Synthesis of purine nucleosides from D -glucuronic acid derivatives and evaluation of their cholinesterase-inhibitory activities

Article abstract of DOI:10.1002/ejoc.201301913

Glucuronolactones were used as precursors for N⁹ and N ⁷ purine nucleosides containing glucuronic acid derivatives in their structures. Acetylated N-benzylglucofuran- and glucopyranuronamides were synthesized in a few steps from glucofuranurono-6,3-lactone. They were converted into the corresponding furanosyl and pyranosyl uronamide-based nucleosides by N-glycosylation with silylated 2-acetamido-6-chloropurine in the presence of trimethylsilyl triflate. The triacetylated bicyclic lactone was coupled itself with the nucleobase to give bicyclic N⁹,N⁷ nucleosides. Tri-O-acetylglucopyranurono-6,1-lactone was used for the first time as a glycosyl donor for N-glycosylation, and led to β-configured N⁹- and N⁷-linked purinylglucuronides under reaction conditions similar to those used with the 1-O-acetyl-substituted glycosyl donors. The cholinesterase inhibitory profiles of the synthetic nucleosides bearing glucuronic acid derivatives as glycons were evaluated, and they showed moderate selective acetylcholinesterase inhibitory activities (K_i = 14.78-50.53 μM). The best inhibition was shown by the furanosyl N ⁹-linked uronamide-based purine nucleoside. The synthesis of furanosyl and pyranosyl N⁹ and N⁷ purine nucleosides containing glucofuranurono-6,3-lactone, N-benzylglucuronamide, and glucuronic acid moieties is reported. Glucuronolactones were used as glycosyl donors or converted into suitable 1-O-acetyl derivatives for purine glycosylation. Some nucleosides showed moderate and selective inhibition of acetylcholinesterase. Copyright

Full text of DOI:10.1002/ejoc.201301913

FULL PAPER
DOI: 10.1002/ejoc.201301913
Synthesis of Purine Nucleosides from -Glucuronic Acid Derivatives and
D
Evaluation of Their Cholinesterase-Inhibitory Activities
Nuno M. Xavier,*[a] Stefan Schwarz,[a,b] Pedro D. Vaz,[a] René Csuk,[b] and
Amélia P. Rauter[a]
Keywords: Nucleosides / Nucleobases / Carbohydrates / Glycosylation / Biological activity / Uronic acids
Glucuronolactones were used as precursors for N9 and N7
purine nucleosides containing glucuronic acid derivatives in
their structures. Acetylated N-benzylglucofuran- and gluco-
pyranuronamides were synthesized in a few steps from gluc-
ofuranurono-6,3-lactone. They were converted into the cor-
responding furanosyl and pyranosyl uronamide-based nu-
cleosides by N-glycosylation with silylated 2-acetamido-6-
chloropurine in the presence of trimethylsilyl triflate. The tri-
acetylated bicyclic lactone was coupled itself with the nu-
cleobase to give bicyclic N9,N7 nucleosides. Tri-O-acetyl-
glucopyranurono-6,1-lactone was used for the first time as a
glycosyl donor for N-glycosylation, and led to β-configured
N9- and N7-linked purinylglucuronides under reaction condi-
tions similar to those used with the 1-O-acetyl-substituted
glycosyl donors. The cholinesterase inhibitory profiles of the
synthetic nucleosides bearing glucuronic acid derivatives as
glycons were evaluated, and they showed moderate selective
acetylcholinesterase inhibitory activities (Ki
50.53 μ ). The best inhibition was shown by the furanosyl
N9-linked uronamide-based purine nucleoside.
=
14.78–
M
been accomplished, syntheses of their core structures or of
synthetic analogues have been reported.[7,8] Synthetic nu-
cleosides bearing uronic acid moieties or their derivatives
are molecules of potential pharmacological interest, bearing
in mind the biological profiles of their naturally-occurring
counterparts. Wolfrom and McWain[9] reported the first
synthesis of nucleosides containing d-glucuronic acid deriv-
atives as glycons, namely the N9 adenine nucleoside of
methyl tri-O-acetyl-d-glucuronate. They achieved the syn-
thesis by coupling methyl 2,3,4-tri-O-acetyl-α-d-glucopyr-
anosyluronate bromide with 6-acetamido-9-chloromercuri-
purine. Treatment of the uronate-based nucleoside with
methanolic ammonia gave the corresponding glucuron-
amide-containing nucleoside. Uronic acid, uronate, or uron-
amide-based nucleosides bearing pyrimidine,[10] uracil,[11]
cytosine,[11] and purine,[12–15] moieties have been synthe-
sized, and some of these compounds were found to have
significant biological activities. Methods for their synthesis
involve the N-glycosylation of silylated nucleobases with
glucuronyl donors such as glycosyl halides, 1-O-acetyl do-
nors,[11b,12] or methyl glycosides,[15] and the oxidation of the
C-5Ј hydroxy group of nucleosides.[11c,16] Among the re-
ported biologically active compounds, 2-acetamido-6-
chloropurine nucleosides containing a bicyclic uronate moi-
ety were found to be cholinesterase (ChE) inhibitors, with
high degrees of inhibition and high selectivities towards
butyrylcholinesterase (BChE).[15] Inhibition of ChEs, which
hydrolyse the neurotransmitter acetylcholine, is one of the
major rational pharmacological strategies for the manage-
ment of Alzheimer’s disease.[17] These results encouraged us
to explore the synthesis of a variety of new purine nucleo-
Introduction
Naturally-occurring nucleosides with uronic acid moie-
ties or their derivatives in their structures are reported to
have a variety of biological properties, namely antibacterial,
antiviral, or antitumor activities.[1] Most of these com-
pounds are pyrimidine nucleosides. Examples include the
cytosine nucleosides Gougerotin and Blasticidin S, which
contain, respectively, a 4-amido-glucuronamide motif and a
2,3-unsaturated 4-amido-uronic acid moiety.[2] Both of
these molecules have broad-spectrum antibacterial activi-
ties, as well as antiviral and antitumor effects. Blasticidin S
is also a fungicide, and it has been used against Pyricularia
oryzae, which causes the serious rice blast disease in Asia.[3]
The total synthesis of these natural compounds has been
reported.[4] Among the few known natural purine nucleo-
sides containing uronic acid moieties are the antibiotics ami-
purimycin and the miharamycins,[5,6] which also effectively
inhibit the growth of Pyricularia oryzae. The distinguishing
feature of these compounds is the presence of higher and
branched-chain uronic acids in their structures. Although
the total synthesis of these complex nucleosides has not yet
[a] Centro de Química e Bioquímica/Departamento de Química e
Bioquímica, Faculdade de Ciências, Universidade de Lisboa,
Edificio C8, 5° Piso,
Campo Grande, 1749-016 Lisboa, Portugal
E-mail: nmxavier@fc.ul.pt
http://webpages.fc.ul.pt/~nmxavier/
[b] Bereich Organische Chemie, Martin-Luther-Universität Halle-
Wittenberg,
Kurt-Mothes-Str. 2, 06120 Halle (Saale), Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301913.
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Synthesis of Purine Nucleosides
sides with a uronic-acid-derived backbone with the aim of ured nucleosides typically show greater biological activities
studying the potential of this family of compounds as anti- than their α-anomers.[18]
cholinesterase agents further.
Hence, benzylation of 1,2-O-isopropylidene-α-d-xylofur-
In this paper, we report the synthesis of N9 and N7 pur- anose (1) was followed by removal of the acetonide func-
ine nucleosides containing glucofuranurono-6,3-lactone, tionality to give known compound 2.[19] Upon acetylation,
N-benzylglucuronoamide, and glucuronic acid moieties, this compound provided α- and β-configured diacetylated
and the assessment of their cholinesterase (ChE) inhibitory derivatives 3α and 3β in 20 and 80% yields, respectively,
activities.
which could be separated by column chromatography.
1-O-Acetylglucuronyl derivatives, including tetra-O-acet-
To further evaluate and compare the biological activities
ylated furanosyl- and pyranosyl-glucuronoamides derived of both N9 and N7 nucleosides, an N-glycosylation method
from glucofuranurono-6,3-lactone, and triacetylated gluco- with 1-O-acetyl donors that would allow the formation of
furanurono-6,3-lactone itself, were used as glycosyl donors N7 nucleosides, which are products of kinetic control, in a
for the microwave-assisted Lewis-acid-promoted coupling reasonable amount relative to the thermodynamic N9 re-
to the silylated nucleobase. Direct N-glucuronidation of gioisomers, was also envisaged. Moreover, N7 nucleosides
glucopyranurono-6,1-lactone, leading to glucopyranuronic- were reported to have higher ChE inhibitory activities than
acid-based purine nucleosides, is described for the first time. their N9 counterparts.[15]
Coupling of 3 with silylated 2-acetamido-6-chloropurine
was carried out in the presence of trimethylsilyl trifluoro-
Results and Discussion
methanesulfonate (TMSOTf) in acetonitrile at 65 °C, condi-
tions that are reported to favour the formation of N7 purine
nucleosides,[7c] and under microwave irradiation (MW) at
150 W maximum power for 30 min (Scheme 1). Thus, β-
linked N9 purine nucleoside 4 and its N7 regioisomer 5 were
obtained in 52 and 12% yields, respectively. HMBC experi-
ments allowed the unambiguous assignment of 4 and 5,
providing conclusive evidence about the regiochemistry of
the nucleosidic linkage. In the N9 regioisomer (i.e., 4), the
anomeric proton (H-1Ј) correlates with C-4 of the purine,
while in the N7 nucleoside (i.e., 5), H-1Ј correlates with
C-5 of the nucleobase. Among the distinguishing features of
Chemistry
The synthesis of a dibenzylated 1-O-acetyl-xylofuranosyl
donor 3 (Scheme 1) was first undertaken. This would allow
us to test the N-glycosylation conditions, but also the nu-
cleosides derived from this donor would be included in the
panel of compounds for assessment against ChE, along
with those derived from glucuronic acid. The presence of a
C-2 participating group to make the coupling of the nucleo-
base to the sugar stereoselective was also desired. β-Config-
1
the H and 13C NMR spectra of 4 and 5 are the significant
chemical shift differences of H-1Ј, H-8, C-4 and C-8, which
are deshielded in the N7 isomer, and of C-5, which is deshi-
elded in the N9-substituted purine.
