Syntheses of 2′-deoxy-2′-fluoro-β-d-arabinofuranosyl purine nucleosides via selective glycosylation reactions of potassium salts of purine derivatives with the glycosyl bromide

Article abstract of DOI:10.1016/j.tetlet.2015.11.091

Syntheses of 9-(2-deoxy-2-fluoro-β-d-arabinofuranosyl)-guanine (1) and -adenine (2) were accomplished from readily available 1,3,5-tri-O-benzoyl-2-deoxy-2-fluoro-α-d-arabinofuranose (3). A new and efficient approach for the synthesis of 1-α-bromide was developed using the mild bromination of α-1-O-benzoate (3). Selective coupling reactions of the bromosugar with purine potassium salts followed by derivatization/and or deprotection of the intermediate blocked 2′-fluoro β-arabinonucleosides resulted in formation of the target compounds with high overall yields.

Full text of DOI:10.1016/j.tetlet.2015.11.091

Tetrahedron Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Syntheses of 20-deoxy-20-fluoro-b-D-arabinofuranosyl purine
nucleosides via selective glycosylation reactions of potassium salts
of purine derivatives with the glycosyl bromide
Grigorii G. Sivets
Institute of Bioorganic Chemistry, The National Academy of Sciences of Belarus, 5 Acad. Kuprevicha, Minsk 220141, Belarus
a r t i c l e i n f o
a b s t r a c t
Article history:
Syntheses of 9-(2-deoxy-2-fluoro-b-
D
-arabinofuranosyl)-guanine (1) and -adenine (2) were accomplished
-arabinofuranose (3). A new and efficient
-bromide was developed using the mild bromination of -1-O-benzoate
Received 17 June 2015
Revised 20 November 2015
Accepted 28 November 2015
Available online xxxx
from readily available 1,3,5-tri-O-benzoyl-2-deoxy-2-fluoro-
approach for the synthesis of 1-
a-D
a
a
(3). Selective coupling reactions of the bromosugar with purine potassium salts followed by derivatiza-
tion/and or deprotection of the intermediate blocked 20-fluoro b-arabinonucleosides resulted in forma-
tion of the target compounds with high overall yields.
Keywords:
Bromosugar
Ó 2015 Elsevier Ltd. All rights reserved.
Purines
Potassium tert-butoxide
Nucleobase anion glycosylation
20-Fluoro arabinonucleosides
Purine nucleosides with 20-ara-fluorine substitution are of
special interest because the location of the fluorine atom in the
b-orientation of the carbohydrate moiety increases their metabolic
stability to degradation by purine nucleoside phosphorylase, pro-
vides chemical stability of the glycosidic bond under acidic condi-
tions and has an impact on the antiviral and anticancer activities of
agents in coupling reactions with various purine bases;1,2,8–12 (b)
chemo-enzymatic approaches including the synthesis of 2-deoxy-
2-fluoro-D-arabinofuranosyl-a-1-phosphate and enzymatic glyco-
sylation reactions by nucleoside phorsphorylases;13,14 (c) direct
introduction of the C20-b-fluorine atom via nucleophilic displace-
ment of an activated C20-
a-hydroxyl in selectively protected
fluorinated nucleoside analogues. 9-(20-Deoxy-20-fluoro-b-
D-arabi-
ribonucleoside using DAST.15–17
nofuranosyl)guanine (1, 20-F-araG) was found to display anticancer
activity against human leukaemic T-cell lines,1,2 and its adenine
analogue (2, 20-F-araA) (Fig. 1) has recently been shown to be more
active against the protozoan parasite Trichomonas vaginalis3 than
metronidazole, the current drug for treatment of trichomoniasis.
Various routes to synthesise 20-deoxy-20-fluoro-b-
D-arabinofu-
ranosyl nucleosides of the natural purines have been explored.
Most of the known methods comprise of the coupling of glycosyl
bromides with purine derivatives and further transformations of
the intermediate protected N9-b-glycosides to guanine or adenine
20-F-araN.1,2,11,18 The major synthetic challenge to be solved for the
development of efficient chemical approaches to purine 20-F-araNs,
in contrast to pyrimidine analogues,19 is the stereo- and regio-
selective formation of a N-b-glycosidic bond in 20-b-fluoro nucleo-
sides during the glycosylation reaction.
