2-Amino-6-(1,2,4-triazol-4-yl)-purine: A useful intermediate in the synthesis of 9-alkylguanines

Article abstract of DOI:10.1016/S0040-4039(00)01236-3

2-Amino-6-(1,2,4-triazol-4-yl)purine, prepared from 2,6-diaminopurine, was regioselectively alkylated at position 9. Subsequent alkaline hydrolysis afforded 9-alkylguanines in high yield. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01236-3

Tetrahedron Letters 41 (2000) 7211±7215
2-Amino-6-(1,2,4-triazol-4-yl)-purine: a useful intermediate in
the synthesis of 9-alkylguanines
Karine Alarcon, Alain Martelli, Martine Demeunynck* and Jean Lhomme
Â
Chimie Bioorganique, LEDSS, UMR CNRS 5616, Universite Joseph Fourier, BP 53, 38041 Grenoble cedex 9, France
Received 23 June 2000; accepted 21 July 2000
Abstract
2-Amino-6-(1,2,4-triazol-4-yl)purine, prepared from 2,6-diaminopurine, was regioselectively alkylated at
position 9. Subsequent alkaline hydrolysis a€orded 9-alkylguanines in high yield. # 2000 Elsevier Science
Ltd. All rights reserved.
Keywords: guanine; 2,6-diaminopurine; 1,2,4-triazole.
The continuous interest in antiviral drugs such as acyclovir and ganciclovir explains the constant
search for new regioselective synthetic methods of 9-alkylguanines.1,2 The major problem
encountered is the control of N9 versus N7 alkylation of guanine. To overcome this problem,
various methods were reported, involving either alkylation of 6-substituted 2-aminopurines, or
ring closure of pyrimidine or imidazole derivatives. In the direct alkylation of purines, the
presence of bulky substituents was shown to dramatically increase N9 regioselectivity and hence
6-chloro and 6-alkoxy derivatives have been successfully used. 2,6-Diaminopurine has also been
used as starting material to prepare carbocyclic guanosine analogues. In 1984, Ogilvie3 reported
the synthesis of ganciclovir by alkylation of N2,N6-diacetyl-2,6-diaminopurine in 55% yield. The
following steps involved the hydrolysis of the acetyl groups and the enzymatic conversion to
ganciclovir using adenosine deaminase. In 1991, Jones4 applied direct alkylation of 2,6-diamino-
purine to prepare carbovir a carbocyclic nucleoside analogue. Introduction of the alkyl group was
achieved regioselectively in 50% yield using sodium hydride in DMF. After selective protection of
the 2-amino group by acetylation, the 6-amino group was converted into 6-hydroxyl via diazo-
tation reaction. Herein, we present a new route to 9-alkylated guanines from 2,6-diaminopurines,
by conversion of the 6-amino group to a bulky easily hydrolysable group, 1,2,4-triazole. Conversion
of an amino group to 1,2,4-triazole has been previously reported to convert adenine5 or adenosine
analogues6 to the corresponding 6-dimethylamino-, 6-methylthio- or 6-methoxy purines or
nucleosides. A similar strategy was also used to functionalyze cytidine.7 The (1,2,4-triazol-4-yl)
* Corresponding author. Tel: +33 476 51 44 29; fax: +33 476 51 43 82; e-mail: martine.demeunynck@ujf-grenoble.fr
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01236-3

7212
intermediates were obtained by treatment of the corresponding amino-heterocycles with
1,2-bis[(dimethylamino)methylene]hydrazine following the Bartlett and Humphrey's method.8
Reaction of 2,6-diaminopurine with 1,2-bis[(dimethylamino)methylene]hydrazine 18 gave a
single product in 85% yield. No other reaction product was detected. The NMR and mass spectra
suggested the incorporation of one triazole moiety but the data were not sucient to determine
the regioselectivity of the reaction as both amino groups are susceptible to react. Furthermore,
the literature data concerning the relative reactivity of the two amino groups of 2,6-diaminopurine
are rather unclear. Acetylation in acetic acid4 or using trichlorophenyl phenoxyacetate9 were
reported to occur at position 2, but Watkins and Rapoport described the regioselective mono-
protection of the amine at position 6 by a CBZ group.10 Nitrosation occurs preferentially at
position 2.11 To achieve the structural determination and distinguish between structures 2A and
2B (Scheme 1), we performed the alkaline hydrolysis of the amino-triazolo purine 2A/2B and the
reaction was followed by HPLC using a diode array detector. Depending on the position of the
triazole at either position 6 or 2, guanine or isoguanine were expected to be formed respectively.
As shown in Fig. 1, the retention time and the UV spectrum of guanine and isoguanine are very
characteristic. Fig. 2 shows the HPLC pro®le obtained after hydrolysis of the amino-triazolo
intermediate 2A/2B. One peak is observed, and both the retention time and the UV spectrum
correspond to guanine. Alkylation of 2A was performed by reacting the sodium salt of 2A with
either methyliodide or 3-bromopropane in DMF. In both cases, a single alkylation product was
formed in good to excellent yields (78 and 93%). The resulting compounds 3a and 3b were
characterised by NMR and mass spectrometry. Alkaline hydrolysis of 3a and 3b was performed
in 1N NaOH to give the corresponding 9-methyl or 9-propylguanines in good yields (67 and
85%). Their spectroscopic properties were identical to those reported in the literature data.12 14
Scheme 1. (i) 1, DMF, 180ꢀC, 18 h; (ii) ICH3 or BrC3H7, NaH, DMF, rt, 1 h; (iii) 1N NaOH, 100ꢀC, or DMSO±1N
NaOH, 50ꢀC
As a conclusion, the commercially available 2,6-diaminopurine was converted into 2-amino-6-
(1,2,4-triazol-4-yl)purine, which may be considered as a cheaper alternative to 2-amino-6-
chloropurine. The alkylation of this key-intermediate occurs regioselectively at position 9, this is
probably due to steric hindrance introduced by the triazole that precludes alkylation at position 7.