HMBC experiments were also used to definitively assign
the structures of further new N9,N7 nucleosides whose syn-
thesis is described below.
Glucofuranurono-6,3-lactone (6; Scheme 2) was used as
the precursor for purine nucleosides containing N-benzyl-
glucuronamide moieties, and its triacetylated derivative was
converted into bicyclic purine nucleosides.
Acetylation of 6 with acetic anhydride and pyridine at
room temperature provided tri-O-acetylglucofuranurono-
6,3-lactone in 92% yield as an anomeric mixture (7α/7β ra-
tio, 1:0.39). Alternatively, treatment of 6 with acetic an-
hydride and iodine at 115 °C under MW irradiation
Scheme 1. Reagents and conditions: a) BnBr, NaH, DMF, room
temp., 3 h; b) AcOH (70% aq.), reflux, 2 h, 80% over two steps;
c) Ac2O/py, room temp., 16 h, quant.; d) silylated 2-NHAc-6-Cl-
purine, CH3CN, 65 °C, MW, max. 150 W, 30 min, 52% (4) and
12% (5).
Scheme 2. Reagents and conditions: a) Ac2O/py, room temp., 16 h, 92%; b) Ac2O/I2, 115 °C, MW, max. 300 W, 40 min, 62% (7β); c) sil-
ylated 2-NHAc-6-Cl-purine, CH3CN, 65 °C, MW, max. 150 W, 30 min, 61% (8) and 9% (9).
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FULL PAPER
(300 W) for 40 min gave the β-anomer (i.e., 7β) in 62% lactone-based nucleosides, which only had a very slight dif-
yield. N-Glycosylation of bis-silylated 2-acetamido-6- ference between their Rf values, only N9 nucleoside 14 was
chloropurine with 7 under reaction conditions similar to isolable in pure form by flash chromatography.
those used with 3 gave glucofuranurono-6,3-lactone-based
The synthesis of a 6-N-benzyladenine nucleoside ana-
N9 purine nucleoside 8 and N7 compound 9 in 70% total logue of 14 was also carried out in order to further evaluate
yield, and with an N9/N7 ratio of 6.8:1. Although pure 8 and compare the ChE inhibitory activities of two com-
could be obtained by subjecting the regioisomeric mixture pounds whose structures differ only in the purine C-6 sub-
to flash column chromatography, its regioisomer 9 was in- stituent. Treatment of bicyclic N9 purine nucleoside 8 with
separably contaminated with compound 8 and could not be N-benzylamine at room temperature for 48 h effected both
isolated, despite repeated separation attempts.
lactone ring opening and nucleophilic displacement at C-6,
For the synthesis of glucuronamide-based nucleosides and subsequent acetylation gave 16, albeit in modest yield
(Scheme 3), acetonide-protected glucofuranurono-6,3-lact- (25%).
one 10 was subjected to lactone ring opening with N-benz-
The replacement of the chlorine atom by a benzylamino
ylamine to give N-benzylglucofuranuronamide derivative 11 group at C-6 of the nucleoside was confirmed by the signifi-
in quantitative yield. Acetylation of 11 (Ac2O/pyridine) cant changes in the chemical shifts of the signals of the
gave 3,5-di-O-acetyl derivative 12, which was treated with purine moiety in the 13C NMR spectrum. In particular,
TFA (72% aq.) at room temperature to cleave the 1,2-O- C-4, C-5, and C-8 of nucleoside 16 are shielded relative to
isopropylidene group. Subsequent acetylation (Ac2O/pyr- those of 14; they appeared at δ = 149.6, 116.3, and
idine) provided N-benzyltetra-O-acetyl-glucofuranuron- 137.4 ppm, respectively.
amide 13α,β in 80% yield as a 1:0.75 mixture of α and β
Pyranosyl counterparts of nucleosides 14 and 15 were
anomers. This anomeric mixture was subjected to MW- then synthesized (Scheme 4). Peracetylated N-benzylgluco-
assisted TMSOTf-promoted N-glycosylation with silylated pyranuronamide was accessed as an anomeric mixture (17α/
2-acetamido-6-chloropurine to give furanuronamide-based 17β, ratio 1:0.3) by acidic removal of the acetonide moiety
N9 purine nucleoside 14 and its N7 counterpart 15 in 60% of N-benzylfuranuronamide 11, which resulted in ring ex-
total yield, with a 2.8:1 N9/N7 ratio. As for the bicyclic- pansion, followed by acetylation. Subsequent MW-assisted
Scheme 3. Reagents and conditions: a) H2SO4 (0.5%), acetone, quant.; b) BnNH2, CH2Cl2, room temp., 16 h, quant; c) Ac2O/py, room
temp., 16 h, 95%; d) TFA (72% aq.), room temp., 3 h; e) Ac2O/py, room temp., 20 min, 80% over two steps; f) silylated 2-NHAc-6-Cl-
purine, CH3CN, 65 °C, MW, max. 150 W, 40 min, 44% (14) and 16% (15); g) BnNH2, CH2Cl2, room temp., 48 h; h) Ac2O/py, room
temp., 45 min, 25% over two steps.
Scheme 4. Reagents and conditions: a) TFA (60% aq.), room temp., 2.5 h; b) Ac2O/py, room temp., 10 min, 71% over two steps; c) silylated
2-NHAc-6-Cl-purine, CH3CN, 65 °C, MW, max. 150 W, 30 min, 42% (18) and 29% (19).
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Synthesis of Purine Nucleosides
Scheme 5. Reagents and conditions: a) Ac2O/I2, 300 W, 115 °C, 20 min, 75%;[20] b) silylated 2-NHAc-6-Cl-purine, CH3CN, 65 °C, MW,
max. 150 W, 50 min, 52% (22) and 13% (23).
coupling of 17 with the silylated nucleobase gave N9 purine
nucleoside 18 and its N7 regioisomer 19 in 71% yield, with chloropurine and galanthamine hydrobromide, a standard
a 1.5:1 N9/N7 ratio.
drug commonly used in the treatment AD, were evaluated
The synthetic nucleosides, along with 2-acetamido-6-
The generation of related nucleosides incorporating a as inhibitors of ChEs. Their inhibition constants Ki and the
glucuronic acid moiety was subsequently attempted. For type of inhibition are given in Tables 1 and 2.
this purpose, 2,3,4-tri-O-acetyl glucopyranurono-6,1-lact-
Table 1. Cholinesterase inibitory activities of purine nucleosides of
di-O-benzylxylofuranose.
one (21; Scheme 5), easily synthesized by acetylation of
glucuronic acid in the presence of iodine under microwave
(MW) irradiation,[20] was used as glycosyl donor in the
TMSOTf-promoted N-glycosylation of silylated 2-acet-
amido-6-chloropurine. Using coupling conditions similar to
those used previously, N9-linked purinylglucuronide 22 and
N7-linked compound 23 were obtained in 52 and 13%
yields, respectively. No α-linked nucleosides were detected,
showing that under these conditions, the presence of the
2-O-acetyl group in 21 is more important than the fused
lactone for the reaction outcome. Although the lactone ring
promotes an SN2 pathway, it acts as an anomeric leaving
group releasing the uronic acid functionality, while partici-
pation of the group at C-2 with oxocarbenium ring forma-
tion drives the β-stereoselectivity of the N-glycosylation re-
action. The presence of the carboxylic acid in nucleosides
22 and 23 was proved by HRMS, which showed prominent
peaks for the corresponding [M – H + 2Na]+ molecular
ions.
Although O- and C-glycosylation reactions using anom-
eric 6,1-lactones, in particular glucopyranurono-6,1-lact-
one, have been reported,[20–22] their use as glycosyl donors
for nucleoside synthesis is, to the best of our knowledge,
The Ki values of di-O-benzylxylofuranosyl-purine deriva-
unprecedented.
tives 4 and 5 (Table 1) could not be determined, due to the
low solubility of these compounds at concentrations above
20 μm; no inhibitory effect from either compound was de-
tected at lower concentrations.
Cholinesterase Inhibition Assessment
All the glucuronic-acid-derived nucleosides showed mod-
The in vitro inhibition of ChEs by the newly synthesized erate inhibitory activity and selectivity towards AChE, but
nucleosides was evaluated by the method of Ellman[23] using they showed lower AChE inhibition than galanthamine hy-
commercially available acetylcholinesterase (AChE) from drobromide by two orders of magnitude. None of them sig-
Electrophorus electricus, and butyrylcholinesterase (BChE) nificantly inhibited BChE, i.e., at concentrations below
from equine serum. Although AChE has been regarded as 100 μm (Table 2). Most of the compounds showed competi-
the most promising therapeutic target for the treatment of tive inhibition of AChE, with the exception of purinylgluc-
Alzheimer’s disease (AD),[17a] BChE inhibition may provide uronides 22 and 23, whose inhibition type was not deter-
additional benefits. Its activity increases over the course of mined.
the disease, while AChE activity progressively declines.[24]
The bicyclic glucofuranuronolactone-based nucleoside
Consequently, as the severity of dementia progresses, ACh (i.e., 8) had a Ki value similar to that of unsubstituted 2-
regulation may become increasingly dependent on BChE, acetamido-6-chloropurine.
so a dual inhibitory action may offer more sustained effi-
cacy in advanced AD.