It is also worth noting that 20-deoxy-20-fluoro-b-
D-arabinofura-
nosyl nucleosides (20-F-araNs) are key materials for the prepara-
tion of building blocks for the synthesis of arabinose-derived
oligonucleotides which exhibit very promising gene silencing
properties. 20-Deoxy-20-fluoro- -arabinonucleic acids (2F0-ANA)
D
can improve the activity of small interfering RNA and possess the
In a continuation of our interest in the synthetic methodology
of 20-fluorosubstituted nucleosides, this letter deals with new
syntheses of the purine 20-F-araNs 1–2 via selective anion
glycosylation of the purine potassium salt with a bromosugar
as the key step starting from available 2-deoxy-2-fluoro-1,3,5-tri-
enhanced stability of duplexes (2F0-ANA/RNA).4–7
The synthetic or chemo-enzymatic routes towards purine 20-b-
fluoro nucleosides have been developed using three main
approaches: (a) convergent syntheses utilising 1-
a-bromo, -O-acyl
derivatives of 2-deoxy-2-fluoro-D-arabinofuranose as glycosylating
O-benzoyl-a-D-arabinofuranose (3).
Firstly, an alternative method to the known procedure to pre-
pare bromide 4 from 3 using HBr/AcOH19 was studied. Bromination
of the a
-1-O-benzoate (3) with TMSBr20 in anhydrous CH2Cl2 in the
E-mail address: gsivets@mail.ru
http://dx.doi.org/10.1016/j.tetlet.2015.11.091
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Sivets, G. G. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.11.091

2
G. G. Sivets / Tetrahedron Letters xxx (2015) xxx–xxx
Table 1
O
N
NH2
Reactions of K-salt of 6-chloro-2-aminopurine with the bromosugar 4
N
N
N
N
N
NH
NH2
a
Entry
Solvent
Ratio (b:
a)
5/6
Yieldb (%)
N
HO
HO
O
O
F
1
2
MeCN
MeCN/DCE
9.2:1
14:1
61
71
F
1
2
OH
OH
a
Determined by 1H NMR spectroscopy of the crude product.
Isolated yield of protected purine N9-b-nucleoside.
b
Figure 1. Structures of biologically active purine 20-b-fluoro nucleosides.
chromatography on silica gel (b/a ratio – 9.2/1). The heterogeneous
nucleobase anion glycosylation reaction in a mixture of acetoni-
trile/1,2-dichloroethane (DCE) (entry 2, Table 1) at rt provided a
higher yield of 5 (71%) after purification with better b/a selectivity
(14/1) compared to previous conditions (entry 1) in acetonitrile
and the known synthetic routes described for 5.9,11,18 The high
anomeric ratio (5/6) achieved in a mixture of the polar acetonitrile
and DCE of the lower polarity (via a preferred SN2 pathway)
allowed us to separate well the desired b-anomer from a-anomer
by flash chromatography on silica gel.
Furthermore, three methods for the displacement of the 6-
chloro atom of 5 and 7 to give the 6-oxo-functional group were
investigated for the preparation of 20-F-araG (1) (Scheme 2). The
treatment of nucleoside 5 with 5 equiv of anhydrous NaOAc in a
mixture of Ac2O-AcOH gave rise to the acylated derivative of 9-
presence of catalytic ZnBr2, generated the desired 1-a-bromide 4
which was isolated as a colourless oil in 96% yield (Scheme 1).
An efficient method developed for synthesis of the glycosyl bro-
mide 4 from perbenzoylated 2-deoxy-2-fluoro-D-arabinofuranose
3 under mild conditions resulted in a single bromo anomer in high
yield as in the case of the bromination method19 with 30% HBr in
acetic acid. The explored method will be of use in preparing 1-
bromo derivatives from different acylated fluorodeoxy sugars.
