Efficient solid-phase synthesis of isocytosine derivatives

Article abstract of DOI:10.1007/s10593-010-0434-0

We have investigated the potential use of a solid-phase synthesis of novel 6-(arylmethyl)-5-methyl-isocytosine derivatives based on the immobilization of the corresponding 2-thiothymine on a Merrifield resin, oxidation of the immobilized form to the sulfo

Full text of DOI:10.1007/s10593-010-0434-0

Chemistry of Heterocyclic Compounds, Vol. 45, No. 11, 2009
EFFICIENT SOLID-PHASE SYNTHESIS
OF ISOCYTOSINE DERIVATIVES
I. A. Novakov1, B. S. Orlinson1, R. V. Brunilin1, M. B. Nawrozkij1*,
E. A. Gordeeva1, E. N. Savel'ev1, and E. N. Gerasimov1
We have investigated the potential use of a solid-phase synthesis of novel 6-(arylmethyl)-5-methyl-
isocytosine derivatives based on the immobilization of the corresponding 2-thiothymine on a Merrifield
resin, oxidation of the immobilized form to the sulfone, and aminolysis of the latter under mild
conditions.
Keywords: isocytosines, "addition-elimination" mechanism, antiviral activity, solid-phase synthesis.
Isocytosine derivatives are characterized by a number of practically useful properties amongst which a
prominent place is occupied by the antiviral activity of specific members of this series which contain specific
hydrocarbon radicals at atoms C-5 and C-6 of the pyrimidine ring and also at the exocyclic nitrogen atom [1].
The corresponding 2-N-unsubstituted and disubstituted derivatives [1, 2] can be readily prepared via
condensation of the corresponding guanidine derivatives with 3-oxoesters in a basic medium. At the same time
the synthesis of 2-N-monosubstituted members of this series unavoidably leads to the formation of a difficult to
separate mixture of condensation products [3]. In this connection the preparation of the corresponding
derivatives broadly involves the use of indirect methods based on the aminolysis of the corresponding
S-alkylated 2-thiouracil derivatives [1, 2, 4-6]. This process generally occurs under rigid conditions and
demands the use of a cosolvent [2, 4-6]. The absence of the latter usually leads to a lower yield of the target
isocytosine derivatives and to difficulties in their separation and purification [1].
In this connection we have studied the use of a solid-phase synthetic strategy which is well established
in the chemistry of peptides [7] in order to prepare the corresponding isocytosine derivatives. It should be noted
that a solid-phase synthesis has already been successfully used in the chemistry of pyrimidine derivatives [3, 8].
The choice of starting materials and target products in the synthesis was carried out on the basis of a
previously established structure – antiviral activity in this series of compounds [1].
We have used a Merrifield resin as solid carrier which contained 1-1.3 milliequivalents of chloromethyl
groups per gram and 1% of divinylbenzene cross-links. Reaction of this resin with the sodium salts of the
corresponding 2-thiothymines 1a-c in anhydrous DMF medium caused the heterocyclic compound to be
attached to the resin through the sulfur atom. The sulfides obtained 2a-c were oxidized by m-chloro-perbenzoic
acid [8, 9] to the corresponding sulfones 3a-c and then treated with excess amine in anhydrous THF
_______
* To whom correspondence should be addressed, e-mail: kholstaedt@yandex.ru.
1 Volgograd State Technical University, Volgograd 400131, Russia.
__________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1697-1702, November, 2009.
Original article submitted September 27, 2008; revised version submitted March 11, 2009.
0009-3122/09/4511-1365©2009 Springer Science+Business Media, Inc.
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medium to give high yields of the target isocytosine derivatives 4a-j (Scheme 1). The yields of the target
products are given in the Experimental section. The starting 2-thiothymines 1a-c were obtained using methods
reported in the literature [10-12].
The synthetic scheme developed is characterized by a series of advantages over known analogs. In the
first place its realization does not demand rigid conditions. In the second, S-methylation of the 2-thiouracil
derivatives is not employed thus avoiding working with iodomethane which is both a toxic and a volatile
compound.
