2,6-Dichloropurine is an interesting new nucleoside which gave regioselectively various 2-derivatized or 6-derivatized purines by using a secondary amines. An efficient, simple and regioselective synthesis of C⁶ morpholine, N⁹ alkylated purine nucleoside derivatives were attained via chloro-amine coupling reaction between 2,6-dichloropurine with morpholine followed by commercial alkylation method using DMF and K2CO3. Over the traditionally used protocols and procedure, it have been exhibited advance benefits such as admirable yield, simple reaction conditions and modest influence.

Full text of DOI:10.14233/ajchem.2019.22235

Asian Journal of Chemistry; Vol. 31, No. 12 (2019), 2871-2874
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2019.22235
High Yielding, Base Catalyzed C6 Regioselective Amination and N9 Alkylation in Purine Nucleotide
1
2,*,
1
1
1
GAUTAMKUMAR DHUDA , KHUSHAL KAPADIYA
, PARESH LADWA , BHAVNA GODHANIYA and JAYESH MODHA
1Department of Chemistry, Maharshi Dayanand Science College (Affilated to Bhakta Kavi Narsinh Mehta University), Porbandar-360575, India
2Department of Chemistry, B.R.C.C. Laboratory, School of Science, R K University, Rajkot-360020, India
*Corresponding author: E-mail: khushal_kapadiya06@yahoo.com
Received: 3 June 2019;
Accepted: 31 July 2019;
Published online: 16 November 2019;
AJC-19634
2,6-Dichloropurine is an interesting new nucleoside which gave regioselectively various 2-derivatized or 6-derivatized purines by using
a secondary amines. An efficient, simple and regioselective synthesis of C6 morpholine, N9 alkylated purine nucleoside derivatives were
attained via chloro-amine coupling reaction between 2,6-dichloropurine with morpholine followed by commercial alkylation method
using DMF and K2CO3. Over the traditionally used protocols and procedure, it have been exhibited advance benefits such as admirable
yield, simple reaction conditions and modest influence.
Keywords: 2,6-Dichloropurine, Regioselective, Alkylation, Amination.
biological and pharmacological properties [6,7]. The well-
studied nucleoside i.e. purines nucleosides and their various
novel substituted adducts are found biologically efficient and
strongly dispersed heterocycles having N-atoms in the core
ring structure.Various types of biologically diverse study have
been exhibited by purine motif like anticancer, anti-HIV, anti-
hypertensive, etc. [8]. It has been identified that purine with
substitution at 9th position in imidazole ring side shows unbeat-
able medicinal properties [6,7,9,10] and many of its derivatives
are commercial available as active drug molecules like acycl-
ovir and famciclovir (antiviral) [11,12], 6-mercaptopurine and
thioguanine (anticancer), neuropsychiatric disorders, neuro-
degenerative diseases, inflammatory diseases (theophylline,
azathioprine,6′-mercaptopurine), tuberculosis or impotence [13].
Traditionally, the protection using halogenated substance,
mesylate or tosylate is an imperative synthetic route for the
modification on purine to generate its derivatives but it typically
suffers from deprived regioselectivity, resultant in a mixture
of N9 and N7 isomers [8,14]. Literature shows that the desired
N9 compound is normally the major product, but formation of
significant amounts of the N7 isomer as well as other alkylation
products is often observed [15]. Addition of acyclic/hetero-
cycles analogues in to the nucleosides have remained major
biomedical research targets in current drug discovery process
and it has been becomes fast after the discovery of potent anti-
INTRODUCTION
Organic chemist is performing one of the most significant
C-N bond formations in organic synthesis [1]. The variety of
amines and its derivatives are utilized as solvents, drug interme-
diates, agrochemicals, medicines and chemical agent for various
chemical reactions. The SN reaction between different alkyl
halides and primary/secondary amines is convenient for the
research of tertiary amines but the downside of this pathway
is to longer reaction time and generate binary product of secon-
dary and tertiary amines. On the other hand, thermal irradiated
reaction using base catalyst needs more reaction time and low
yield of target molecules. To minimize the above drawbacks,
cooper and palladium catalyzed coupling reactions have been
performed for the successful amination [2].
For such transformation two concepts are utilizing by
researchers i.e. microwave and traditional method [3]. Out of
these two, microwave irradiation has concerned substantial
attention for quick synthesis of a library of organic amalgams
but due to initiation of side reaction and other byproduct form-
ation, it is not used in diverse range of reaction [4]. Traditional
heating method and combinatorial synthesis for the lead mole-
cule identification is well developed and used [5].
Recent literature shows that modified nucleosides are
becoming sizzling area of research due to their captivating
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2872 Dhuda et al.
Asian J. Chem.
viral activity of 9-[(2-hydroxyethoxy)methyl]guanine, the
standard drug for the treatment of herpes viral infections [2].