Among the uronamide-based nucleosides, pyranosyl N7
nucleoside 19 showed better AChE inhibition (Ki
=
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FULL PAPER
Table 2. Cholinesterase inhibitory activities of purine nucleosides lactone precursors. 1-O-Acetylglucofuranosyl or glucopyr-
containing glucuronic acid derivatives as glycons.
anosyl-uronamide donors as well as triacetylated glucofur-
anuronolactone were coupled with persilylated 2-acet-
amido-6-chloropurine in the presence of TMSOTf to give
N9 and N7 nucleosides. Exclusive β-nucleoside formation
was observed in all cases due to neighbouring-group par-
ticipation. The regioisomeric N9/N7 ratios obtained in the
N-glycosylation were shown to be dependent on the glyc-
osyl donor structure. In the synthesis of furanosyl nucleo-
sides, higher N9/N7 ratios were observed when bicyclic lact-
one 7 was coupled with the nucleobase; in this reaction, the
N9 product was favoured over the N7 compound by a factor
of about seven. The β-N-glycosylation achieved with 7 is
driven by the participation of the 2-O-acetyl group. How-
ever, the sterically crowded nature of the β face, along with
the strong conformational rigidity of 7 due to its fused ring
system, probably result in a higher control of the regioselec-
tivity in favour of the N9 linkage, leading to the nucleoside
in which the purine substituents are more distant. Purine β-
N-glycosylation with furanuronamide 13, whose structure is
considerably less strained than 7, with only a moderately
bulky substituent (i.e., an N-benzyl group) on the furanose
β face, gave the lowest N9/N7 ratio (2.8:1) of those obtained
with glycofuranosyl donors. The coupling of the silylated
purine with acetylated pyranuronamide 17, whose structure
is more flexible than that of 13 due to its six-membered
ring, gave an increased amount of N7 nucleoside and a N9/
N7 ratio of 1.5:1.
A straightforward method for the stereoselective synthe-
sis of glucuronic acid-containing nucleosides was explored,
using acetylated glucopyranurono-6,1-lactone as a glycosyl
donor for purine N-glycosylation. The fused lactone can be
seen as a tethered anomeric acetate, with the uronic acid
group acting as intramolecular leaving group. The C-2 acet-
ate promoted the stereoselective formation of β-linked nu-
cleosides.
The nucleosides containing glucuronic acid derivatives as
18.72 μm) than N9 compound 18 (Ki = 30.98 μm). Compar-
glycons showed moderate and selective inhibition of acetyl-
ing glucuronamide N9-linked nucleosides 14 and 18, the in-
cholinesterase. The lowest Ki value for AChE inhibition (Ki
hibitory effect of the furanosyl derivative (i.e., 14; Ki =
= 14.78 μm) of the compounds tested was obtained with
14.78 μm) was about twice as high as that of its pyranosyl
N9-linked purinyl glucofuranuronamide 14. This interesting
isomer (i.e., 19), and it was the best AChE inhibitor of the
result motivates future structural optimization and the syn-
compounds tested. An N-benzylamino group at C-6 of the
thesis of analogues to obtain improved biological activities.
purine moiety of 16, led to a threefold decrease of the enzy-
matic inhibition (Ki = 50.53 μm) relative to that of the 6-
chloro derivative (i.e., 14).
Experimental Section
In terms of the glucuronic acid-based nucleosides, N9 nu-
General Methods: All reactions were monitored by TLC on Merck
cleoside 22 showed a lower Ki value than its N7 regioisomer
60 F254 silica gel aluminium plates with detection under UV light
23. The inhibitory effect of 22 was not significantly different
(254 nm) and/or by spraying with a solution of H2SO4 (10% in
EtOH) or with a solution of cerium(IV) sulfate (0.2% w/v) and
ammonium molybdate (5% w/v) in H2SO4 (6% aq.). Flash column
from that of its glucuronamide-based counterpart (i.e., 18).
The N7-purinylglucuronic acid derivative (i.e., 23) was, how-
ever, less active than the corresponding amide (i.e., 19) by
a factor of two.
chromatography was carried out on silica gel 60G (0.040–
0.063 mm, E. Merck). Microwave-assisted synthesis was carried out
with a CEM Discover system. NMR spectra were recorded with a
1
Bruker Avance 400 spectrometer operating at 400.13 MHz for H
and 100.62 MHz for 13C spectra. Chemical shifts are expressed in
parts per million relative to tetramethylsilane. Spectra were cal-
ibrated using internal tetramethylsilane in the case of CDCl3, or
Conclusions
Purine nucleosides containing d-glucuronic acid deriva-
tives as sugar moieties were synthesized from glucurono- using the residual solvent peak as a reference. HRMS spectra were
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Synthesis of Purine Nucleosides
acquired with an Apex Ultra FTICR Mass Spectrometer equipped
with an Apollo II Dual ESI/MALDI ion source from Bruker Dal-
tonics, and a 7T actively shielded magnet from Magnex Scientific.
Optical rotations were measured with a Perkin–Elmer 343 polari-
meter at 20 °C (589 nm, sodium D line). Melting points were deter-
mined with a Stuart SMP 30 apparatus.
2-Acetamido-6-chloro-9-(2-O-acetyl-3,5-di-O-benzyl-β-
anosyl)purine (4) and 2-Acetamido-6-chloro-7-(2-O-acetyl-3,5-di-O-
benzyl-β- -xylofuranosyl)purine (5): According to the general pro-
D-xylofur-
D
cedure, and starting from 1,2-di-O-acetyl-3,5-di-O-benzyl-d-xylo-
furanose (3; 160 mg, 0.39 mmol) and 2-acetamido-6-chloropurine
(123 mg, 0.58 mmol), the N-glycosylation of the corresponding sil-
ylated purine with 3 in the presence of trimethylsilyl triflate
(0.45 mL, 2.51 mmol), was complete within 30 min. Purification by
flash column chromatography on silica gel (EtOAc/petroleum ether,
from 2:3 to 2:1) gave N9 nucleoside 4 (114 mg, 52%) and its N7
regioisomer 5 (25 mg, 12%) as colourless oils.
1,2-Di-O-acetyl-3,5-di-O-benzyl-α-
D-xylofuranose (3α) and 1,2-Di-
O-acetyl-3,5-di-O-benzyl-β- -xylofuranose (3β): Acetic anhydride
D
(4 mL) was added to a solution of 3,5-di-O-benzyl-α,β-d-xylofur-
anose (100 mg, 0.30 mmol) in pyridine (5 mL), and the solution
was stirred overnight at room temperature. The solvents were co-
evaporated with toluene, and the crude product was purified by
flash column chromatography (petroleum ether/EtOAc, from 6:1 to
5:1) to give 3α (25 mg, 20%) and 3β (100 mg, 80%) as colourless
oils.
Data for 4: Rf = 0.61 (EtOAc/petroleum ether, 7:3). [α]2D0 = +57
(c = 1, CHCl3). 1H NMR (400 MHz, CDCl3): δ = 8.57 (br. s, 1 H,
3
NH), 8.35 (s, 1 H, 8-H), 7.38–7.24 (m, 10 H, Ph), 6.16 (d, J1Ј,2Ј
=
3
3
3.3 Hz, 1 H, 1Ј-H), 5.70 (dd, J1Ј,2Ј = 3.6, J2Ј,3Ј = 4.7 Hz, 1 H, 2Ј-
3
3
2
2
H), 4.69–4.44 (m, J2Ј,3Ј = 4.7, J3Ј,4Ј = 5.4, Ja,b = 11.4, Ja,b
=
Data for 3α: Rf = 0.14 (EtOAc/petroleum ether, 1:7). [α]2D0 = +43
(c = 1, CHCl3). 1H NMR (400 MHz, CDCl3): δ = 7.36–7.25 (m,
3
3
11.8 Hz, 5 H, 3Ј-H, 2 CH2Ph) 4.34–4.29 (ddd, J3Ј,4Ј = 5.4, J4,5Јa
= 2.1, 3J4,5Јb = 3.5 Hz, 1 H, 4Ј-H), 3.82 (dd, part A of ABX system,
3
3
10 H, Ph), 6.40 (d, J1,2 = 4.6 Hz, 1 H, 1-H), 5.15 (dd, J1,2 = 4.6,
3J4,5Јa = 2.1, J5Јa,5Јb = 10.8 Hz, 1 H, 5Јa-H), 3.59 (dd, part B of
2
3J2,3 = 6.2 Hz, 1 H, 2-H), 4.63 (d, part A of AB system, Ja,b
=
2
3
2
ABX system, J4,5Јb = 3.5, J5Јa,5Јb = 10.8 Hz, 1 H, 5Јb-H), 2.49
(s, 3 H, CH3, NHAc), 2.13 (s, 3 H, CH3, OAc) ppm. 13C NMR
(100 MHz, CDCl3): δ = 170.0 (CO, NHAc, CO, Ac), 152.2 (C-4 or
C-2), 152.1 (C-2 or C-4), 151.4 (C-6), 143.4 (C-8), 137.3, 137.3 (2
Cq, Ph), 128.7, 128.6 (CH, Ph), 128.5 (C-5), 128.3, 128.2, 128.1,
128.0 (CH, Ph), 87.7 (C-1Ј), 82.4 (C-4Ј), 76.1 (C-3Ј), 74.8 (C-2Ј),
73.6, 73.5 (2 CH2Ph), 68.6 (C-5Ј), 25.3 (CH3, NHAc), 20.8, (CH3,
OAc) ppm. HRMS: calcd. for C28H28ClN5O6 [M + H]+ 566.1801;
found 566.1787; calcd. for [M + Na]+ 588.1620; found 588.1601.
