A convenient method for the modification of 8-bromoguanine via its N 9-tetrahydrofuranyl derivative

Article abstract of DOI:10.1055/s-2007-966015

A simple and straightforward method for the introduction of some N- and O-protecting groups into 8-bromoguanine has been developed using 2-acetylamino-8-bromo-6-oxo-9-(tetrahydrofuran-2-yl)-purine as a key intermediate. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-966015

PAPER
1325
A Convenient Method for the Modification of 8-Bromoguanine via Its
N9-Tetrahydrofuranyl Derivative
Modification of 8-Brom
a
oguaninervia Its 9
N
i
-Tetra
n
hydrofurany
a
l
D
erivative Madre,* Martins Ikaunieks, Sergey Belyakov
Latvian Institute of Organic Synthesis, Aizkraukles Str. 21, 1006 Riga, Latvia
Fax +371(7)550338; E-mail: madre@osi.lv
Received 4 December 2006; revised 26 February 2007
sis of the N- or O-protected derivatives of 2-acetylamino-
8-bromo-6-oxo-9(7)H-purine has not been described in
the literature. Therefore, the objective of this research was
to develop methods for the introduction of protecting
groups into 8-bromoguanine in order to transform it into a
useful synthetic intermediate.
Abstract: A simple and straightforward method for the introduc-
tion of some N- and O-protecting groups into 8-bromoguanine has
been developed using 2-acetylamino-8-bromo-6-oxo-9-(tetrahy-
drofuran-2-yl)-purine as a key intermediate.
Key words: alkylation, nucleobases, protecting groups, synthesis,
tetrahydrofuranyl substituent
The first protecting group we examined was the N,N-
diphenylcarbamoyl (DPC) functionality, which is widely
used for the O6-protection of guanine and guanosine.4 The
presence of the bulky and lipophilic DPC group in the
guanine molecule may strongly favour its regioselective
N9-alkylation or glycosylation, help to avoid side reac-
tions and make the nucleobase more soluble and, conse-
quently, much easier to manipulate.4 Furthermore, the O-
DPC moiety at position 6 of the purine could be replaced
by other functionalities, such as alkoxy or phenylthio.5
Purines bearing diverse substituents at various positions
of the heterocycle are synthons in the preparation of novel
nucleosides, nucleotides and oligonucleotides. They can
function as antiviral, antibacterial and anticancer agents.
Besides these uses, many possess antagonistic activity on
adenosine receptors and influence numerous other enzy-
matic systems.1
Our research has been focused on the synthesis of new
polyfunctional guanine derivatives. Guanine is the most
problematic nucleobase owing to its poor solubility in
most common organic solvents as well as its tendency
towards unselective chemical transformations caused by
the presence of many reaction centers in the molecule.
There is one general method for the carbamoylation of 6-
oxopurines – their treatment with N,N-diphenylcarbamoyl
chloride (DPC-Cl) and N,N-diisopropylethylamine in py-
ridine at ambient temperature.4 N2,O2¢,O3¢,O5¢-Tetraacet-
ylated 2-amino-6-oxo-9-(b-D-ribofuranosyl)purine and
some of its analogues as well as 2-acetylamino- and 2-
isobutyrylamino-9-acetyl-6-oxopurine have been used as
substrates for this reaction. In the latter case, the 9-acetyl
group served as the protecting group, which was removed
after carbamoylation by solvolysis.4 Unfortunately, the
existing methodology was unsuitable for the carbamoyla-
tion of 2-acetylamino-8-bromo-6-oxo-9(7)H-purine (1).
We failed to convert compound 1 into its 9-acetylated de-
rivative and the reaction of the 9-unprotected form of pu-
rine 1 with DPC-Cl afforded a disappointing mixture.
There was therefore a need to find some other derivative
of 8-bromopurine 1, suitable for the introduction of the
O6-DPC function. To this end, we decided to study 2-
acetylamino-8-bromo-6-oxo-9-(tetrahydrofuran-2-yl)pu-
rine (2), prepared previously through the reaction of 1
with 2,3-dihydrofuran (Scheme 1).6 Treatment of inter-
mediate 2 with DPC-Cl, under standard conditions,4 af-
forded 2-acetylamino-8-bromo-6-(N,N-diphenyl-carbam-
oyloxy)-9-(tetrahydrofuran-2-yl)purine (3) in 78% yield,
after purification by column chromatography. An attempt
to recrystallize product 3 from hot ethanol revealed that
the tetrahydrofuranyl moiety was removed under these
conditions and the corresponding 2-acetylamino-8-bro-
One of the methods we have extensively used for guanine
modification is alkylation, which is one of the principal
procedures of purine chemistry. However, in the case of
guanine-type bases, it often suffers from serious draw-
backs such as the formation of poorly separable isomeric
mixtures.2 Our previous research into the alkylation of 2-
acetylamino-8-bromo-6-oxo-9(7)H-purine was motivated
by the assumption that the electron-withdrawing bromine
at position 8 of the purine cycle would not only improve
the yield and regioselectivity of the guanine alkylation but
it could also serve as a good leaving group in further
chemical transformations of the alkylation products.3
However, the use of 2-acetylamino-8-bromo-6-oxo-
9(7)H-purine as the alkylating substrate did not solve the
problem of process selectivity. Usually, a mixture of N7/
N9-substituted products was formed and, in some cases, a
significant amount of bis-alkylated derivatives was isolat-
ed.3
It is well documented that the regioselectivity of alkyla-
tion and glycosylation of nucleobases can be controlled by
appropriate protection of the hydroxyl and amino groups
present in their molecules. To our knowledge, the synthe-
mo-6-(N,N-diphenylcarbamoyloxy)-9(7)H-purine
precipitated from the ethanolic solution in quantitative
yield. Further experiments showed that product 4 could
(4)
SYNTHESIS 2007, No. 9, pp 1325–1332
Advanced online publication: 18.04.2007
DOI: 10.1055/s-2007-966015; Art ID: P14306SS
x
x
.