On the basis of above modifications, researchers turn their
attention to insert saturated heterocycles to the nucleoside to
generate active lead molecules. It has been also noted that the
morpholine/aromatic amines analogous to the nucleosides build
up their supreme space in the current medicinal chemistry [16].
In continuation of our interest on the modification of nucleo-
sides [17] and obtain these diverse scaffolds in higher yields
with shorter reaction times and under milder reaction conditions,
we turned our attention on insertion of acyclic part (morpholine
and alkyl group) in to the purine motif. In this article, we report
the modified purine nucleosides of 2-chloro-9-alkylated-6-
morpholino-9H-purine via highly regioselective C6 acylation
of purines with morpholine catalyzed by Hunig’s base (DIPEA-
N,N-diisopropylethylamine).Alkylation of morpholine-purine
was oriented to N9 using DMF/K2CO3 with 89 % regioselectivity
[18].
of the reaction, it was poured into crushed ice water, dried over
vaccum buchner funnel and purified by column chromatography
using silica 60-120 mesh size as stationary phase [mobile
phase: hexane (60 %):ethyl acetate (40 %)] (Scheme-I). The
isolated purine derivatives were characterized and identified
by various spectroscopic techniques like MS, IR, 1H and 13C NMR.
9-Butyl-2-chloro-6-morpholino-9H-purine (4a): Dark
yellow, yield: 92 %; m.p.: 152 ºC; IR (KBr, cm-1, νmax): 3024.84
(arom. ring C-H str.), 2966.76 (aliph. C-H str. asym.), 2935.25
(aliph. C-H str. sym.), 1584.21, 1514.38, 1487.24 (arom. ring
skeleton), 1258.11 (C-N str.), 1163.33 (C-O str.); 1H NMR (400
MHz, DMSO-d6) δ ppm: 8.23 (s, 1H, CH of imidazole ring),
4.52-4.33 (broad-m, 4H, morpholine -CH2), 4.13-3.95 (m, 2H,
morpholine -CH2), 3.73-3.51 (m, 2H, morpholine -CH2), 1.79-
1.71 (m, 2H, -CH2 of alkyl chain), 1.29-1.18 (m, 2H, -CH2 of
alkyl chain), 1.19-1.17 (t, 3H, -CH3 of alkyl chain), 0.91-0.85
(m, 2H, N-CH2 of alkyl chain); 13C NMR (101 MHz, DMSO-d6)
δ ppm: 153.32, 152.40, 151.73, 140.69, 117.98, 66.01, 31.17,
22.07, 19.15; MS (m/z): 295.76 (M+); Anal. calcd. (found) %
for C13H18N5OCl: C, 52.79 (52.80); H, 6.13 (6.15); N, 23.68
(23.67); O, 5.41 (5.40); Cl, 11.99 (11.98).
EXPERIMENTAL
All the chemicals and reagents were received from Sigma-
Aldrich and Merck. Silica gel plate G60 F254 (Merck) was used
in thin layer chromatography to monitor the completion of
the reaction. Visualization was made under UV light (254 and
365 nm). Infrared spectra of the compounds were recorded
2-Chloro-6-morpholino-9-(prop-2-ynyl)-9H-purine (4b):
Bright yellow, yield: 96 %; m.p.: 140 ºC; IR (KBr, cm-1, νmax):
3101.51 (arom. ring C-H str.), 2979.73 (aliph. C-H str. asym.),
2967.96 (aliph. C-H str. sym.), 2128.56 (C≡C str.), 1584.09,
1514.40, 1460.71 (arom. ring skeleton), 1261.00 (C-N str.),
1
on IR Affinity-1S spectrophotometer (Shimadzu). H (400
MHz) and 13C (101.1 MHz) NMR spectra were recorded on a
Bruker AVANCE II spectrometer in DMSO-d6. Mass spectro-
meter GCMS-QP 2010 (Shimadzu) was used to resolute the
mass spectra of compounds and rotary evaporator was used
for drying the compounds. Melting points were measured by
open capillary method and are uncorrected.
1
1140.71 (C-O str.); H NMR (400 MHz, DMSO-d6) δ ppm:
8.27 (s, 1H, CH of imidazole ring), 5.04-5.03 (m, 2H, propargyl
-CH2), 4.43 (broad, 4H, morpholine-CH2), 3.91-3.84 (m, 4H,
morpholine -CH2), 3.53 (s, 1H, ≡CH of propargyl); 13C NMR
(101 MHz, DMSO-d6) δ ppm: 152.72, 151.26, 139.94, 118.22,
77.77, 76.28, 65.98, 32.63; MS (m/z): 277.70 (M+);Anal. calcd.
(found) % for C12H12N5OCl: C, 51.90 (51.93); H, 4.36 (4.38);
Cl, 12.77(12.75); N, 25.22 (25.20); O, 5.76 (5.75).