12.0 Hz, 1 H, a-H, CH2Ph), 4.57–4.41 (m, 3 H, Bn), 4.32 (dt, 1 H,
3
4-H), 4.14 (dd, 3J2,3 = 6.2, J3,4 = 4.2 Hz, 1 H, 3-H), 3.51 (dd, part
3
2
A of ABX system, J4,5a = 3.5, J5a,5b = 10.5 Hz, 1 H, 5a-H), 3.40
3
2
(dd, part B of ABX system, J4,5b = 3.6, J5a,5b = 10.5 Hz, 1 H,
5b-H), 2.13, 2.12 (2 s, 6 H, 2 CH3, OAc) ppm. 13C NMR
(100 MHz, CDCl3): δ = 170.2, 170.2 (2 CO, OAc), 137.8, 137.8 (2
Cq, Ph), 128.6, 128.5, 128.0, 127.9, 127.9, 127.8 (CH, Ph), 94.7
(C-1), 83.5 (C-4), 75.5 (C-3), 73.6, 73.2 (2 CH2Ph), 71.4 (C-2), 69.2
(C-5), 21.3, 20.7 (2 CH3, 2 OAc) ppm. HRMS: calcd. for C23H26O7
[M + Na]+ 437.1571; found 437.1560; calcd. for [M + K]+ 453.1310;
found 453.1299.
Data for 5: Rf = 0.25 (EtOAc/petroleum ether, 7:3). [α]2D0 = +40 (c
1
= 0.5, CHCl3). H NMR (400 MHz, CDCl3): δ = 8.92 (s, 1 H, 8-
3
H), 8.18 (br. s, 1 H, NH), 7.42–7.20 (m, 10 H, Ph), 6.58 (d, J1Ј,2Ј
Data for 3β: Rf = 0.23 (EtOAc/petroleum ether, 1:7). [α]2D0 = +40
(c = 1.6, CHCl3). 1H NMR (400 MHz, CDCl3): δ = 7.36–7.24
3
3
= 2.5 Hz, 1 H, 1Ј-H), 5.58 (dd, J1Ј,2Ј = 2.5, J2Ј,3Ј = 4.2 Hz, 1 H,
3
3
2
2
2Ј-H), 4.64–4.26 (m, J2Ј,3Ј = 4.2, J3Ј,4Ј = 6.5, Ja,b = 11.9, Ja,b
=
3
(m, 10 H, Ph), 6.12 (br. s, 1 H, 1-H), 5.30 (d, J2,3 = 4.3 Hz, 1 H,
11.1 Hz, 6 H, 3Ј-H, 4Ј-H, 2 CH2Ph) 3.91 (br. d, part A of ABX
system, 2J5Јa,5Јb = 10.8 Hz, 1 H, 5Јa-H), 3.55 (br. d, part B of ABX
system, 2J5Јa,5Јb = 10.8 Hz, 1 H, 5Јb-H), 2.60 (s, 3 H, CH3, NHAc),
2.18 (s, 3 H, CH3, OAc) ppm. 13C NMR (100 MHz, CDCl3): δ =
169.6 (CO, NHAc, CO, Ac), 163.9 (C-4), 152.7 (C-2 or C-6), 147.8
(C-8), 143.0 (C-2 or C-6), 137.0, 137.0 (2 Cq, Ph), 128.9, 128.7,
128.5 128.4, 128.3, 128.2 (CH, Ph), 118.2 (C-5), 89.2 (C-1Ј), 82.3
(C-4Ј), 75.6 (C-2Ј), 75.0 (C-3Ј), 73.7, 73.3 (2 CH2Ph), 67.3 (C-5Ј),
25.4 (CH3, NHAc), 20.8, (CH3, OAc) ppm. HRMS: calcd. for
C28H28ClN5O6 [M + H]+ 566.1801; found 566.1782; calcd. for [M
+ Na]+ 588.1620; found 588.1603.
3
2-H), 4.62 (d, part A of AB system, Ja,b = 11.6 Hz, 1 H, a-H,
2
CH2Ph), 4.53 (d, part A of AB system, Ja,b = 12.2 Hz, 1 H, a-H,
CH2Ph), 4.49–4.42 (m, 4 H, 2 b-H, 2 CH2Ph), 4.30 (dd, 3J2,3 = 4.3,
3
3
3
3J3,4 = 7.7 Hz, 1 H, 3-H), 4.22 (ddd, J3,4 = 7.7, J4,5a = 2.9, J4,5b
3
= 4.0 Hz, 1 H, 4-H), 3.68 (dd, part A of ABX system, J4,5a = 2.9,
2J5a,5b = 11.0 Hz, 1 H, 5a-H), 3.54 (dd, part B of ABX system,
3J4,5b = 4.0, J5a,5b = 11.0 Hz, 1 H, 5b-H), 2.12, 1.91 (2 s, 6 H, 2
2
CH3, OAc) ppm. 13C NMR (100 MHz, CDCl3): δ = 169.9, 169.2
(2 CO, OAc), 138.1, 137.3 (2 Cq, Ph), 128.5, 128.3, 128.1, 128.0,
127.6, 127.5 (CH, Ph), 98.5 (C-1), 81.4 (C-4), 76.4 (C-3), 73.6
(C-2), 73.2 (2 CH2Ph), 69.0 (C-5), 20.9, 20.8 (2 CH3, 2 OAc) ppm.
HRMS: calcd. for C23H26O7 [M + Na]+ 437.1571; found 437.1563;
calcd. for [M + K]+ 453.1310; found 453.1301.
1,2,5-Tri-O-acetyl-α,β-D-glucofuranurono-6,3-lactone (7α,7β)
Method 1: A mixture of glucofuranurono-6,3-lactone (250 mg,
1.42 mmol), pyridine (7 mL), and acetic anhydride (5 mL) was
stirred overnight at room temp. After coevaporation with toluene,
the residue was purified by flash column chromatography (EtOAc/
petroleum ether, 1:4) to give triacetylated glucuronolactone 7α,7β
(393 mg, 92%; α/β ratio = 1:0.39) as a white solid. 1H NMR
General Procedure for N-Glycosylation with 1-O-Acetylglycosyl Do-
nors: 2-Acetamido-6-chloropurine (123 mg, 0.58 mmol) was sus-
pended in anhydrous acetonitrile (2.5 mL) under N2, and N,O-
bis(trimethylsilyl)acetamide (0.28 mL, 1.16 mmol) was added. The
mixture was stirred at room temp. for 20 min. Then, a solution
of 1-O-acetylglycosyl donor (0.39 mmol) in anhydrous acetonitrile
3
(400 MHz, CDCl3): δ = 6.55 (d, J1,2(α) = 4.8 Hz, 0.72 H, 1α-H),
3
(2.5 mL) was added, followed by dropwise addition of trimethylsilyl 6.17 (br. s, 0.28 H, 1β-H), 5.53 (d, J4,5(α) = 5.2 Hz, 0.72 H, 5α-H),
triflate (0.45 mL, 2.51 mmol). The reaction mixture was exposed to
microwave irradiation at 150 W (Pmax = 250 Psi), and it was contin-
uously stirred at 65 °C for 30–40 min. The mixture was then diluted
5.36–5.21 (m, 1.56 H, 2α-H, 2β-H, 4β-H, 5β-H), 5.16–5.03 (m, 1.72
H, 3α-H, 4α-H, 3β-H), 2.22 (s, 2.16 H, CH3, Ac), 2.16, 2.14, 2.13
(3 s, 3.84 H, 2 β-CH3, β-Ac, α-CH3, α-Ac), 2.09 (s, 2.16 H, CH3,
with dichloromethane, and washed with satd. aq. NaHCO3 solu- α-Ac), 2.06 (s, 0.84 H, CH3, β-Ac) ppm. 13C NMR (100 MHz,
tion. The aqueous phase was extracted with dichloromethane (3ϫ),
and the combined organic phases were dried with MgSO4. After
filtration and evaporation of the solvent, the residue was purified
by column chromatography on silica gel.
CDCl3): δ = 170.4, 169.7, 169.6, 169.3, 169.2, 169.2, 168.7, 168.6
(CO, α, β), 98.5 (C-1β), 94.8 (C-1α), 82.6 (C-3α), 81.7 (C-3β), 77.9
(C-2β), 76.8 (C-2α), 76.2 (C-4β), 75.4 (C-4α), 68.5 (C-5α), 68.0 (C-
5β), 20.6, 20.6, 20.4, 20.1, 20.1, 19.8 (CH3, Ac, α, β) ppm.
Eur. J. Org. Chem. 2014, 2770–2779
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2775

N. M. Xavier, S. Schwarz, P. D. Vaz, R. Csuk, A. P. Rauter
N-Benzyl-1,2-O-isopropylidene-α- -glucofuranuronamide (11): 1,2-
FULL PAPER
Method 2: Alternatively, acetylation of glucofuranurono-6,3-lact-
one with Ac2O/I2 under MW irradiation (300 W) gave only the β-
anomer. I2 (0.2 g) was added to a mixture of glucofuranurono-6,3-
lactone (0.5 g, 2.84 mmol) and acetic anhydride (4 mL). The mix-
D
O-Isopropylidene-α-d-glucofuranurono-6,3-lactone (10; 150 mg,
0.69 mmol) was dissolved in anhydrous dichloromethane (5 mL)
under nitrogen, and benzylamine (0.08 mL, 0.73 mmol) was added.