7213
Figure 1. HPLC pro®le of a reference mixture of guanine and isoguanine (m-bondapak-C18 reversed-phase column,
methanol±water (pH 2.5, H3PO4) gradient)
Figure 2. HPLC pro®le obtained after hydrolysis of 2A

7214
The corresponding substituted guanines are easily obtained by hydrolysis. Nucleophilic
substitution of the triazole to prepare 9, N6-dialkyl-2,6-diaminopurine is currently under investigation.
Synthetic procedures. 2-Amino-6-(1,2,4-triazol-4-yl)-9-H-purine 2: A suspension of 2,6-diami-
nopurine (0.2 g, 1.3 mmol) and 18 (0.38 g, 1.7 mmol) in DMF (50 mL) was re¯uxed for 8 h. The
mixture was then cooled and the solvent removed under reduced pressure. The residue was
co-evaporated with methanol three times. The solid was suspended in methanol, ®ltered and dried
to give 2 (0.223 g, 85%), mp: >360ꢀC. H NMR (200 MHz, DMSO-d6): ꢀ ppm=13 (br s, NH),
1
9.42 (2H, s, H triazole), 8.22 (1H, s, H-8), 6.77 (2H, s, NH2). 13C NMR (200 MHz, DMSO-d6): ꢀ
ppm=160.0 (C-2), 157.1 (C-4), 142.6 (C-6), 142.0 (C-8), 140.6 (2C, C triazole), 114.9 (C-5); ms
(CI, ammoniac+isobutane): M=202, m/z: 203 ((M+1)+).
2-Amino-9-alkyl-6-(1,2,4-triazol-4-yl)-9H-purin 3: Sodium hydride (60%, 0.025 g, 0.60 mmol)
was added to compound 2 (0.1 g, 0.50 mmol) dissolved in DMF (2.5 mL) under inert atmosphere.
After 1 h stirring at room temperature, methyl iodide or 3-bromopropane (0.75 mmol) was added
to the solution and the resulting mixture was stirred for 3 h. After dilution with water, the desired
product was extracted with CH2Cl2. The organic layers were dried over sodium sulphate and
concentrated. Precipitation was achieved by slow addition of diethylether.
ꢀ
1
Compound 3a: (93% yield) mp:>350 C. H NMR (200 MHz, TFA-D): ꢀ ppm=10.13 (2H, s,
H triazole), 8.94 (1H, s, H-8), 4.09 (3H, s, CH3). 13C NMR (50 MHz, TFA-D): ꢀ ppm=156.6,
150.6, 141.8, 139.2, 138.8, 138.3, 106.8, 28.6 (N-CH3).
Compound 3b: (78% yield) mp: 248±250ꢀC. 1H NMR (200 MHz, CD3OD): ꢀ ppm=9.62 (2H,
s, H triazole), 8.11 (1H, s, H-8), 4.15 (2H, t, N-CH2), 1.92 (2H, m, CH2-CH3), 0.96 (3H, t, CH3).
1H NMR (200 MHz, DMSO-d6): ꢀ ppm=9.41 (2H, s, H triazole), 8.27 (1H, s, H-8), 6.90 (2H, s,
NH2), 4.05 (2H, t, N-CH2), 1.81 (2H, q, CH2-CH3), 0.86 (3H, t, CH3). 13C NMR (50 MHz,
DMSO-d6): ꢀ ppm=160.0, 156.2, 144.0, 142.9, 140.7 (2C, triazole), 115.2, 44.5 (N-CH2), 22.4
(CH2-CH3), 10.9 (CH2-CH3); ms (FAB(+),NBA): M=244, m/z: 245 (100, (M+1)+), 176 (M+-tri-
azole).
9-Propylguanine 4b: A suspension of 9-propyl-6-triazolyl derivative 3b (0.3 mmol) in 1N
NaOH (1 mL) was stirred at 100ꢀC for 10 min (or in DMSO±1N NaOH, 1:1 mixture at 50ꢀC for
30 min). When all starting material had dissolved, the solution was cooled at room temperature
and acetic acid was slowly added until precipitation of 4b, which was thus isolated in 85% yield.
9-Methylguanine 4a: The procedure described above was used for conversion of 3a. Due to
poor solubility of 3a, hydrolysis required 45 min in boiling 1N NaOH. Compound 4a was
obtained in 67% yield. The structures of 4a and 4b were con®rmed by comparison of their MS
and NMR data with literature data.12 14
Acknowledgements
The `Association pour la Recherche sur le Cancer' (ARC) is gratefully thanked for its support.
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