To improve the known synthetic methods for the preparation of
1
1,2,10,11, the synthesis of the purine analogue 59,18 from 1-
a-bro-
mide 4 (Scheme 1) and different conditions for the efficient conver-
sion of the key intermediate to the guanine derivative were
examined. Previous investigations on the glycosylations of modi-
fied purine bases with halogenoses have provided evidence for
nucleobase-anion glycosylation12,21 preceding with high anomeric
selectivity and good yields for the b-glycosylation products. The
coupling of potassium salts of halogenated 7-deazapurines or
8-aza-7-deazapurines, generated using powdered KOH12,22–24 in
the presence of tris[2-(2-methoxy)ethyl]amine as catalyst
(2-deoxy-2-fluoro-b-D-arabinofuranosyl)guanine 10.
The chemical stability of the glycosidic bond in 5 made it possi-
ble to carry out the conversions of the nucleobase under heating
(120–125 °C, 150 min) resulting in the formation of guanine nucle-
oside 10 in high yield (90–95%). The proposed mechanism for this
transformation involves the generation of intermediate acetylated
product 8 in the first step followed by selective 6-O-deacylation via
formation of adduct 9 which gives the blocked nucleoside 10 after
releasing a molecule of acetic anhydride (Scheme 2, i–iii). The
structure of the latter was confirmed by 1H, 13C, 19F NMR data
and HRMS. It is noteworthy that Mansuri et al.26 have briefly
reported the preparation of 10 through the condensation of bro-
mide 4 with the silylated N2-acetylguanine albeit without detailed
experimental procedures and comprehensive NMR spectral data.
in MeCN, with bromide 4 gave rise to protected b-D-nucleosides
in 50–85% isolated yields. It should be also noted that application
of the solid–liquid phase-transfer glycosylation reactions of
different 7-deazapurine bases resulted in the stereoselective or
stereospecific formation of the b-D-anomers; however the main
drawback of this approach is the formation of 30-debenzoylated
derivatives of N1-b-nucleosides (9–21%)12,23 owing to a large
excess of potassium hydroxide used in the glycosylation step. In
the development of practical synthetic methods to purine
20-F-araNs, Bauta and co-workers have thoroughly studied the con-
densation reactions of halogenated purines with bromide 4 in the
presence of potassium t-butoxide18,25 in various solvents to afford
20-b-fluoro N9-b-nucleosides as main products, with the highest
HO
C
O
C
anomeric ratios (b/a) achieved in mixtures of solvents. The above
results prompted us to employ the potassium salt glycosylation
procedure for the synthesis of 5. The coupling of 4 with the potas-
sium salt of 2-amino-6-chloropurine, produced by the reaction
with potassium t-butoxide in 1,2-dimethoxyethane (DME) at rt,
was performed in anhydrous acetonitrile for 18 h at rt (Table 1).
This reaction resulted in the formation of N9-b-anomer 5 and its
CH3
OAc
O
CH3
O
N
N
N
N
N
N
NHAc
N
NHAc
N
ii
BzO
BzO
O
F
O
F
9
8
OBz
OBz
a-anomer 6 in 61% and 6% yield, respectively, after column
a (i)
iii
Cl
N
O
N
N
N
N
NH
NHAc
BzO
BzO
BzO
O
F
a
O
F
N
NH2
NH2
N
10
O
BzO
HO
BzO
Br
OBz
3
O
F
O
a
F
OBz
OBz
4
5
b
Cl
N
OBz
b
OBz
b
c
Cl
N
N
N
N
N
N
N
NH
NH2
N
NH2
+
BzO
O
F
O
F
N
N
d
HO
O
F
O
F
N
NH2
N
N
6
5
OBz
OBz
1
7
N
OH
OH
Cl
Scheme 2. Reagents and conditions: (a) 5.2 equiv AcONa in Ac2O/AcOH (1:1, v/v),
120–125 °C; (b) saturated NH3/MeOH, rt (1, 70% overall yield from 5); (c) 4.3 equiv
HSCH2CH2OH, 4.2 equiv MeONa in MeOH, reflux, (1, 72%); (b) saturated NH3/MeOH,
rt (7, 75%); (d) ADA, potassium phosphate buffer pH 7.4, rt (1, 87%).