The solution was stirred overnight at room temp. The solvent was
removed under vacuum, and the residue was purified by flash col-
umn chromatography on silica gel (ethyl acetate/petroleum ether,
9:1) to give compound 11 (224 mg, quant.) as a white solid. Rf =
0.42 (EtOAc/petroleum ether, 9:1), m.p. 132.9–134.8 °C. [α]2D0 = –9
ture was exposed to microwave irradiation at 300 W (Pmax
=
200 Psi) with continuous stirring at 115 °C for 40 min. The mixture
was then diluted with CH2Cl2, and it was successively washed with
satd. aq. Na2S2O3, satd. aq. NaHCO3, and brine. The organic
phase was dried with MgSO4. After filtration and concentration
under vacuum, the residue was purified by flash column
chromatography on silica gel (ethyl acetate/petroleum ether, 1:4) to
give 1,2,5-tri-O-acetyl-β-d-glucofuranurono-6,3-lactone (532 mg,
62%) as white crystals.
1
(c = 1.2, CHCl3). H NMR (400 MHz, CDCl3): δ = 7.37–7.23 (m,
3
3
5 H, Ph), 7.10 (t, JNH,a-H = JNH,b-H = 5.9 Hz, 1 H, NH), 5.96 (d,
3J1,2 = 3.4 Hz, 1 H, 1-H), 4.60–4.45 (m, J1,2 = 3.4, Ja,b = 15.0,
3
2
3JNH,a-H = JNH,b-H = 5.9 Hz, 3 H, 2-H, CH2Ph), 4.40 (d, J3,4
=
3
3
2.4 Hz, 1 H, 3-H), 4.36 (d, 3J4,5 = 7.1 Hz, 1 H, 5-H), 4.28 (dd, 3J3,4
Data for 1,2,5-Tri-O-acetyl-β-d-glucofuranurono-6,3-lactone (7β):
M.p. 193.4–194.7 °C. [α]2D0 = +70 (c = 0.7, CHCl3). 1H NMR
(400 MHz, CDCl3): δ = 6.17 (br. s, 1-H), 5.31 (br. s, 2-H), 5.28–
3
= 2.4, J4,5 = 7.1 Hz, 1 H, 4-H), 1.47, 1.31 (2 s, 6 H, 2 CH3, iPr)
ppm. 13C NMR (100 MHz, CDCl3): δ = 172.1 (CO), 137.7 (Cq,
Ph), 128.9, 128.7, 128.5 (CH, Ph), 112.4 (Cq, iPr), 105.6 (C-1), 85.1
(C-2), 81.4 (C-4), 75.5 (C-3), 69.2 (C-5), 43.7 (CH2Ph), 27.1, 26.3
(2 CH3, iPr) ppm. HRMS: calcd. for C16H21NO6 [M + H]+
324.1442; found 324.1436; calcd. for [M + Na]+ 346.1261; found
346.1255.
3
3
3
5.20 (m, J3,4 = 4.6, J4,5 = 7.3 Hz, 2 H, 4-H, 5-H), 5.09 (d, J3,4
=
4.6 Hz, 1 H, 3-H), 2.17, 2.15, 2.08 (s, 3 CH3, Ac) ppm. 13C NMR
(100 MHz, CDCl3): δ = 170.2, 169.5, 169.2, 168.9 (4 CO), 98.7 (C-
1), 81.8 (C-3), 78.2 (C-2), 76.3 (C-4), 68.2 (C-5), 20.9, 20.7, 20.1 (3
CH3, Ac) ppm. * HRMS: calcd. for C12H14O9 [M + Na]+ 325.0530;
found 325.0525; calcd. for [M + K]+ 341.0269; found 341.0265.
*Data were in accordance with the published data.[25]
N-Benzyl-2,5-di-O-acetyl-1,2-O-isopropylidene-α-D-glucofuranuron-
amide (12): N-Benzyl-1,2-O-isopropylidene-α-d-glucofuranurona-
mide (11; 100 mg, 0.31 mmol) was dissolved in pyridine (4 mL),
1-(2-Acetamido-6-chloropurin-9-yl)-2,5-di-O-acetyl-β-
urono-6,3-lactone (8) and 1-(2-Acetamido-6-chloropurin-7-yl)-2,5-di- stirred overnight at room temp. The solvents were then coevapo-
O-acetyl-β- -glucofuranurono-6,3-lactone (9): According to the ge- rated with toluene, and the residue was purified by flash column
neral procedure, and starting from 1,2,5-tri-O-acetyl-α,β-d-glucof- chromatography (ethyl acetate/petroleum ether, 3:1) to give com-
D-glucofuran- and acetic anhydride (3 mL) was added. The reaction mixture was
D
uranurono-6,3-lactone (7; 170 mg, 0.56 mmol) and 2-acetamido-6-
chloropurine (150 mg, 0.71 mmol), the N-glycosylation of the cor-
responding silylated purine with 7 in the presence of trimethylsilyl
triflate (0.55 mL, 3.06 mmol), was complete within 30 min. Flash
column chromatography on silica gel (EtOAc/petroleum ether, from
5:1 to 6:1) gave 8 and 9 (176 mg, 70% total yield) in a ratio of
6.8:1. Pure N9 nucleoside 8 (72 mg) was obtained as a white solid,
while the N7 compound (i.e., 9) could not be isolated and was ob-
tained in a mixture containing 8 (105 mg; ratio 8/9, 3.6:1).
pound 12 (120 mg, 95%) as a white solid. Rf = 0.71 (EtOAc/petro-
leum ether, 9:1), m.p. 129.2–131.0 °C. [α]2D0 = +11 (c = 0.9, CHCl3).
1H NMR (400 MHz, CDCl3): δ = 7.38–7.22 (m, 5 H, Ph), 7.53 (t,
3
3
3JNH,a-H = JNH,b-H = 5.9 Hz, 1 H, NH), 5.96 (d, J1,2 = 3.6 Hz, 1
3
3
H, 1-H), 5.41 (d, J3,4 = 2.7 Hz, 1 H, 3-H), 5.16 (d, J4,5 = 9.7 Hz,
3
3
1 H, 5-H), 4.63 (dd, J3,4 = 2.7, J4,5 = 9.7 Hz, 1 H, 4-H), 4.59–
4.42 (m, J1,2 = 3.6, Ja,b = 15.0, 3JNH,a-H = 3JNH,b-H = 5.9 Hz, 3 H,
3
2-H, CH2Ph), 2.12, 2.04 (2 s, 6 H, CH3, OAc), 1.52, 1.32 (2 s, 6 H,
2 CH3, iPr) ppm. 13C NMR (100 MHz, CDCl3): δ = 169.8, 169.4,
167.2 (3 CO), 137.9 (Cq, Ph), 128.8, 127.7, 127.6 (CH, Ph), 113.1
(Cq, iPr), 105.6 (C-1), 83.0 (C-2), 77.5 (C-4), 75.3 (C-3), 69.2 (C-
5), 43.7 (CH2Ph), 26.9, 26.4 (2 CH3, iPr), 20.8, 20.7 (2 CH3, OAc)
ppm. HRMS: calcd. for C20H25NO8 [M + H]+ 408.1653; found
408.1646; calcd. for [M + Na]+ 430.1472; found 430.1466.
Data for 8: Rf = 0.26 (EtOAc), m.p. 185.5–187.2 °C. [α]2D0[α] = +130
(c = 1, CHCl3). 1H NMR (400 MHz, CDCl3): δ = 8.61 (br. s, 1 H,
3
NH), 8.08 (s, 1 H, 8-H), 6.17 (d, J1Ј,2Ј = 2.7 Hz, 1 H, 1Ј-H), 5.82
3
3
(d, J1Ј,2Ј = 2.7 Hz, 1 H, 2Ј-H), 5.63 (d, J4Ј,5Ј = 4.8 Hz, 1 H, 5Ј-
3
3
H), 5.28–5.21 (m, J3Ј,4Ј = 3.7, J4Ј,5Ј = 4.8 Hz, 2 H, 3Ј-H, 4Ј-H),
2.49 (s, 3 H, CH3, NHAc), 2.20, 2.17 (2 s, 6 H, CH3, OAc) ppm.
13C NMR (100 MHz, CDCl3): δ = 170.2 (CO, NHAc), 169.7, 169.6
(CO), 152.4 (C-2 or C-6), 151.9 (C-2 or C-6), 151.8 (C-4), 142.8
(C-8), 128.5 (C-5), 91.4 (C-1Ј), 82.4 (C-3Ј), 80.1 (C-2Ј), 77.9 (C-
4Ј), 69.5 (C-5Ј), 25.3 (CH3, NHAc), 20.6, 20.4 (2 CH3, OAc) ppm.
HMBC: 1Ј-H correlates with 151.8 (C-4) and 142.8 (C-8); 8-H cor-
relates with 128.5 (C-5). HRMS: calcd. for C17H16ClN5O8 [M + H]+
454.0760; found 454.0748; calcd. for [M + Na]+ 476.0580; found
476.0567.