x
x
.2
0
0
7
© Georg Thieme Verlag Stuttgart · New York

1326
M. Madre et al.
PAPER
N(Ph)2
O
N
O
N
O
N
O
N
N
N
N
Ref. 6
85%
HN
N
R
a
HN
HN
Br
Br
Br
N
78%
N
H
HN
HN
1
O
O
2
O
O
3, 8
O
80–90%
N(Ph)2
b
Ref. 6
86%
O
N
HN
O
O
N
Br
N
H2N
N
c
N
a
77%
Br
5
O
N
H
HN
N
4, 9
83%
O
O
R
O
N
3, 4 R = Me
8, 9 R = Ph
N
N
N
b
HN
HN
Br
Br
87%
N
H
HN
HN
N
O
O
Ph
O
Ph
7
6
Scheme 1 Reagents and conditions: (a) DPC-Cl, DIPEA, py, r.t., 2.5–4 h; (b) EtOH, reflux, 20 min; (c) TMSCl, py, CH2Cl2, 0 °C→r.t., 30
min, then BzCl, 0 °C→r.t., 4 h.
also be obtained from crude 3, although in lower yield. base 9 was more soluble than derivative 4 and, further-
Therefore, 9-tetrahydrofuranyl derivative 2 turned out to more, it could be recrystallized from ethanol.
be a convenient intermediate for the O6-carbamoylation of
purine 1.
Apart from acyl groups, dialkyl formamidines (especially
N,N-dimethylformamidine) are widely used as protecting
Taking into account the fact that in some reactions the N2- groups for the amino function of nucleosides. Taking into
benzoylated guanine derivatives are preferred over those account the possible influence of N-amidine-type protec-
that are N2-acetylated, the next objective of our research tion on the further alkylation of purine 1,9 the next object
was to synthesize 2-benzoylamino-8-bromo-6-(N,N- of our synthesis was 8-bromo-6-(N,N-diphenylcarbamoyl-
diphenylcarbamoyloxy)-9(7)H-purine (9), using the same oxy)-9(7)H-purine, with amidine protection at the amino
intermediate 2 (Scheme 1). For benzoylation of the exo- group. The preparation of an analogous guanosine deriva-
cyclic amino group of compound 5, obtained from depro- tive has been described in literature10 and, using this ap-
tection of 2,6 instead of the conventional benzoyl proach, 8-bromo-2-(N,N-dimethyliminoformamido)-6-
chloride–pyridine method,7 we decided to use the tran- oxo-9-(tetrahydrofuran-2-yl)purine (10) was generated
sient silylation approach developed recently for the prep- from compound 5 using N,N-dimethylformamide dimeth-
aration of N2-acetyl- and N2-phenacetyl derivatives of ylacetal in DMF (Scheme 2).
guanosine.8 The approach worked well in our case and al-
lowed us to synthesize 8-bromo-2-benzoylamino-6-oxo-
9-(tetrahydrofuran-2-yl)purine (6) from nucleobase 5 in a
three-step one-pot reaction (i.e. silylation with trimethyl-
silyl chloride, acylation with benzoyl chloride, de-silyla-
tion with aqueous sodium bicarbonate). Product 6 could
The carbamoylation of purine 10 afforded 8-bromo-6-
(N,N-diphenylcarbamoyloxy)-2-formylamino-9-(tetrahy-
dro-2-furanyl)purine (11) as the sole reaction product in-
stead of the expected 2-(N,N-dimethyliminoformamido)
derivative. It is likely that the formamide obtained result-
ed from a partial hydrolysis of the formamidine function
be treated with hot ethanol, affording 2-benzoylamino-8-
via the intermediate hemiorthoamide.11 A similar trans-
bromo-6-oxo-9(7)H-purine (7), or it could be carbamoy-
lated to obtain 2-benzoylamino-8-bromo-6-(N,N-diphe-
formation was also observed by us during the O6-tosyla-
tion of 9-(2-acetoxyethoxymetyl)-2-(N,N-dimethylimino-
formamido)-6-oxopurine.12 The reasons behind this hy-
drolysis are not immediately obvious as the carbamoyla-
tion of 10 was carried out in anhydrous solvents under
anhydrous conditions. Deprotection of 11 (ethanolic) and
crystallization afforded the previously unknown 8-bromo-
nylcarbamoyloxy)-9-(tetrahydrofuran-2-yl)purine
(8)
which, in turn, was easily transformed into 2-benzoyl-
amino-8-bromo-6-(N,N-diphenylcarbamoyloxy)-9(7)H-
purine (9). The purine 8 was less stable than the corre-
sponding 2-acetylamino analogue 3 and partial loss of its
tetrahydrofuranyl moiety occurred during the isolation
process. On the other hand, the 2-benzoylamino nucleo-
Synthesis 2007, No. 9, 1325–1332 © Thieme Stuttgart · New York

PAPER
Modification of 8-Bromoguanine via Its N9-Tetrahydrofuranyl Derivative
1327
N(Ph)2
N(Ph)2
O
O
O
N
O
N
N
N
c
N
Br
N
Br
87%
N
N
O
N
H
H
N
O
11
H
O
12
b
79%
O
Ph
O
N
O
N
N
d
a
HN
Br
Br
5
N
H
72%
82%
N
N
N
N
N
N
N
13
O
10
Scheme 2 Reagents and conditions: (a) N,N-dimethylformamide dimethylacetal, DMF, r.t., 4 h; (b) DPC-Cl, DIPEA, py, r.t., 6 h; (c) EtOH,
reflux, 20 min; (d) BzCl, py, MeCN, r.t., 72 h.