Scheme 1
O
O
Me
Me
NH
NH
ResinCH2Cl
DMF
N
R1
SNa
N
R1
S
Resin
O
R
R
2a–c
O
1a–c
Me
Me
NH
NH
R2NH2
THF
R2
m-CPBA
N
R1
SO2 Resin
N
R1
N
CH2Cl2
H
R
R
4a–j
3a–c
4 a, d−j R = R1 = H; b R = R1 = F, c R = F, R1 = Cl; a−c R2 = PhCH2CH2,
d R2 = 4-FC6H4CH2CH2, e R2 = 4-MeOC6H4CH2CH2, f R2 = PhCH2CH2CH2,
g R2 = 1-AdCH2, h R2 = 1-AdCH2CH2, i R2 = 1-AdCH2CH(Me),
j R2 = 3-(1-naphthyl)bicyclo[2.2.1]hept-2-yl; m-CPBA = m-chloroperbenzoic acid
The main feature of the proposed method is the use of an immobilized form of 2-thiouracil which
permits a series of operations connected with the removal of reaction side products to be achieved without loss
of the starting compound. In addition, activation of the immobilized form of the 2-thiouracil is accomplished via
oxidation of the sulfide group to a sulfone. The high lability of the sulfone towards nucleophilic attack (when
compared with the starting sulfide) is achieved through the presence the significantly greater positive charge on
the C-2 atom of the pyrimidine ring and also the improved properties of the leaving group. The key solvolysis
reaction is likely realized through an "addition-elimination" reaction.
Hence we can conclude that a solid-phase synthesis is a convenient route for the preparation of
differently structured isocytosine derivatives. The target products are formed in good yield and purity under
mild conditions.
EXPERIMENTAL
1H NMR spectra were recorded on a Varian Mercury 300B (300 MHz) instrument in CDCl3 (compounds
4a-j) with HMDS as internal standard. The melting points of the synthesized compounds were measured on a
Cole-Palmer instrument.
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Reagents from Alfa Aesar (2-phenylethylamine, 99%; 2-(4-methoxyphenyl)ethylamine, 98%; 3-phenyl-
propylamine, 98%; m-chloroperbenzoic acid, ~ 70%; and Merrifield resin) and from Acros Organics (2-(4-
fluorophenyl)ethylamine, 99%) were used in the synthesis. Adamant-1-ylmethylamine [13], 2-adamant-1-yl-
ethylamine [13], 1-adamant-1-ylpropyl-2-amine [14], and trans-[3-(1-naphthyl)bicyclo[2.2.1]hept-2-yl]amine
[15] were prepared by methods reported before. Solvent purification and drying was carried out according to
standard methods [16].
6-Benzyl-5-methyl-2-[(2-phenylethyl)amino]pyrimidin-4(3H)-one (4a). The Merrifield resin (9 g)
was stirred with anhydrous DMF (45 ml) until swollen, excess solvent was decanted off, and there was added
the sodium salt of 6-benzyl-2-thiothymine (1a), (prepared by treating 6-benzyl-2-thiothymine (2 g, 8.6 mmol)
with an alcohol solution of sodium hydroxide (380 mg, 9.5 mmol) and subsequent evaporation of the reaction
product). An additional volume of DMF (120 ml) was added. The reaction mixture was stirred at room
temperature to full conversion of the starting compound (TLC monitoring of an acidified sample in the system
3% MeOH in CH2Cl2). Solvent was decanted off and the residue was washed with DMF (3×100 ml), a mixture
of CH2Cl2–ethanol (19:1 by volume, 3×100 ml), CH2Cl2 (3×100 ml), and petroleum ether (40-70ºC, 3×100 ml).