N-Benzyl-1,2,3,5-tetra-O-acetyl-α,β-D-glucofuranuronamide
(13α,13β): A solution of N-benzyl-2,5-di-O-acetyl-α-d-glucofuran-
uronamide (70 mg, 0.17 mmol) in aq. trifluoroacetic acid (72%;
3.6 mL) was stirred at room temp. for 3 h. After coevaporation with
toluene, the residue was stirred in pyridine (1.5 mL) and acetic an-
hydride (1 mL) at room temp. for 20 min. The solvents were then
coevaporated with toluene, and the residue was purified by flash
column chromatography (ethyl acetate/petroleum ether, 1:1) to give
13α,13β (62 mg, 80% over two steps; anomeric mixture, α/β ratio,
1:0.75) as a colourless oil. Rf = 0.62 (EtOAc/petroleum ether, 4:1).
1
Data for 9: Rf = 0.17 (EtOAc). H NMR (400 MHz, CDCl3)*: δ = 1H NMR (400 MHz, CDCl3): δ = 7.38–7.22 (m, 5 H, Ph), 6.51–
3
3
8.61 (br. s, 1 H, NH), 8.41 (s, 1 H, 8-H), 6.67 (d, J1Ј,2Ј = 2.1 Hz,
6.40 (m, J1,2(α) = 4.6 Hz, 1.57 H, 1α-H, NH α, NH β), 6.15 (br. s,
3
3
3
1 H, 1Ј-H), 5.74–5.70 (m, J1Ј,2Ј = 2.1, J4Ј,5Ј = 4.6 Hz, 2 H, 2Ј-H, 0.43 H, 1β-H), 5.63 (dd, 3J = 3.9, J = 5.2 Hz, 0.57 H, 3α-H), 5.47
3
3
3
3
5Ј-H), 5.35–5.31 (m, J3Ј,4Ј = 3.3, J4Ј,5Ј = 4.6 Hz, 1 H, 4Ј-H), 5.20 (dd, J2,3(β) = 1.1, J3,4(β) = 4.7 Hz, 0.43 H, 3β-H), 5.33–5.22 (m,
3
(d, J3Ј,4Ј = 3.3 Hz, 1 H, 3Ј-H), 2.54 (s, 3 H, CH3, NHAc), 2.22,
1.57 H, 2α-H, 5α-H, 5β-H), 5.17 (br. s, 0.43 H, 2β-H), 4.82–4.74
(m, 1 H, 4α-H, 4β-H), 4.58–4.49 (m, 1 H, aα-H, aβ-H, Bn), 4.47–
2.21 (2 s, 6 H, CH3, OAc) ppm. 13C NMR (100 MHz, CDCl3)*: δ
= 169.5, 169.2, 169.0 (CO), 163.4 (C-4), 152.9 (C-2 or C-6), 146.8 4.37 (m, 1 H, bα-H, bβ-H, Bn), 2.13, 2.13, 2.13, 2.10, 2.08, 2.08,
(C-8), 143.1 (C-2 or C-6), 118.4 (C-5), 91.8 (C-1Ј), 81.9 (C-3Ј), 79.9 2.07, 2.04 (each s, 12 H, CH3, OAc, α,β) ppm. 13C NMR
(C-2Ј), 79.0 (C-4Ј), 69.6 (C-5Ј), 25.2 (CH3, NHAc), 20.5, 20.3 (2
CH3, OAc) ppm. *Data extracted from the spectrum of the 8/9
regioisomeric mixture.
(100 MHz, CDCl3): δ = 169.7, 169.6, 169.6, 169.5, 169.3, 169.2,
169.1, 169.1, 166.8, 166.6 (CO, α,β), 137.6, 137.5 (Cq, Ph α,β),
128.9, 128.9, 127.7, 127.7, 127.7 (CH, Ph, α,β), 99.4 (C-1β), 93.5
2776
www.eurjoc.org
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 2770–2779

Synthesis of Purine Nucleosides
(C-1α), 80.0 (C-4β), 79.1 (C-2β), 77.1 (C-4α), 75.8 (C-2α), 73.9 (C- 5Ј-H), 4.84–4.69 (m, J3Ј,4Ј = 4.0, J4Ј,5Ј = 8.8 Hz, 3 H, 4Ј-H,
3
2
3
3
3α), 73.2 (C-3β), 70.3 (C-5α), 70.2 (C-5β), 43.8, 43.6 (CH2Ph, α,β),
20.9, 20.7, 20.7, 20.7, 20.6, 20.6, 20.5 (CH3, Ac, α,β) ppm. HRMS:
calcd. for C21H25NO10 [M + H]+ 452.1551; found 452.1540; calcd.
for [M + Na]+ 474.1371; found 474.1359.
CH2Ph), 4.49–4.36 (m, Ja,b = 14.9, JNH,a-H = 5.6, JNH,b-H =
5.4 Hz, 2 H, CH2Ph, amide), 2.46 (s, 3 H, CH3, NHAc), 2.15, 2.12,
2.05 (3 s, 9 H, 3 CH3, OAc) ppm. 13C NMR (100 MHz, CDCl3): δ
= 169.8, 169.1, 168.8 (CO), 166.5 (CO, amide), 149.6 (C-4)*, 137.4,
137.2 (C-8, Cq, Ph), 128.9, 128.8 127.7, 127.7 (CH, Ph), 116.3 (C-
5)*, 87.9 (C-1Ј), 79.2 (C-4Ј), 79.1 (C-2Ј), 73.8 (C-3Ј), 69.7 (C-5Ј),
44.9 (NHCH2Ph)**, 43.7 (CH2Ph, amide), 25.3 (CH3, NHAc),
20.7, 20.7 (CH3, OAc). *inferred from HMBC, ** inferred from
HMQC. HRMS: calcd. for C33H35N7O9 [M + H]+ 674.2569; found
674.2554; calcd. for [M + Na]+ 696.2389; found 696.2369.
N-Benzyl-1-(2-acetamido-6-chloropurin-9-yl)-2,3,5-tri-O-acetyl-β-
glucofuranuronamide (14) and N-Benzyl-1-(2-acetamido-6-chloro-
purin-7-yl)-2,3,5-tri-O-acetyl-β- -glucofuranuronamide (15): Ac-
D-
D
cording to the general procedure, and starting from N-benzyl-
1,2,3,5-tetra-O-acetyl-α,β-d-glucofuranuronamide (13; 48 mg,
0.11 mmol) and 2-acetamido-6-chloropurine (34 mg, 0.16 mmol),
the N-glycosylation of the corresponding silylated purine with 13
in the presence of trimethylsilyl triflate (0.13 mL, 0.69 mmol), was
complete within 40 min. Flash column chromatography on silica
gel (EtOAc/petroleum ether, from 4:1 to 5:1) gave 14 and 15 (38 mg,
60% total yield) in a ratio of 2.8:1. Pure N9 nucleoside 14 (10 mg)
was obtained as a white solid, while the N7 compound (i.e., 15)
could not be isolated and was obtained in a mixture containing 14
(28 mg, ratio 14/15, 1.8:1).
N-Benzyl-1,2,3,4-tetra-O-acetyl-α,β-D-glucopyranuronamide
(17α,17β): A solution of N-benzyl-1,2-O-isopropylidene-α-d-gluco-
furanuronamide (11; 110 mg, 0.34 mmol) in aq. trifluoroacetic acid
(60%; 2.3 mL) was stirred at room temp. for 2.5 h. After coevapo-
ration with toluene, the residue was stirred in pyridine (2 mL) and
acetic anhydride (1.2 mL) at room temp. for 10 min. The solvents
were then coevaporated with toluene, and the residue was purified
by flash column chromatography (ethyl acetate/petroleum ether,
1:1) to give 17α,17β (109 mg, 71% over two steps; anomeric mix-
ture, α/β ratio, 1:0.65) as a white solid. Rf = 0.35 (EtOAc/petroleum
ether, 1:1). 1H NMR (CDCl3, 400 MHz): δ = 7.41–7.22 (m, 5 H,
Data for 14: Rf = 0.41 (EtOAc), m.p. 198.8–200.0 °C. [α]2D0= +9 (c
1
= 1, CHCl3). H NMR (400 MHz, CDCl3): δ = 8.22 (s, 1 H, 8-H),
8.08 (s, 1 H, NH), 7.26–7.10 (m, 5 H, Ph), 6.67 (br.t, 1 H, NH),
3
3
3
3
3
6.14 (d, J1Ј,2Ј = 2.1 Hz, 1 H, 1Ј-H), 5.67 (dd, J2Ј,3Ј = 1.7, J3Ј,4Ј
=
Ph), 6.68 (t, JNH,aα-H = JNH,bα-H = 5.7 Hz, 0.61 H, NH α), 6.61
3
3
3
3
4.0 Hz, 1 H, 3Ј-H), 5.53–5.44 (m, 1 H, 2Ј-H, 5Ј-H), 4.86 (dd, J3Ј,4Ј
= 4.0, J4Ј,5Ј = 8.5 Hz, Ja,b = 14.9, JNH,a-H = 5.8 Hz, 1 H, 4Ј-H),
4.47 (dd, part A of ABX system, 1 H, a-H from , JNH,b-H = 5.5,
(t, JNH,aβ-H = JNH,bβ-H = 5.5 Hz, 0.39 H, β-NH), 6.34 (d, J1,2(α)
3
3
3
3
= 3.7 Hz, 0.61 H, 1α-H), 5.75 (d, J1,2(β) = 8.3 Hz, 0.39 H, 1β-H),
3
3
5.54 (t, J2,3(α)
=
3J3,4(α) = 10.1 Hz, 0.61 H, 3α-H), 5.36–5.18 (m,
3Ja,b = 14.9 HzCH2Ph), 4.39 (dd, part B of ABX system, 1 H, b-
H from CH2Ph), 2.47 (s, 3 H, CH3, NHAc), 2.17, 2.14, 2.07 (3 s,
9 H, 3 CH3, OAc) ppm. 13C NMR (100 MHz, CDCl3): δ = 169.7,
169.3, 168.7, 166.3 (CO), 152.1 (C-4), 151.9 (C-2, C-6), 142.6 (C-
8), 137.3 (Cq, Ph), 128.8 (CH, Ph), 127.8, 127.8, 127.8 (CH, Ph,
C-5), 88.2 (C-1Ј), 79.7 (C-4Ј), 79.3 (C-2Ј), 73.6 (C-3Ј), 69.7 (C-5Ј),
43.8 (CH2Ph), 25.3 (CH3, NHAc), 20.7, 20.7, 20.3 (CH3, OAc)
ppm. HRMS: calcd. for C26H27ClN6O9 [M + H]+ 603.1601; found
603.1605; calcd. for [M + Na]+ 625.1420; found 625.1422.