Scheme 1. Reagents and conditions: (a) TMSBr, ZnBr2, CH2Cl2, 0 °C ? rt, 96%; (b) 4,
K-salt of 2-amino-6-chloropurine, generated using t-BuOK in DME, solvent, 18 h, rt
(Table 1, entries 1 and 2).
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G. G. Sivets / Tetrahedron Letters xxx (2015) xxx–xxx
3
AdeR
glycosylation reaction of N6-pivaloyladenine in a mixture of ace-
tonitrile/DCE at rt provided good b/ selectivity (14:1) (entry 2,
Table 2) and high yield of N9-b-nucleoside 11 (61%) along with
its -anomer 12 (5%) after separation by silica gel column
BzO
BzO
a or b
O
F
O
F
a
AdeR
4
+
OBz
OBz
a
c
11 R=Piv
12 R=Piv
chromatography.
13 R=Bz
14 R=Bz
The nucleobase anion glycosylation of N6-benzoyladenine with
4 under the similar reaction conditions afforded a more complex
reaction mixture from which the N9-b-anomer 13 (48–50%) and
NH2
N
d
N
N
O
N
its
on silica gel (b/
glycosylation reactions are the formation of N9-b- and -
a
-anomer 14 (5%) were isolated by column chromatography
ratio – 10.3/1, entry 4). The limitations of studied
-anomers,
HO
F
a
a
2
OH
and a low solubility of the purine salts in the tested solvents that
involve the need for a difficult separation of the isomeric protected
nucleosides and require longer reaction times compared to the
glycosylation methods explored for 7-deazapurines22,23 in the
presence of KOH, respectively. The pure adenine b-nucleosides 11
and 13 were isolated by column chromatography on silica gel.
After crystallisation, the key intermediates were obtained as
crystalline products.
From the above glycosylation reactions of purine derivatives it
was found that the coupling of 4 with the potassium salts of
N6-pivaloyladenine or 2-amino-6-chloropurine proceeded in a
stereoselective manner, primarily via an SN2 pathway, using a
mixture of solvents of different types and polarities (acetonitrile/
DCE) at ambient temperature to afford good isolated yields of
protected b-nucleosides. In addition, the best stereochemical
Scheme 3. Reagents and conditions: (a) 4, K-salt of N6-pivaloyladenine, generated
using t-BuOK in DME, MeCN, CaH2, rt; (b) 4, K-salt of N6-pivaloyladenine or N6-
benzoyladenine, generated using t-BuOK in DME, MeCN/DCE, CaH2, rt; (c) 4, K-salt
of N6-pivaloyladenine or N6-benzoyladenine/THF, reflux (Table 2, entries 1–5); (d)
saturated NH3/MeOH, rt or MeONa/MeOH, heating (2 from 11, 82%; 2 from 13, 80%).
Compound 10 could be used for the deprotection step without
purification.
Removal of the acyl protective groups in 10 with ammonia in
methanol for 48 h yielded arabinoside 1 in 70% yield over two
steps. N9-b-D
-arabinonucleoside 5 was also converted into 20-F-
araG (1) by treating with 2-mercaptoethanol and sodium methox-
ide in refluxing methanol to the target nucleoside 1 in 72% yield
after column chromatography on a column of Silica Gel Woelm
(20% water). The chlorine atom at the 6th position of purine 7
was converted to a hydroxyl group by utilising the hydrolase
activity of adenosine deaminase. Treatment of intermediate 7 with
adenosine deaminase from calf intestinal mucosa (ADA) in a
phosphate buffer at rt afforded nucleoside 1 in 87% yield after
purification on silica gel (Scheme 2). Thus, guanine nucleoside 1
was obtained from halogenose 4 and 2-amino-6-chloropurine in
two or three steps with 46–51% overall yields. The nucleobase
anion glycosylation27 with potassium t-butoxide has also been
utilized in the simultaneous study on the synthesis of purine
outcomes (b/a ratio – 86/1 and 32/1) were achieved by carrying
out the condensation reactions of potassium salts of N6-pivaloyl-
or -benzoyladenine with 4 at reflux in the lower polarity solvent
THF (entries 3 and 5, Table 2), which is not favourable for SN1
mechanism leading to b- and
a-anomeric nucleosides with a low
selectivity via oxonium ion intermediates. b-Anomers 11 and 13
were isolated in moderate yields of 49% and 46%, respectively, after
flash chromatographic purification. The deprotection of 11 or 13
gave arabinoside 2 in 80–82% yields. The guanine and adenine
nucleosides 1–2 were prepared in 36–48% overall yield from 3
using commercially available nucleobases and reagents.