6-(N,N-diphenylcarbamoyloxy)-2-formylamino-9(7)H-
purine (12).
application in various substitution and hydrogenation re-
actions and also appear to be superior substrates in some
cross-coupling reactions.16 The sulfonylated products can
be prepared easily and rapidly by treatment of the suitably
protected guanosine derivatives with either arenesulfonyl
chlorides in pyridine or with the triethylamine/4-(N,N-
dimethylamino)pyridine (DMAP)/dichloromethane sys-
tem.16 The latter conditions have also been applied for the
arenesulfonylation of 9-(2-acetoxyethoxymethyl)-2-
acetylamino-6-oxopurine.17 Since, to the best of our
knowledge, there are no literature references to O6-sulfo-
nylated 8-bromoguanine or its derivatives, we therefore
decided to apply our methodology to the synthesis of this
compound. The reaction of purine 2 with 4-toluenesulfo-
nyl chloride, under standard conditions, afforded 2-acetyl-
amino-8-bromo-6-[(4-methylphenyl)sulfonyloxy]-9-
(tetrahydrofuran-2-yl)purine (14) in 74% yield
(Scheme 3).
The next step of our research was to evaluate the possibil-
ity of protecting position 6 of purine 1 with a benzoyl
group. Although guanine residues usually react with car-
boxylic acid chlorides and anhydrides at N2, it has been re-
ported that guanosine and its N2-acyl derivatives can
undergo acylation at O6 when treated with an excess of
substituted benzoyl chlorides in pyridine solution.13 Un-
fortunately, our attempts at treating nucleobases 1 or 5
with benzoyl chloride in pyridine, led to a dark mixture of
compounds. On the other hand, benzoylation of purine 5
in an acetonitrile/pyridine system14 gave a single reaction
product in reasonable yield. The product turned out to be
1-benzoyl-8-bromo-2-(N,N-dimethyliminoformamido)-
6-oxo-9(7)H-purine (13; Scheme 2). Therefore, it was ev-
ident that, alongside the benzoylation, cleavage of the tet-
rahydrofuranyl moiety occurred in the course of this
reaction. The benzoylation of the guanine molecule at po-
sition 1 has not been previously described in literature.
Recently, a similar pterin derivative, 2-(N,N-dimethylim-
inoformamido)-6-formyl-3-(o-toluoyl)pterin, was synthe-
sized from 6-formylpterin in a three-component reaction
[o-TolCl, DIPEA, DMF].15 When we attempted to apply
these reaction conditions to substrate 5 the only isolated
product was derivative 10. Evidently, in this case the re-
action failed to go to completion and stopped at the second
step, i.e. the interaction of the Vilsmeyer-type activated
complex with the amino group.15 This was possibly
brought about by the extremely poor solubility of interme-
diate 10 in the reaction mixture. The same reason may
also be responsible for the slow benzoylation of 5 to 13;
about 72 hours were necessary to complete this conver-
sion.
O
O
Tol
Tol
S
S
O
N
O
N
O
N
O
N
a
b
N
N
2
Br
+
1
Br
74%
N
HN
N
H
HN
O
14
O
15
22%
O
Scheme 3 Reagents and conditions: (a) TsCl, Et3N, DMAP,
CH2Cl2, r.t., 24–48 h; (b) EtOH, reflux, 20 min.
However, when intermediate 14 was treated with boiling
ethanol, in order to prepare the target 2-acetylamino-
8-bromo-6-[(4-methylphenyl)sulfonyloxy]-9(7)H-purine
(15), mostly starting material 1 was recovered, with deriv-
ative 15 only produced as a side product of the process.
It is well documented that O6-sulfonates can be useful
synthetic intermediates for the transformation of gua-
nosine derivatives. These compounds, being particularly
susceptible to nucleophilic substitution at C-6, have found
Finally, to broaden the scope of our methodology, we ap-
plied it to the synthesis of purine derivative 1, benzylated
Synthesis 2007, No. 9, 1325–1332 © Thieme Stuttgart · New York

1328
M. Madre et al.
PAPER
O
N
O
N
N
N
N
a
Ph
N
b
Ph
N
Br
Br
10
68%
81%
N
H
N
N
N
N
O
16
17
Scheme 4 Reagents and conditions: (a) BnBr, K2CO3, DMF, r.t., 36 h; (b) EtOH, PTSA, reflux, 20 min.
in the pyrimidine part of the molecule. The selection of bond could therefore be excluded. The 1H NMR spectra of
the benzyl group was motivated by its possible use for 11 and 12 were also very similar to that of the previously
site-modification of the purine cycle, as well as for tempo- described 9-(2-acetoxyethoxymethyl)-2-formylamino-6-
rary protection of the 1,6-lactam function. Furthermore, it [(4-methylphenyl)sulfonyloxy]purine, for which the
was of interest to evaluate the influence of the benzyl structure was unambiguously assigned using X-ray crys-
group on the N9–C bond stability in compound 2.
tallographic analysis.12 The multiplet at d = 7.5–7.8 ppm
in the spectrum of 13, corresponded to the protons of the
benzoyl group introduced into the molecule. Furthermore,
a notable upfield shift of the singlet signal from one NCH3
group of 13, in comparison with the corresponding singlet
for the starting material 10, was also a distinguishing fea-
ture of the 1H NMR spectrum. As the proton spectrum of
13 gave little information about the benzoylation site, X-
ray crystallographic analysis was used for this purpose
(Figure 1).