3J2,3(β) = 3J3,4(β) = 9.3, 3J3,4(α) = 3J4,5(α) = 10.1, J4,5(β) = 9.6 Hz, 1.39
3
3
H, 3β-H, 4α-H, 4β-H), 5.15–5.01 (m, J1,2(α) = 3.7, J2,3(α) = 10.1,
J1,2(β) = 8.3, 3J2,3(β) = 9.3 Hz, 1 H, 2α-H, 2β-H), 4.54–4.43 (m, 2Ja,b
3
3
= 14.7, JNH,aα-H = 5.7, JNH,aβ-H = 5.5 Hz, 1 H, aα-H, aβ-H, Bn),
2
3
3
3
4.39–4.30 (m, Ja,b = 14.7, JNH,bα-H = 5.7, JNH,bβ-H = 5.5, J4,5(α)
= 10.1 Hz, 1.61 H, bα-H, bβ-H, 5α-H), 4.13 (d, J4,5(β) = 9.6 Hz,
0.39 H, 5β-H), 2.19 (s, 1.83 H, CH3, OAc, α), 2.11, 2.09, 2.09,
2.04, 2.04, 2.02 (each s, 10.17 H, CH3, OAc, α,β) ppm. 13C NMR
(100 MHz, CDCl3): δ = 170.0, 170.0, 169.9, 169.9, 168.9, 168.9,
167.9, 166.5, 166.0 (CO, α,β), 137.5 (Cq, Ph, β), 137.4 (Cq, Ph, α),
128.9, 128.9, 128.1, 128.0, 127.9, 127.8 (CH, Ph, α,β), 91.4 (C-1β),
88.4 (C-1α), 73.1 (C-5β), 72.1 (C-3β), 70.5 (C-5α), 70.3 (C-2β), 69.3
(C-4α), 69.1 (C-2α, C-4β), 69.0 (C-3α), 43.2, 43.2 (CH2Ph, α,β),
20.9, 20.8, 20.7, 20.7, 20.6 (CH3, Ac, α,β) ppm. HRMS: calcd. for
C21H25NO10 [M + H]+ 452.1551; found 452.1542; calcd. for [M +
Na]+ 474.1371; found 474.1360.
1
Data for 15: Rf = 0.31 (EtOAc). H NMR (400 MHz, CDCl3)*: δ
= 8.65 (s, 1 H, H-8), 8.29 (s, 1 H, NH), 7.26–7.10 (m, 5 H, Ph),
3
6.71 (br. s, 1 H, NH), 6.58 (br. s, 1 H, 1Ј-H), 5.59 (br. d, J3Ј,4Ј
=
3
3.2 Hz, 1 H, 3Ј-H), 5.45 (d, J4Ј,5Ј = 8.6 Hz, 1 H, 5Ј-H), 5.33 (br. s,
1 H, 2Ј-H), 4.89 (dd, 1 H, 4Ј-H, J3Ј,4Ј = 3.2, J4Ј,5Ј = 8.6 Hz), 4.48
3
(d, J = 5.6 Hz, 2 H, CH2Ph), 2.59 (s, 3 H, CH3, NHAc), 2.22, 2.13,
2.01 (3 s, 9 H, 3 CH3, OAc) ppm. 13C NMR (100 MHz, CDCl3)*:
δ = 147.5 (C-8), 90.4 (C-1Ј), 80.9 (C-4Ј), 80.2 (C-2Ј), 73.5 (C-3Ј),
69.4 (C-5Ј) ppm. * Data extracted from the spectrum of the 14/15
regioisomeric mixture.
N-Benzyl-1-(2-acetamido-6-chloropurin-9-yl)-2,3,4-tri-O-acetyl-β-
glucopyranuronamide (18) and N-Benzyl-1-(2-acetamido-6-chloro-
purin-7-yl)-2,3,4-tri-O-acetyl-β- -glucopyranuronamide (19): Ac-
D-
D
N-Benzyl-1-(2-acetamido-6-N-benzyladenin-9-yl)-2,3,5-tri-O-acetyl-
β-D-glucofuranuronamide (16): 2-Acetamido-6-chloro-9-(2Ј,5Ј-tri-
cording to the general procedure, and starting from N-benzyl-
1,2,3,4-tetra-O-acetyl-α,β-d-glucopyranuronamide (17; 63 mg,
0.14 mmol) and 2-acetamido-6-chloropurine (43 mg, 0.2 mmol),
the N-glycosylation of the corresponding silylated purine with 17
in the presence of trimethylsilyl triflate (0.16 mL, 0.88 mmol), was
complete within 40 min. Purification by flash column chromato-
graphy on silica gel (EtOAc/petroleum ether, from 2.5:1 to 4:1) gave
N9 nucleoside 18 (35 mg, 42%) and its N7 regioisomer 19 (24 mg,
29%) as white solids.
O-acetyl-β-d-glucofuranosylurono-6Ј,3Ј-lactone)purine (8, 19 mg,
42 μmol) was dissolved in anhydrous dichloromethane (2.5 mL),
and benzylamine (50 μL, 0.46 mmol) was added. The reaction mix-
ture was stirred at room temp. for 2 d. The solvent was then evapo-
rated, the residue was dissolved in pyridine (1.6 mL) and acetic
anhydride (1 mL), and the solution was stirred at room temp. for
45 min. After coevaporation with toluene, the residue was purified
by flash column chromatography on silica gel (ethyl acetate/petro-
leum ether, 5:1) to give compound 16 (7 mg, 25% over two steps)
Data for 18: Rf = 0.3 (EtOAc/petroleum ether, 5:1), m.p. 134.1–
as a white solid. Rf = 0.48 (EtOAc/MeOH, 19:1), m.p. 202.4– 136.0 °C. [α]2D0 = –3 (c = 0.3, CHCl3). 1H NMR (400 MHz, CDCl3):
204.3 °C. [α]2D0 = +3 (c = 0.6, CHCl3). 1H NMR (400 MHz,
CDCl3): δ = 7.85 (br. s, 2 H, NHAc, 8-H), 7.40–7.11 (m, 10 H,
Ph), 6.67 (br. s, 1 H, CONHBn), 6.43 (br. s, 1 H, NHBn), 6.17 (d,
δ = 8.35 (br. t, 1 H, NH), 8.10 (s, 1 H, 8-H), 7.34–7.15 (m, 5 H,
3
Ph), 7.02 (t, 1 H, NH), 5.86 (d, J1Ј,2Ј = 9.3 Hz, 1 H, 1Ј-H), 5.74
3
3
(t, J2Ј,3Ј
=
3J1Ј,2Ј = 9.3 Hz, 1 H, 2Ј-H), 5.52 (t, J2Ј,3Ј
=
3J3Ј,4Ј
=
3J1Ј,2Ј = 1.7 Hz, 1 H, 1Ј-H), 5.62 (dd, J2Ј,3Ј = 1.6, J3Ј,4Ј = 4.0 Hz, 9.3 Hz, 1 H, 3Ј-H), 5.44 (t, J3Ј,4Ј
=
3J4Ј,5Ј = 9.3 Hz, 1 H, 4Ј-H),
3
3
3
3
3
2
3
1 H, 3Ј-H), 5.58 (br. t, 1 H, 2Ј-H), 5.43 (d, J4Ј,5Ј = 8.8 Hz, 1 H, 4.47–4.31 (m, JNH,a-H
=
3JNH,b-H = 5.8, Ja,b = 15.2, J4Ј,5Ј
=
Eur. J. Org. Chem. 2014, 2770–2779
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2777

N. M. Xavier, S. Schwarz, P. D. Vaz, R. Csuk, A. P. Rauter
9.3 Hz, 3 H, 2 H, CH2Ph, 5Ј-H), 2.42 (s, 3 H, CH3, NHAc), 2.11, 1.80 (3 s, 9 H, 3 CH3, OAc) ppm. 13C NMR (100 MHz, [D4]meth-
FULL PAPER
2.05, 1.85 (3 s, 9 H, 3 CH3, OAc) ppm. 13C NMR (100 MHz,
CDCl3): δ = 169.9, 169.7, 169.1, 165.5 (CO), 152.7 (C-4), 152.5,
152.0 (C-2, C-6), 142.3 (C-8), 137.7 (Cq, Ph), 128.8 (CH, Ph), 128.2
(C-5), 128.0, 127.7 (CH, Ph), 81.1 (C-1Ј), 75.5 (C-5Ј), 72.2 (C-3Ј),
69.4 (C-2Ј), 69.2 (C-4Ј), 43.3 (CH2Ph), 25.3 (CH3, NHAc), 20.8,
20.6, 20.3 (3 CH3, OAc) ppm. HRMS: calcd. for C26H27ClN6O9
[M + H]+ 603.1601; found 603.1583; calcd. for [M + Na]+ 625.1420;
found 625.1401.
anol): δ = 171.4, 171.1, 170.7 (CO), 154.1, 150.6 (C-8), 120.0 (C-
5), 81.7 (C-1Ј), 78.4 (C-5Ј), 74.1 (C-3Ј), 73.0 (C-2Ј), 71.2 (C-4Ј), 24.6
(CH3, NHAc), 20.7, 20.5, 20.0 (CH3, OAc) ppm. HRMS: calcd. for
C19H20ClN5O10 [M + Na]+ 536.0791; found 536.0779; calcd. for
[M – H + 2Na]+ 558.0610; found 558.0595.