The 6-benzyl-2-thiothymine immobilized on the polymer matrix was evacuated and suspended in anhydrous
CH2Cl2 (25 ml) and a solution of m-chloroperbenzoic acid (~ 70%, 6.4 g, 25.8 mmol) in dry CH2Cl2 (125 ml)
was added. The reaction product was stirred at room temperature for 24 h, solvent was decanted, and the residue
was washed with 2-propanol (3×90 ml), petroleum ether (40-70ºC, 3×90 ml), ether (3×90 ml), and anhydrous
THF (3×90 ml). The washed residue was evacuated and suspended in anhydrous THF (75 ml) and the mixture
obtained was treated with 2-phenylethylamine (3.2 ml, 3.07 g, 25.3 mmol) and stirred for 24 h at room
temperature. The solution was decanted and evaporated, the residue was washed with a mixture of CH2Cl2 and
ethanol (19:1 by volume, 3×150 ml), and the solution obtained was mixed with the residue after evaporation of
THF. The obtained solution was treated with a 10% aqueous solution of citric acid monohydrate (2×75 ml) and
water (3×50 ml), and dried over MgSO4. After filtration and evaporation of solvent the residue crystallized.
Yield 2.33 g (85%); mp 146-148ºC (MeCN). 1H NMR spectrum, δ, ppm (J, Hz): 1.77 (3H, s, CH3); 2.78 (2H, t,
J = 7.3, NHCH2CH2); 3.49-3.56 (2H, m, NHCH2CH2); 3.77 (2H, s, CH2); 6.33 (1H, br. s, NH); 7.06-7.24 (10H,
m, 2С6H5); 11.55 (1H, br. s, NH pyrimidine). Found, %: C 74.90; H 6.70; N 12.81. C20H12N3O. Calculated, %:
C 75.21; H 6.63; N 13.16.
6-(2,6-Difluorobenzyl)-5-methyl-2-[(2-phenylethyl)amino]pyridimin-4(3H)-one (4b) was prepared
similarly to compound 4a apart from the use of 6-(2,6-difluorobenzyl)-2-thiothymine in place of the 6-benzyl-
2-thiothymine. Yield 81%; mp 175-176ºC (MeCN). 1H NMR spectrum, δ, ppm (J, Hz): 1.83 (3H, s, CH3); 2.59
(2H, t, J = 7.3, NHCH2CH2); 3.26-3.32 (2H, m, NHCH2CH2); 3.82 (2H, s, CH2); 6.17 (1H, t, J = 6.2, NH); 6.79
(2H, t, J = 7.7, H-3,5 2,6-F2C6H3); 6.98 (2H, d, J = 7.3, H-2,6 Ph); 7.07-7.22 (4H, m, H-4 2,6-F2C6H3, H-3,4,5
Ph); 11.47 (1H, br. s, NH pyrimidine). Found, %: C 67.39; H 5.20; F 10.69; N 12.20. C20H19F2N3O. Calculated,
%: C 67.59; H 5.39; F 10.69; N 11.82.
6-(6-Chloro-2-fluorobenzyl)-5-methyl-2-[(2-phenylethyl)amino]pyrimidin-4(3H)-one (4c) was
prepared similarly to compound 4a apart from the use of 6-(2-fluoro-6-chlorobenzyl)-2-thiothymine in place of
1
the 6-benzyl-2-thiothymine. Yield 84%; mp 160.5-161.5ºC (MeCN). H NMR spectrum, δ, ppm (J, Hz): 1.83
(3H, s, CH3); 2.54 (2H, t, J = 7.3, CH2Ph); 3.20-3.26 (2H, m, CH2NH); 3.93 (2H, s, 2-F-6-ClC6H3CH2); 6.27
(1H, br. s, NH); 6.88-6.96 (3H, m, H-3,4 2-F-6-ClC6H3, H-4 Ph); 7.03-7.20 (5H, m, H-5 2-F-6-ClC6H3,
H-2,3,5,6 Ph); 11.41 (1H, br. s, NH pyrimidine). Found, %: C 65.00; H 4.99; Cl 9.53; F 5.11; N 10.92.
C20H19ClFN3O. Calculated, %: C 64.60; H 5.15; Cl 9.53; F 5.11; N 11.30.