Initial attempts at treating nucleobase 2 with benzyl bro-
mide, in the presence of potassium carbonate, resulted in
a complex mixture of compounds. The only isolated prod-
ucts were 2-acetylamino-7-benzyl- and 2-acetylamino-9-
benzyl-8-bromo-6-oxopurine,3 which indicated that dur-
ing this reaction the benzylation may be accompanied by
the loss of the tetrahydrofuranyl residue. On the other
hand, the interaction of 2-(N,N-dimethyliminoformami-
do)purine 10 with benzyl bromide proceeded smoothly
with the formation of a single product to which the struc-
ture of 1-benzyl-8-bromo-2-(N,N-dimethyliminoform-
amido)-6-oxo-9-(tetrahydrofuran-2-yl)purine (16) was
assigned (Scheme 4).
The result obtained was in accordance with the literature
data on the formation of an analogous 1-substituted deriv-
ative in the benzylation of 2¢-deoxyguanosine bearing
amidine-type protection at N2.18 Intermediate 16 turned
out to be stable in boiling ethanol and, in order to remove
the tetrahydrofuranyl residue and prepare the target
1-benzyl-8-bromo-2-(N,N-dimethyliminoformamido)-6-
oxo-9(7)H-purine (17), a catalytic amount of 4-toluene-
sulphonic acid had to be added to the ethanolic solution.
The structures of all the synthesized products were veri-
1
fied by their H NMR spectra. The presence of the DPC
function in compounds 3, 4, 8, 9, 11 and 12, and of the
benzoyl function in compounds 6–9 were confirmed by
the presence of the corresponding aromatic protons sig-
nals. The chemical shift of the exocyclic NH-group proton
singlet (d = ~10 ppm) in 3, 4, 8 and 9 supported their struc-
tures as O6,N2-disubstituted purine derivatives rather than
N1-substituted.19 The spectra of compounds 11 and 12
contained doublets (d = 9.3 and 11.1 ppm) characteristic
of the formamidine group. It is known that some mono-
substituted formamides can exist in solution as cis and
trans isomers, due to the restricted rotation of the single
C–N bond. In such cases, two sets of signals for the pro-
tons of the amine group and of the formyl group are ob-
Figure 1 X-ray crystal structure of compound 13.
Position 1 was excluded as the tosylation site in product
14 on the basis of the established fact that the presence of
a substituent at position 1 in 2-acetylamino-6-oxopurines
causes significant shift of the signal arising from the pro-
tons of the acetylamino function.19 Such changes were not
observed in the spectrum of 14. The benzyl group in com-
pound 16 was characterized by a multiplet at d = ~7.2 ppm
and by a two-proton singlet at d = 5.37 ppm. The benzyla-
tion site in product 16 was proven by NOE measurements;
irradiation of the protons at C-10 showed a strong nuclear
Overhauser effect with the proton at C-12 as well with the
protons of the NCH3 group (Figure 2).
1
served in the H NMR spectra, with minor coupling
constant (~0–3 Hz) for the cis NH–CH protons and major
coupling constant (~10–13 Hz) for the corresponding
trans protons.20 Since only one set of signals, with cou-
pling constants of ~9.5 Hz, were present in the spectra of
derivatives 11 and 12, a restricted rotation of the amide
Synthesis 2007, No. 9, 1325–1332 © Thieme Stuttgart · New York

PAPER
Modification of 8-Bromoguanine via Its N9-Tetrahydrofuranyl Derivative
1329
14
pressure and the residue was purified by flash chromatography on
silica gel (CH2Cl2–EtOH, 100:1.5) to afford chromatographically
and spectrally pure 3 (1.47 g, 78.2%).
1H NMR (200 MHz, DMSO-d6): d = 1.85–2.10 (s, 1 H, H-4¢), 2.15
(s, 3 H, CH3), 2.30–2.54 (m, 1 H, H-3¢), 2.55–2.73 (m, 1 H, H-4¢),
2.74–2.93 (m, 1 H, H-3¢), 3.84–3.98 (m, 1 H, H-5¢), 4.21–4.34 (m,
1 H, H-5¢), 6.18–6.28 (m, 1 H, H-2¢), 7.20–7.78 (m, 10 H, ArH),
10.79 (br s, 1 H, NH).
13
12
15
16
O
6
11
10
7
N
18
N1
5
19 N
Br
8
4
3
N
2
9N
2'
N
17
O1'
3'
5'
4'
2-Acetylamino-8-bromo-6-(N,N-diphenylcarbamoyloxy)-
9(7)H-purine (4)
Figure 2 NOE interactions in compound 16.
Compound 3 (1.45 g, 2.7 mmol) was dissolved in warm EtOH (35
mL) and heated at reflux temperature. Within 5 min, a precipitate
began to separate. After heating for an additional 15 min, the reac-
tion mixture was allowed to cool to r.t. The precipitated solid was
collected by filtration, washed with EtOH and dried in air to afford
4.