Synthesis of 2-chloro-6-morpholino-9H-purine (3)
(step I): To a n-butanol containing round bottom flask, 2,6-
dichloropurine (1) (50 mmol), N,N-diisopropylethylamine
(DIPEA) (10 mmol) and morpholine (2) (50 mmol) were added
and was refluxed at 80 ºC for 2 h to generate a reactive inter-
mediate (3). Thin layer chromatography [hexane(7):ethyl acetate
(3)] was utilized to identify the complete formation of product.
After 12 h stirring at room temperature, it was poured into
ice-cold water and stirred for 1 h to isolate free material in
powder form. The separated product was filtered and washed
with cold water. The isolated product was dried for next 12 h
at room temperature.
For the purification purpose, hexane stripping was perfor-
med using roteva evaporator at 60 ºC and 500 mm pressure
through 100 rpm speed. The step-I crude product was charact-
erized by 1H NMR and IR techniques to carry out further reactions.
Synthesis of 2-chloro-9-alkylated-6-morpholino-9H-
purine (4a-f) (step II): Into a vaccum dried round bottom flask,
product (3) (3 mmol), powdered form of potassium carbonate
(K2CO3) (10 mmol) and DMF (1 mL) were heated at 80 ºC for
2 h to generate nucleophile at N9 position of purine. After 2 h
heating, various alkyl halides (5.5 mmol) were charged at heating
condition using micropipette to the reaction mixture and continue
to heating for 3 h for the completion of reaction. It was monitored
by TLC using two different mobile phase systems [hexane (7):
ethyl acetate (3) & ethyl acetate (100 %)]. After completion
2-Chloro-9-isopropyl-6-morpholino-9H-purine (4c):
Yellow, yield: 91 %; m.p.: 152 ºC; IR (KBr, cm-1, νmax): 3031.26
(arom. ring C-H str.), 2965.22 (aliph. C-H str. asym.), 2939.20
(aliph. C-H str. sym.), 1589.48, 1521.59, 1496.21 (arom. ring
1
skeleton), 1259.99 (C-N str.), 1156.56 (C-O str.); H NMR
(400 MHz, DMSO-d6) δ ppm: 8.25 (s, 1H, CH of imidazole
ring), 5.01-4.99 (m, 2H, propargyl -CH2), 4.49 (broad, 4H,
morpholine -CH2), 3.99-3.91 (m, 4H, morpholine -CH2), 4.38
(m, 1H, -CH of isopropyl), 1.52-1.51 (d, 6H, -CH3 of iso-propyl);
13C NMR (101 MHz, DMSO-d6) δ ppm: 159.63, 155.11, 147.70,
142.35, 135.34, 65.59, 48.20, 45.92, 45.92, 18.84, 18.84; MS
(m/z): 281.74 (M+);Anal. calcd. (found) % for C12H16N5OCl: C,
51.16 (51.15); H, 5.72 (5.72); N, 24.86 (24.84); O, 5.68 (5.70);
Cl, 12.58 (12.59).
2-Chloro-6-morpholino-9-propyl-9H-purine (4d): Off-
whitish yellow, yield: 92 %; m.p.: 136 ºC; IR (KBr, cm-1, νmax):
3103.20 (arom. ring C-H str.), 2989.01 (aliph. C-H str. asym.),
2941.20 (aliph. C-H str. sym.), 1590.10, 1555.21, 1489.04(arom.
ring skeleton), 1269.23 (C-N str.), 1151.22 (C-O str.); 1H NMR
(400 MHz, DMSO-d6) δ ppm: 8.29 (s, 1H, CH of imidazole
ring), 4.45-4.39 (broad-m, 4H, morpholine -CH2), 4.20-4.12
(m, 2H, morpholine-CH2), 3.88-3.78 (m, 2H, -CH2 alkyl chain),
3.70-3.63 (m, 2H, morpholine-CH2), 1.83-1.79 (m, 2H, -CH2
alkyl chain), 1.04-1.01 (t, 3H, -CH3 alkyl chain); 13C NMR (101

Vol. 31, No. 12 (2019)
High Yielding, Base Catalyzed C6 Regioselective Amination and N9 Alkylation in Purine Nucleotide 2873
O
O
Cl
N
N
N
O
N
N
a & a'
b & b'
N
N
N
N
N
N
N
Cl
N
H
Cl
N
Cl
N
H
H
R
1
2
3
4a-f
O
N
O
O
N
a & a'
Cl
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Cl
Cl
N
H
N
Cl
O
3'
4b;
96 %
4a: 92 %
4c;
91 %
O
O
O
Reaction and conditions:
a) DIPEA, n-butanol, 80 oC;
a') conc. HCl, IPA, 80 oC;
N
N
N
N
N
N
N
N
b) DMF, K2CO3, R-Br, 70-80 oC;
b') DMF, NaH, R-Br, 70-80 oC
N
N
N
Cl
N
N
N
Cl
N
Cl
4f; 86 %
4e; 93 %
4d;
92 %
Scheme-I: Synthetic route for the Synthesis of N9-alkyl C6-purine morpholine (4a-f)
MHz, DMSO-d6) δ ppm: 155.69, 153.82, 151.85, 142.59, 135.19,
66.52, 49.23, 46.56, 20.86, 11.99; MS (m/z): 281.74 (M+);
Anal. calcd. (found) % for C12H16N5OCl: C, 51.16 (51.12); H,
5.72 (5.73); N, 24.86 (24.87); O, 5.68 (5.69); Cl, 12.58 (12.59).