20,30-difluoro-
D-arabinofuranosyl nucleosides, including 2-amino-
In summary, simple and effective routes for the synthesis of
6-chloropurine and guanine derivatives.
purine 20-F-araNs from benzoate 3 were described. A new method
Next, 20-F-araA (2) was prepared by analogy to the study using
2-amino-6-chloropurine (Scheme 3). Heterogeneous reactions of
commercially available N6-benzoyladenine or N6-pivaloy-
for preparation of the 1-
a-bromosugar 4 was developed. It has
been shown that the -1-O-benzoate 3 can be converted to 4 in
a
ladenine28 and 1-
a-bromide 4 were studied. The potassium salt
high yield by treating with TMSBr in the presence of ZnBr2 as
catalyst. The potassium salt glycosylation reactions of 2-amino-6-
chloropurine and N6-acyladenines with the glycosyl bromide pro-
ceeded under mild conditions with high anomeric b-selectivity
providing access to N9-b-glycosylated products as key intermedi-
ates in the synthesis of the target nucleosides. Convenient methods
for the conversions of glycosides of 2-amino-6-chloropurine to the
guanine nucleoside were explored. This synthetic methodology of
of N6-pivaloyladenine was produced by the treatment of the corre-
sponding purine with potassium t-butoxide in DME at 0 °C fol-
lowed by removal of the solvent under reduced pressure. The
glycosylation reaction of the prepared salt of N6-pivaloyladenine
with 4 in anhydrous acetonitrile in the presence of calcium hydride
for 18 h at rt resulted in the formation of b-anomer 11 and its
anomer 12, which were isolated by column chromatography in
54% overall yield (b/ ratio 7.4:1, entry 1, Table 2). Calcium hydride
a-
purine nucleosides with the 20-fluoro-b-
D-arabinofuranosyl moiety
a
will be useful for their practical synthesis and suitable for the
preparation of novel purine 20-fluorinated nucleoside analogues
of biological interest.
was added to reaction mixture as a drying agent to remove trace
amounts of water from the solvents and increase the anomeric
ratio during the glycosylation step.25 The heterogeneous anion
Table 2
Reactions of potassium salts of adenine derivatives with the bromosugar 4 under various conditions according to Scheme 3
a
Entry
K-salt of purine
Conditions
Time (h)
Anomer ratios (b:
a)
of prepared nucleosides
Yieldb (%)
1
2
3
4
5
N6-pivaloyladenine
N6-pivaloyladenine
N6-pivaloyladenine
N6-benzoyladenine
N6-benzoyladenine
MeCN, CaH2, rt
MeCN/DCE, CaH2, rt
THF, 74–75 °C
MeCN/DCE, CaH2, rt
THF, 74–75 °C
19
19
4
24
4
7.4:1 (11/12)
14:1 (11/12)
86:1c (11/12)
10.3:1 (13/14)
32:1c (13/14)
48
61
49
50
46
a
b
c
Determined by 1H NMR spectroscopy of the crude product in CDCl3.
Isolated yield of protected purine N9-b-nucleoside.
Reaction mixture also contained unreacted bromide 4 (45–50% by 1H NMR) after work-up.
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4
G. G. Sivets / Tetrahedron Letters xxx (2015) xxx–xxx
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This work was supported from Belarus State Program of FOI
‘‘Chempharmsynthesis” (Grants 2.19 and 4.20).
Supplementary data
Supplementary data (experimental procedures, characteriza-
tion data) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.tetlet.2015.11.091. These
data include MOL files and InChiKeys of the most important com-
pounds described in this article.
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Please cite this article in press as: Sivets, G. G. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.11.091