The loss of the tetrahydrofuranyl residue in products 4, 7,
9, 12, 13, 15 and 17 was confirmed by the disappearance
of all the corresponding proton signals and the appearance
of the N9-H and/or N7-H proton singlets at d = 13–14 ppm
in their 1H NMR spectra. 13C NMR spectra of nucleobases
4, 7, 9, 12, 13, 15 and 17 turned out to be unsuitable for
the characterization of these compounds. The aromatic
purine carbon atom signals were very broad, evidently
due to the equilibrium between the N(7)H and N(9)H tau-
tomeric forms in solution. The purity of all the compounds
synthesized, except for the rather unstable intermediates
3, 6, 8, 11 and 14, were confirmed by elemental analysis.
Yield: 1.19 g (93.7%); mp 249–251 °C.
1H NMR (200 MHz, DMSO-d6): d = 2.13 (s, 3 H, CH3), 7.25–7.53
(m, 10 H, ArH), 10.69 (s, 1 H, NH), 14.39 (br s, 1 H, NH).
Anal. Calcd for C20H15BrN6O3: C, 51.41; H, 3.24; N, 17.98. Found:
C, 51.47; H, 3.10; N, 17.65.
2-Benzoylamino-8-bromo-6-oxo-9-(tetrahydrofuran-2-yl)pu-
rine (6)
A suspension of 5 (0.6 g, 2.0 mmol) in a mixture of CH2Cl2 (40 mL)
and pyridine (8 mL) was cooled to 0 °C in an ice-bath and TMSCl
(1.5 mL, 11.8 mmol) was added, dropwise, over 2 min with stirring.
The reaction was allowed to come to r.t. over 30 min, during which
a clear solution was obtained. The flask was cooled again to 0–5 °C
and BzCl (0.26 mL, 2.2 mmol) was added. The reaction mixture
was stirred for 4 h at r.t., diluted with CH2Cl2 (20 mL) and poured
into 5% aq NaHCO3 (40 mL). The organic layer was separated,
washed with 5% aq NaHCO3 (20 mL), H2O (2 × 20 mL), brine (20
mL) and dried over MgSO4. The solvent was removed under re-
duced pressure to give a thick oil that was purified by flash chroma-
tography on silica gel (CH2Cl2–EtOH, 100:2) to afford
chromatographically and spectrally pure 6 (0.67 g, 82.7%).
In conclusion, we have developed an efficient method for
the modification of 8-bromoguanine, using its 9-(tetrahy-
drofuran-2-yl) derivative as the key intermediate in the
process. The method enables the introduction of several
protecting groups into the guanine molecule and, there-
fore, allows access to a series of new nucleobases suitable
for further chemical transformations. Furthermore, we be-
lieve that the elaborated procedure is general enough to be
successfully used for the preparation of more novel purine
derivatives.
1H NMR (200 MHz, DMSO-d6): d = 1.85–2.07 (m, 1 H, H-4¢),
2.31–2.51 (m, 1 H, H-3¢), 2.53–2.71 (m, 1 H, H-4¢), 2.72–2.90 (m,
1 H, H-3¢), 3.82–3.96 (m, 1 H, H-5¢), 4.18–4.32 (m, 1 H, H-5¢),
6.12–6.22 (m, 1 H, H-2¢), 7.43–7.72 (m, 3 H, ArH), 7.90–8.00 (m,
2 H, ArH).
All reagents were purchased from Acros Organics and used from
freshly opened containers without any further purification. Reac-
tions involving air- or moisture-sensitive reagents were carried out
in anhydrous solvents under anhydrous argon. Merck TLC silica gel
60 F254 plates were used for TLC analyses and the products were
visualized by UV irradiation. Column chromatography was carried
out with Merck silica gel 60, particle size 0.040–0.063 mm. Melting
points were determined on a Boetius table and are uncorrected. El-
emental analyses were carried out on a Carlo Erba Elemental Ana-
lyzer. NMR spectra were taken on Varian Mercury 200 and Varian
Mercury 400 spectrometers with HMDS as internal standard
(d = 0.055 ppm). The X-ray structure determination was performed
on a Bruker-Nonius Kappa CCD automated diffractometer. Crys-
tals of compound 13 were obtained by crystallization from ethanol.
2-Benzoylamino-8-bromo-6-oxo-9(7)H-purine (7)
Compound 6 (0.40 g, 1.0 mmol) was dissolved in warm EtOH (10
mL) and heated at reflux temperature for 15 min. The reaction mix-
ture was cooled in a refrigerator and the precipitated solid was col-
lected by filtration, washed with EtOH and dried in air to afford 7.
Yield: 0.29 g (86.8%); mp >280 °C (decomp.).
1H NMR (200 MHz, DMSO-d6): d = 7.46–7.70 (m, 3 H, ArH),
7.97–8.06 (m, 2 H, ArH), 11.97 (s, 1 H, NH), 12.34 (s, 1 H, NH),
13.92 (br s, 1 H, NH).
2-Acetylamino-8-bromo-6-(N,N-diphenylcarbamoyloxy)-9-(tet-
rahydrofuran-2-yl)purine (3)
Anal. Calcd for C12H8BrN5O2: C, 43.14; H, 2.41; N, 20.96. Found:
C, 43.28; H, 2.30; N, 20.76.
To a suspension of 2 (1.20 g, 3.5 mmol) in pyridine (18.0 mL) were
added DIPEA (1.3 mL, 7.5 mmol) and DPC-Cl (0.93 g, 4.00 mmol).