6-Benzyl-2-{[(4-fluorophenyl)ethyl]amino}-5-methylpyrimidin-4(3H)-one (4d) was prepared similarly to
compound 4a apart from the use of 2-(4-fluorophenyl)ethylamine in place of 2-phenylethylamine. Yield 78%; mp
190-191ºC (MeCN). 1H NMR spectrum, δ, ppm (J, Hz): 1.77 (3H, s, CH3); 2.74 (2H, t, J = 7.3, 4-FC6H4CH2); 3.49
(2H, m, NHCH2); 3.77 (2H, s, CH2Ph); 6.24 (1H, br. s, NH); 6.86 (2H, t, J = 8.8, H-3,5 4-FC6H4); 6.96-7.06 (2H, m,
H-2,6 Ph); 7.08-7.24 (5H, m, H-2,6 4-FC6H4, H-3,4,5 Ph); 11.54 (1H, br. s, NH pyrimidine). Found, %: C 71.01;
H 6.00; F 5.64; N 12.12. C20H20FN3O. Calculated, %: C 71.20; H 5.97; F 5.63; N 12.45.
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6-Benzyl-2-{[2-(methoxyphenyl)ethyl]amino}5-methylpyrimidin-4(3H)-one (4e) was prepared
similarly to compound 4a apart from the use of 2-(4-methoxyphenyl)ethylamine in place of 2-phenylethylamine.
Yield 79%; mp 156-157ºC (MeCN). 1H NMR spectrum, δ, ppm (J, Hz): 1.79 (3H, s, CH3); 2.71 (2H, t, J = 7.7,
4-MeOC6H4CH2); 3.48 (2H, m, NHCH2); 3.69 (3H, s, OCH3); 3.77 (1H, s, CH2Ph pyrimidine ); 6.20-6.31 (1H,
m, NH); 6.73 (2H, d, J = 8.1, H-3,5 4-MeOC6H4); 7.01 (2H, d, J = 8.1, H-2,6 4-MeOC6H4); 7.10-7.27 (5H, m,
C6H5); 11.56 (1H, br. s, NH pyrimidine). Found, %: C 72.20; H 6.64; N 12.10. C21H23N3O2. Calculated, %:
C 72.18; H 6.63; N 12.03.
6-Benzyl-5-methyl-2-[(3-phenylpropyl)amino]pyrimidin-4(3H)-one (4f) was prepared similarly to
compound 4a using 3-phenylpropylamine in place of the 2-phenylethylamine. Yield 75%; mp 124-125ºC
1
(MeCN). H NMR spectrum, δ, ppm (J, Hz): 1.72-1.80 (2H, m, CH2CH2CH2Ph); 1.84 (3H, s, CH3); 2.45-2.62
(2H, m, CH2CH2Ph); 3.21-3.37 (2H, m, CH2NH); 3.78 (2H, s, CH2Ph pyrimidine); 7.03-7.19 (11H, m, 2C6H5,
NH). Found, %: C 75.61; H 6.89; N 12.74. C21H23N3O. Calculated, %: C 75.65; H 6.95; N 12.60.
2-[(Adamant-1-ylmethyl)amino]-6-benzyl-5-methylpyrimidin-4(3H)-one (4g) was prepared similarly
to compound 4a apart from the use of adamant-1-ylmethylamine in place of the 2-phenylethylamine. Yield 97%;
mp 228-229ºC (MeCN). 1H NMR spectrum, δ, ppm (J, Hz): 1.27-1.43 (6H, m, CH2 Ad); 1.46-1.62 (6H, m, CH2
Ad); 1.76-1.93 (6H, m, CH3, H-3,5,7 Ad); 2.92-3.02 (2H, m, CH2NH); 3.75 (2H, s, CH2Ph); 6.07-6.23 (1H, m,
NH); 7.09-7.26 (5H, m, С6H5); 11.48 (1H, br. s, NH pyrimidine). Found, %: C 76.16; H 8.04; N 11.96.
C23H29N3O. Calculated, %: C 76.00; H 8.04; N 11.56.