9-sec-Butyl-2-chloro-6-morpholino-9H-purine (4e):
Dark yellow, yield: 93 %; m.p.: 165 ºC; IR (KBr, cm-1, νmax):
3019.96 (arom. ring C-H str.), 2960.88 (aliph. C-H str. asym.),
2946.06 (aliph. C-H str. asym.), 1601.22, 1556.55, 1452.42
(arom. ring skeleton), 1261.88 (C-N str.), 1170.55 (C-O str.);
1H NMR (400 MHz, DMSO-d6) δ ppm: 8.25 (s, 1H, CH of
imidazole ring), 4.50-4.43 (m, 4H, morpholine-CH2), 4.26-
4.21 (m, 2H, morpholine -CH2), 3.89-9.83 (m, 2H, morpholine
-CH2), 3.70-3.62 (m, 1H, -CH of alkyl chain), 1.69-1.65 (m,
2H, -CH2 of alkyl chain), 1.53-1.51 (d, 3H, -CH3 of alkyl chain),
0.99-1.02 (t, 3H, -CH3 of alkyl chain); 13C NMR (101 MHz,
DMSO-d6) δ ppm: 160.02, 157.54, 152.01, 147.85, 141.91,
68.12, 59.00, 29.62, 18.59, 15.62; MS (m/z): 295.76 (M+);
Anal. calcd. (found) % for C13H18N5OCl: C, 52.79 (52.78); H,
6.13 (6.13); N, 23.68 (23.69); O, 5.41 (5.41); Cl, 11.99 (11.99).
2-Chloro-9-cyclopropyl-6-morpholino-9H-purine (4f):
Off-whitish yellow, yield: 86 %; m.p.: 181 ºC; IR (KBr, cm-1,
1H, CH of imidazole ring), 4.50-4.41 (broad-m, 4H, morp-
holine -CH2), 4.00-3.96 (m, 2H, morpholine -CH2), 3.70-3.62
(m, 2H, morpholine -CH2), 2.42-2.38 (m, 1H, -CH of cyclo-
propane ring), 0.88-0.84 (m, 2H, -CH2 of cyclopropane ring),
0.88-0.84 (m, 2H, -CH2 of cyclopropane ring); 13C NMR (101
MHz, DMSO-d6) δ ppm: 165.59, 160.22, 155.23, 149.05,
140.00, 68.93, 47.22, 39.56, 12.97. MS (m/z): 279.72 (M+);
Anal. calcd. (found) % for C12H14N5OCl: C, 51.52 (51.49); H,
5.04 (5.05); N, 25.04 (25.06); O, 5.72 (5.71); Cl, 12.67 (12.68).
RESULTS AND DISCUSSION
For the purpose of functionalizing the 6th positions in
one pot, 2,6-chloropurine was treated with one equivalent of
morpholine in the presence of DIPEA. This led to various
6-substituted purines 3 as outlined in Scheme-I. It should be
noticed that the yield decreased when using more than 1 equiv.
of DIPEA. On the other hand, the same reaction with same
mole proportion of purine and morpholine, in the presence of
acidic catalyst was no longer gives product. The C6 chlorine
atom of purine was substituted by a -NH group of morpholine
followed by reaction of 3 with various alkyl halides gives N9
alkylation in 89 % yields [18]. The amination of compound 1
(Scheme-I) with morpholine (2), under changing the reaction
condition i.e. using conc. HCl and isopropyl alcohol (IPA)
were seems to be not very efficient and display poor yield.
ν
max): 3102.33 (arom. ring C-H str.), 2989.02 (aliph. C-H str.
asym.), 2939.22 (aliph. C-H str. sym.), 1598.16, 1519.98,
1455.72 (arom. ring skeleton), 1260.54 (C-N str.), 1170.22
1
(C-O str.); H NMR (400 MHz, DMSO-d6) δ ppm: 8.19 (s,

2874 Dhuda et al.
Asian J. Chem.
Alkylation yields with 2-chloro-9-alkylated-6-morpho-
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