The reaction mixture was stirred for 2.5 h at r.t. whereupon a dark
red solution was formed. H2O (1.5 mL) was added and the mixture
was stirred for another 20 min. All volatiles were removed under re-
duced pressure and the oily residue obtained was co-evaporated
several times with toluene to remove the traces of pyridine and then
taken up in EtOAc (100 mL). The solution was washed with 5% aq
NaHCO3 (30 mL), H2O (3 × 30 mL) and brine (30 mL), dried
(MgSO4) and filtered. The solvent was removed under reduced
2-Benzoylamino-8-bromo-6-(N,N-diphenylcarbamoyloxy)-9-
(tetrahydrofuran-2-yl)-purine (8)
To a suspension of 6 (0.34 g, 0.8 mmol) in pyridine (4 mL) were
added DIPEA (0.3 mL; 1.7 mmol) and DPC-Cl (0.21 g; 0.9 mmol).
The reaction mixture was stirred for 4 h at r.t. whereupon a dark red
solution formed. H2O (0.5 mL) was added and the mixture was
stirred for another 20 min. All volatiles were removed under re-
duced pressure and, after several co-evaporations with toluene to re-
Synthesis 2007, No. 9, 1325–1332 © Thieme Stuttgart · New York

1330
M. Madre et al.
PAPER
8-Bromo-6-(N,N-diphenylcarbamoyloxy)-2-formylamino-
move the traces of pyridine, an oily residue was obtained. The
residue was purified by flash chromatography on silica gel
(CH2Cl2–EtOH, 100:1.5) to afford chromatographically and spec-
trally pure 8 (0.37 g, 77.1%).
1H NMR (200 MHz, DMSO-d6): d = 1.87–2.07 (m, 1 H, H-4¢),
2.31–2.51 (m, 1 H, H-3¢), 2.53–2.71 (m, 1 H, H-4¢), 2.72–2.90 (m,
1 H, H-3¢), 3.84–3.98 (m, 1 H, H-5¢), 4.20–4.34 (m, 1 H, H-5¢),
6.20–6.29 (m, 1 H, H-2¢), 7.25–7.65 (m, 13 H, ArH), 7.89–8.00 (m,
2 H, ArH), 11.17 (s, 1 H, NH).
9(7)H-purine (12)
Compound 11 (0.24 g, 0.53 mol) was dissolved in EtOH (10 mL)
and heated at reflux temperature for 20 min. The solution was
cooled in a refrigerator and the precipitated solid was collected by
filtration, recrystallized from EtOH and dried in air to afford 12.
Yield: 0.21 g (87.5%); mp 264–265 °C.
1H NMR (200 MHz, DMSO-d6): d = 7.23–7.64 (m, 10 H, ArH),
9.27 (d, J = 9.4 Hz, 1 H, CH), 11.08 (d, J = 9.4 Hz, 1 H, NH), 14.37
(br s, 1 H, NH).
2-Benzoylamino-8-bromo-6-(N,N-diphenylcarbamoyloxy)-
9(7)H-purine (9)
Compound 8 (0.28 g, 0.47 mmol) was dissolved in a minimal
amount of boiling EtOH. The solution was cooled in a refrigerator
and the precipitated solid was collected by filtration, recrystallized
from EtOH and dried in air to afford 9.
Anal. Calcd for C19H13BrN6O3·0.5H2O: C, 49.37; H, 3.05; N, 18.18.
Found: C, 49.39; H, 3.03; N, 17.95.
1-Benzoyl-8-bromo-2-(N,N-dimethyliminoformamido)-6-oxo-
9(7)H-purine (13)
To a suspension of 10 (1.24 g, 3.5 mmol) in MeCN (30 mL) were
added pyridine (3.0 mL, 3.9 mmol), DMAP (0.043 g, 0.035 mmol)
and BzCl (1.23 g, 1.0 mL, 8.75 mmol) in a dropwise manner. The
reaction mixture was stirred at r.t. until a clear solution was obtained
and TLC indicated the disappearance of the starting material (~72
h). The reaction mixture was diluted with EtOH (20 mL) and stir-
ring was continued for another 30 min. All volatiles were removed
under reduced pressure and, after several co-evaporations with tol-
uene, a thick yellow oil was obtained. The oil was dissolved in
CHCl3 (100 mL) and any precipitate was removed by filtration. The
filtrate was concentrated and the residue was purified by flash chro-
matography on silica gel (CHCl3–EtOH, 40:1) to afford chromato-
graphically and spectrally pure 9 (0.98 g, 72.1%). An analytical
sample was recrystallized from EtOH and a crystal was submitted
for X-ray crystallographic analysis (Figure 1 and Table 1).
Yield: 0.19 g (79.2%); mp 215–217 °C.
1H NMR (200 MHz, DMSO-d6): d = 7.26–7.64 (m, 13 H, ArH),
7.91–7.99 (m, 2 H, ArH), 11.09 (s, 1 H, NH), 14.46 (br s, 1 H, NH).
Anal. Calcd for C25H17BrN6O3: C, 56.73; H, 3.24; N, 15.88. Found:
C, 56.45; H, 3.06; N, 15.51.
8-Bromo-2-(N,N-dimethyliminoformamido)-6-oxo-9-(tetrahy-
drofuran-2-yl)purine (10)
Compound 5 (1.20 g, 4.0 mmol) was dissolved in DMF (20 mL)
with slight heating, then the solution was allowed to cool to r.t. and
N,N-dimethylformamide dimethylacetal (2.38 g, 20.0 mmol) was
added in one portion. The reaction mixture was stirred at r.t. and,
within 10 min, a white solid began to separate. Stirring was contin-
ued for 4 h then the separated solid was collected by filtration,
washed with EtOH and dried in air to afford 10 (1.16 g, 81.7%). The
product was used for the next step without further purification. An
analytical sample was recrystallized (EtOH–H2O, 3:1) for analysis.