2-[(2-Adamant-1-ylethyl)amino]-6-benzyl-5-methylpyrimidin-4(3H)-one (4h) was prepared similarly
to compound 4a apart from the use of 2-adamant-1-ylethylamine in place of 2-phenylethylamine. Yield 92%;
1
mp 182-184ºC (MeCN). H NMR spectrum, δ, ppm (J, Hz): 1.23 (2H, dd, J1 = 8.4, J2 = 16.5, CH2 Ad); 1.43
(6H, br. s, CH2 Ad); 1.51-1.67 (6H, m, CH2 Ad); 1.79-1.95 (6H, m, CH3, H-3,5,7 Ad); 3.21-3.37 (2H, m,
CH2NH); 3.77 (2H, s, CH2Ph); 6.00 (1H, t, J = 5.5, NH); 7.09-7.26 (5H, m, C6H5); 11.49 (1H, br. s, NH
pyrimidine). Found, %: C 76.70; H 8.28; N 10.75. C24H31N3O. Calculated, %: C 76.35; H 8.28; N 11.13.
2-[(1-Adamant-1-ylprop-2-yl)amino]-6-benzyl-5-methylpyrimidin-4(3H)-one (4i) was prepared
similarly to compound 4a except for the use of 1-adamant-1-ylprop-2-ylamine in place of 2-phenylethylamine.
Yield 97%; mp 228-229ºC (MeCN). 1H NMR spectrum, δ, ppm (J, Hz): 1.06 (4H, d, J = 6.8, CH2 Ad); 1.12 (1H,
d, J = 3.4, CH2 Ad); 1.22 (1H, d, J = 8.5, CH2 Ad); 1.27 (1H, d, J = 8.5, CH2 Ad); 1.35-1.51 (8H, m, CH3CH,
CH2 Ad); 1.53-1.60 (2H, m, CHCH2 Ad); 1.80-1.86 (3H, m, CH3 pyrimidine); 1.93 (3H, s, H-3,5,7 Ad);
3.70-3.87 (2H, m, CH2Ph); 4.16-4.32 (1H, m, CHNH); 5.87 (1H, d, J = 8.5, NH); 7.12 (1H, d, J = 6.8, H-4 Ph);
7.15-7.31 (4H, m, H-2,3,5,6 Ph); 11.23 (1H, br. s, NH pyrimidine). Found, %: C 77.00; H 8.40; N 11.01.
C25H33N3O. Calculated, %: C 76.69; H 8.49; N 10.73.
6-Benzyl-5-methyl-2-{[trans-3-(1-naphthyl)bicyclo[2.2.1]hept-2-yl]amino}pyrimidin-4(3H)-one (4j)
was prepared similarly to compound 4a apart from the use of trans-3-(1-naphthyl)bicyclo[2.2.1]hept-2-ylamine in
1
place of the 2-phenylethylamine. Yield 70%; mp 232.5-233.5ºC (MeCN). H NMR spectrum, δ, ppm (J, Hz):
1.11-1.19 (1H, m, H-7 bicycle); 1.27-1.31 (1H, m, H-6 bicycle); 1.34-1.49 (6H, m, CH3, H-5,6,7 bicycle); 1.56 (1H,
br. s, H-5 bicycle); 1.98-2.14 (1H, m, H-4 bicycle); 2.43 (1H, br. s, H-1 bicycle); 3.34-3.48 (2H, m, CH2Ph);
3.88-4.04 (1H, m, H-2 bicycle); 4.19-4.35 (1H, m, H-3 bicycle); 6.80 (1Н, br. s, NH); 6.88-6.96 (3H, m, H-4 Ph, H-
2,7 C10H7); 6.98-7.10 (7H, m, H-2,3,5,6 Ph, H-3,4,6 C10H7); 7.14-7.18 (1H, m, H-5 C10H7); 7.58 (1H, d, J = 8.5, H-8
C10H7); 10.61 (1H, br. s, NH pyrimidine). Found, %: C 80.00; H 6.72; N 10.02. C29H29N3O. Calculated, %: C 79.97;
H 6.71; N 9.65.
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