Mp >165 °C (decomp.).
1H NMR (200 MHz, DMSO-d6): d = 2.59 (s, 3 H, CH3), 3.05 (s,
3 H, CH3), 7.51–7.83 (m, 5 H, ArH), 8.49 (s, 1 H, CH), 13.57 (s,
1 H, NH), 14.04 (s, 1 H, NH).
Mp 220–223 °C.
1H NMR (200 MHz, DMSO-d6): d = 1.92–2.13 (m, 1 H, H-4¢),
2.27–2.48 (m, 2 H, H-3¢ and H-4¢), 2.36 (s, 3 H, CH3), 2.63–2.84
(m, 1 H, H-3¢), 3.04 (s, 3 H, CH3), 3.15 (s, 3 H, CH3), 3.84–3.97 (m,
1 H, H-5¢), 4.09–4.24 (m, 1 H, H-5¢), 6.09–6.20 (m, 1 H, H-2¢), 8.51
(s, 1 H, CH), 11.49 (s, 1 H, NH).
Anal. Calcd for C15H13BrN6O2·0.5C2H5OH: C, 45.73; H, 3.84; N,
20.00. Found: C, 46.00; H, 3.67; N, 19.92.
2-Acetylamino-8-bromo-6-[(4-methylphenyl)sulfonyloxy]-9-
(tetrahydrofuran-2-yl)purine (14)
To a suspension of 2 (0.34 g, 1.0 mmol) in CH2Cl2 (20 mL) were
added Et3N (0.52 mL, 3.7 mmol), DMAP (0.031 g, 0.26 mmol) and
TsCl (0.50 g, 2.6 mmol). The reaction mixture was stirred at r.t. un-
til a clear solution was obtained and TLC indicated the disappear-
ance of starting material (24–48 h). The reaction mixture was
diluted with CH2Cl2 (80 mL) and washed with 5% aq NaHCO3 (20
mL), H2O (20 mL) and brine (20 mL), dried (MgSO4) and filtered.
The solvent was removed under reduced pressure to give a thick oil
that was purified by flash chromatography on silica gel (CH2Cl2–
EtOH, 100:2.5) to afford chromatographically and spectrally pure
10 (0.37 g, 74.0%).
1H NMR (200 MHz, DMSO-d6): d = 1.87–2.08 (m, 1 H, H-4¢), 2.16
(s, 3 H, CH3), 2.44 (s, 3 H, CH3), 2.31–2.67 (m, 2 H, H-3¢ and H-4¢),
2.69–2.87 (m, 1 H, H-3¢), 3.84–3.97 (m, 1 H, H-5¢), 4.17–4.32 (m,
1 H, H-5¢), 6.17–6.25 (m, 1 H, H-2¢), 7.46–7.55 (m, 2 H, ArH),
8.10–8.18 (m, 2 H, ArH), 10.72 (s, 1 H, NH).
Anal. Calcd for C12H15BrN6O2: C, 40.58; H, 4.26; N, 23.66. Found:
C, 40.39; H, 4.12; N, 23.61.
8-Bromo-6-(N,N-diphenylcarbamoyloxy)-2-formylamino-9-
(tetrahydrofuran-2-yl)purine (11)
To a suspension of 10 (0.36 g, 1.0 mmol) in pyridine (6 mL) were
added DIPEA (0.40 mL, 2.3 mmol) and DPC-Cl (0.25 g, 1.1 mmol).
The reaction mixture was stirred for 6 h at r.t. to obtain a dark red
mixture. All volatiles were removed under reduced pressure and the
oily residue was co-evaporated several times with toluene to re-
move the traces of pyridine and taken up in CH2Cl2 (50 mL). The
CH2Cl2 solution was washed with 5% aq. NaHCO3 (15 mL), H2O
(2 × 15 mL) and brine (15 mL), dried (MgSO4) and filtered. The
solvent was removed under reduced pressure and the residue was
purified by flash chromatography on silica gel (CH2Cl2–EtOH,
50:1) to afford chromatographically and spectrally pure 11 (0.28 g,
78.9%).
2-Acetylamino-8-bromo-6-[(4-methylphenyl)sulfonyloxy]-
9(7)H-purine (15)
1H NMR (200 MHz, DMSO-d6): d = 1.90–2.12 (m, 1 H, CH-4¢),
2.31–2.48 (m, 2 H, CH-4¢ and CH-3¢), 2.69–2.88 (m, 1 H, CH-3¢),
3.85–3.99 (m, 1 H, CH-5¢), 4.08–4.23 (m, 1 H, CH-5¢), 6.19–6.28
(m, 1 H, NCH-2¢), 7.03–7.53 (m, 10 H, ArH), 9.29 (d, J = 9.7 Hz,
1 H, CH), 11.20 (d, J = 9.7 Hz, 1 H, NH).
Compound 14 (0.15 g, 0.30 mmol) was dissolved in EtOH (10 mL)
and heated at reflux temperature for 20 min. The solution was
cooled in a refrigerator and the precipitated solid, consisting mostly
of 1, was removed by filtration. The filtrate was evaporated under
reduced pressure and the residue was purified by flash chromatog-
raphy on silica gel (CHCl3–EtOH, 40:0.5) to afford 15.
Synthesis 2007, No. 9, 1325–1332 © Thieme Stuttgart · New York

PAPER
Modification of 8-Bromoguanine via Its N9-Tetrahydrofuranyl Derivative
1331
1-Benzyl-8-bromo-2-(N,N-dimethyliminoformamido)-6-oxo-9-
(tetrahydrofuran-2-yl)purine (16)
Table 1 Crystallographic Data of Compound 13
To a suspension of 10 (0.43 g, 1.2 mmol) in DMF (15 mL) were
added K2CO3 (0.26 g, 1.9 mmol) and BnBr (0.22 g, 0.16 mL, 1.3
mmol). The reaction mixture was stirred for 24 h at r.t. then an ad-
ditional amount of BnBr (0.05 g, 0.04 mL, 0.03 mmol) was added
and the stirring was continued overnight. The reaction mixture was
filtered and the filtrate was evaporated under reduced pressure to
give a residue that was purified by flash chromatography on silica
gel (CHCl3) and recrystallized from EtOH to afford 16.
Empirical formula
Molecular weight
Crystal form
Crystal size (mm)
Crystal color
Crystal system
a (Å)
C15H17BrN6O4
425.243
Prism
0.26 × 0.23 × 0.14
Colorless
Monoclinic
14.4399(7)
14.9685(7)
18.7679(13)
104.063(2)
1869.08(5)
P 21/c
Yield: 0.36 g (67.9%); mp 168–170 °C.
1H NMR (200 MHz, DMSO-d6): d = 1.95–2.11 (m, 1 H, CH-4¢),
2.30–2.45 (m, 2 H, CH-4¢ and CH-3¢), 2.64–2.87 (m, 1 H, CH-3¢),
3.02 (s, 3 H, CH3), 3.15 (s, 3 H, CH3), 3.85–3.95 (m, 1 H, CH-5¢),
4.11–4.22 (m, 1 H, CH-5¢), 5.37 (s, 2 H, NCH2), 6.12–6.18 (m, 1 H,
CH-2¢), 7.18–7.31 (m, 5 H, ArH).
13C NMR (400 MHz, DMSO-d6): d = 25.96 (C-4¢), 29.86 (C-3¢),
35.46 (N-CH3), 41.17 (N-CH3), 45.11 (C-10), 69.79 (C-5¢), 86.78
(C-2¢), 119.72 (C-5), 122.57 (C-8), 127.21 (C-14), 127.83 (C-12),
128.58 (C-13), 138.81 (C-11), 149.36 (C-4), 156.61 (C-6), 156.69
(C-2), 158.33 (C-17).
b (Å)
c (Å)
b (°)
V (Å3)
Space group
Z
4
1-Benzyl-8-bromo-2-(N,N-dimethyliminoformamido)-6-oxo-
9(7)H-purine (17)
Compound 16 (0.095 g, 0.2 mmol) was dissolved in EtOH (7 mL)
and several crystals of PTSA were added. The reaction mixture was
heated at reflux for 20 min then cooled in a refrigerator. The precip-
itated solid was collected by filtration and washed with EtOH to af-
ford 17.
F(000)
1728
µ (mm–1)
2.121
Density (calc.) (g/cm3)
2qmax for data (°)
Diffractometer
1.436
Yield: 0.065 g (81.3%); mp 243–245 °C.
50.0
1H NMR (200 MHz, DMSO-d6): d = 2.99 (s, 3 H, CH3), 3.14 (s,
3 H, CH3), 5.38 (s, 2 H, NCH2), 7.12–7.33 (m, 5 H, ArH), 8.50 (s,
1 H, CH), 13.32 (br s, 1 H, NH), 13.80 (br s, 1 H, NH).
Nonius Kappa CCD
ϕ and ω scans
–15 ≤ h ≤ 16
–17 ≤ k ≤ 17
–22 ≤ l ≤ 22
9386
Methods of collection
Index ranges
Anal. Calcd for C15H15BrN6O: C, 48.02; H, 4.03; N, 22.40. Found:
C, 47.95; H, 3.85; N, 22.42.
References
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Reflections collected
Independent reflections
Reflections with I > 2σ(I)
Method of solution
6482 (Rint = 0.055)
3553
(3) Madre, M.; Panchenko, N.; Zhuk, R.; Geenevasen, J. A.; van
den Burg, A.; Koomen, G.-J. Synthesis 1999, 775.
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SIR97 [1]
SHELXL97 [2]
469
Method of structure refinement
Number of refined parameters
(D/σ)max
0.001
Final R-factor
0.0794
CCDC deposition number
297189
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Yield: 0.029g (22.3%); mp 176–177 °C.
1972, 2937.
1H NMR (200 MHz, DMSO-d6): d = 2.15 (s, 3 H, CH3), 2.43 (s,
3 H, CH3), 7.48 (d, J = 8.3 Hz, 2 H, ArH), 8.14 (d, J = 8.3 Hz, 2 H,
ArH), 10.66 (s, 1 H, CH), 14.46 (br s, 1 H, NH).
(8) Fan, Y.; Gaffney, B. L.; Jones, R. A. Org. Lett. 2004, 6,
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Sproat, B. J. Chem. Soc., Perkin Trans. 1 1997, 2779.
Anal. Calcd for C14H12BrN5O4S: C, 39.45; H, 2.84; N, 16.43.
Found: C, 39.81; H, 2.89; N, 16.08.
Synthesis 2007, No. 9, 1325–1332 © Thieme Stuttgart · New York

1332
M. Madre et al.
PAPER
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Synthesis 2007, No. 9, 1325–1332 © Thieme Stuttgart · New York