Enzymatic Studies
Spectrophometer and Chemicals: A TECAN SpectraFluorPlus
working in kinetic mode and measuring the absorbance at 415 nm
was used for the enzymatic studies. Acetylcholinesterase (from
Electrophorus electricus), 5,5Ј-dithiobis(2-nitrobenzoic acid)
(DTNB), and acetylthiocholine iodide (ATChI) were purchased
from Fluka. Butyrylcholinesterase (from equine serum) was pur-
chased from Sigma, and butyrylthiocholine iodide (BTChI) was
bought from Aldrich.
Data for 19: Rf = 0.3 (EtOAc), m.p. 137.6–139.3 °C. [α]2D0 = –2 (c
1
= 0.8, CHCl3). H NMR (400 MHz, CDCl3): δ = 8.38 (s, 1 H, 8-
H), 8.32 (br. s, 1 H, NH), 7.32–7.13 (m, 5 H, Ph), 7.00 (br.t, 1 H,
3
NH), 6.16 (br. d, 1 H, 1Ј-H), 5.71 (br.t, 1 H, 2Ј-H), 5.55 (t, J2Ј,3Ј
3
3
3
= J3Ј,4Ј = 9.4 Hz, 1 H, 3Ј-H), 5.48 (t, J3Ј,4Ј ≈ J4Ј,5Ј ≈ 9.4 Hz, 1 H,
2
3
4Ј-H), 4.48 (dd, part A of ABX system, Ja,b = 14.7, JNH,H-a
=
3
6.4 Hz, 1 H, H-a from CH2Ph), 4.39 (d, J4Ј,5Ј = 9.6 Hz, 1 H, 5Ј-
H), 4.31 (dd, part B of ABX system, 2Ja,b = 14.7, JNH,b-H = 5.3 Hz,
1 H, b-H from CH2Ph), 2.50 (s, 3 H, CH3, NHAc), 2.12, 2.07, 1.90
(3 s, 9 H, 3 CH3, OAc) ppm. 13C NMR (100 MHz, CDCl3): δ =
169.9, 169.8 (CO), 165.3 (CO, amide), 153.0, 148.5 (C-8), 137.4
(Cq, Ph), 128.8 (CH, Ph), 128.0, 127.8 (CH, Ph), 75.2 (C-5Ј), 72.5
(C-3Ј),* 69.4 (C-2Ј),** 68.9 (C-4Ј), 43.2 (CH2Ph), 25.3 (CH3,
NHAc), 20.8, 20.6, 20.4 (3 CH3, OAc). * inferred from HMBC,
** inferred from HMQC. HRMS: calcd. for C26H27ClN6O9 [M +
H]+ 603.1601; found 603.1582; calcd. for [M + Na]+ 625.1420;
found 625.1400.
Preparation of Solutions: Preparation of Tris-HCl buffer (50 mm;
pH 8.0): Tris(hydroxymethyl)aminomethane (606 mg) was dis-
solved in double-distilled water (100 mL) and the pH was adjusted
to 8.0Ϯ0.1 by the addition HCl. Buffer was freshly prepared and
stored in the refrigerator. AChE solution (2.005 U/mL): the enzyme
(271 U/mg; 0.037 mg) was dissolved in freshly prepared buffer pH
8.0 (5 mL) containing NaN3 (0.98 mg). BChE solution (2.040 U/
mL): the enzyme (7.54 U/mg, 1.353 mg) was dissolved in freshly
prepared buffer pH 8.0 (5 mL) containing NaN3 (0.98 mg). DTNB
solution (3 mm): DTNB (23.8 mg) was dissolved in freshly prepared
buffer pH 8.0 (20 mL) containing NaCl (116.8 mg) and MgCl2
(38.0 mg). ATChI solution (15 mm): ATChI (43.4 mg) was dis-
solved in double-distilled water (10 mL). BTChI solution (15 mm):
BTChI (47.6 mg) was dissolved in double-distilled water (10 mL).
All solutions were stored in eppendorf vials in the refrigerator or
freezer, if necessary. The pure compounds were initially dissolved
in DMSO. Galantamine hydrobromide as standard was dissolved
in double-distilled water. The final concentrations for the enzymatic
assay were obtained by diluting the stock solution with double-
distilled water. No inhibition by residual DMSO was detected
(Ͻ 0.5%).
1-(2-Acetamido-6-chloropurin-9-yl)-2,3,4-tri-O-acetyl-β-
anuronic Acid (22) and 1-(2-Acetamido-6-chloropurin-7-yl)-2,3,4-
tri-O-acetyl-β- -glucopyranuronic Acid (23): 2-Acetamido-6-chloro-
D-glucopyr-
D
purine (121 mg, 0.57 mmol) was suspended in anhydrous aceto-
nitrile (2.5 mL) under N2, and N,O-bis(trimethylsilyl)acetamide
(0.28 mL, 1.14 mmol) was added. The mixture was stirred at room
temp. for 20 min. Then, a solution of 2,3,4-tri-O-acetyl-β-d-gluco-
pyranurono-6,1-lactone (115 mg, 0.38 mmol) in anhydrous aceto-
nitrile (2.5 mL) was added, followed by dropwise addition of tri-
methylsilyl triflate (0.45 mL, 2.47 mmol). The reaction mixture was
exposed to microwave irradiation at 150 W (Pmax = 250 Psi), and it
was continuously stirred at 65 °C for 50 min. The mixture was then
treated with triethylamine and concentrated. The residue was puri-
fied by flash column chromatography on silica gel (using ethyl acet-
ate to ethyl acetate/methanol, 4:1 to 1:1) to give N9 nucleoside 22
(102 mg, 52%) and its N7 regioisomer 23 (25 mg, 13%) as colour-
less oils.
Enzyme Assays: A mixture of the DTNB solution (125 μL), enzyme
(25 μL), and solutions of test compounds (25 μL, three different
concentrations and once blank water) was prepared, and then incu-
bated at 30 °C for 20 min. The substrate (25 μL, four different con-
centrations) was added to start the enzymatic reaction. The ab-
sorbance data (415 nm) was recorded at a controlled temperature
of 30 °C for 30 min at 1 min intervals. All measurements were per-
formed in triplicate. The final concentrations in the test were as
follows: [AChE] = 2.005 U/mL, [BChE] = 2.040 U/mL, [DTNB]
= 3 mm, [ATChI] = [BTChI] = 0.9375 mm, 0.625 mm, 0.325 mm,
0.1875 mm. The mode of inhibition as well as Ki and KiЈ were deter-
mined using Lineweaver–Burk plot,[26] Dixon plot,[27] and Cornish–
Bowden plot.[28]
Data for 22: Rf = 0.41 (EtOAc/MeOH, 1:1). [α]2D0 = +8 (c = 0.8,
1
MeOH). H NMR (400 MHz, [D4]methanol): δ = 8.81 (s, 1 H, 8-
3
3
H), 6.29 (d, J1Ј,2Ј = 9.1 Hz, 1 H, 1Ј-H), 5.69 (t, J1Ј,2Ј
= =
3J2Ј,3Ј
9.1 Hz, 1 H, 2Ј-H), 5.55 (br. t, 3J2Ј,3Ј = 9.1, J3Ј,4Ј = 9.8 Hz, 1 H, 3Ј-
3
3
H), 5.40 (t, J3Ј,4Ј
= =
3J4Ј,5Ј = 9.8 Hz, 1 H, 4Ј-H), 4.24 (d, J4Ј,5Ј
9.8 Hz, 1 H, 5Ј-H), 2.33 (s, 3 H, CH3, NHAc), 2.05, 2.01, 1.73 (3
s, 9 H, 3 CH3, OAc) ppm. 13C NMR (100 MHz, [D4]methanol):
δ = 173.2 (COOH), 171.4, 171.2, 170.8 (CO), 153.9 (C-4), 151.8
(C-2, C-6), 146.0 (C-8), 128.5 (C-5), 81.6 (C-1Ј), 78.3 (C-5Ј), 74.1
(C-3Ј), 72.2 (C-2Ј), 71.3 (C-4Ј), 24.7 (CH3, NHAc), 20.7, 20.5, 20.0
(CH3, OAc) ppm. HRMS: calcd. for C19H20ClN5O10 [M + Na]+
536.0791; found 536.0780; calcd. for [M – H + 2Na]+ 558.0610;
found 558.0598.
Supporting Information (see footnote on the first page of this arti-
1
cle): Copies of the H and 13C NMR spectra of new compounds.
Acknowledgments
Data for 23: Rf = 0.22 (EtOAc/MeOH, 1:1). [α]2D0 = +30 (c = 1.1,
1
MeOH). H NMR (400 MHz, [D4]methanol): δ = 9.08 (s, 1 H, 8-
The authors thank Fundação para a Ciência e Tecnologia (FCT)
3
3
H), 6.38 (d, J1Ј,2Ј = 8.8 Hz, 1 H, 1Ј-H), 5.67 (br. t, J1Ј,2Ј = 8.8, for financial support through the PEst-OE/QUI/UI0612/2013 pro-
3J2Ј,3Ј = 9.4 Hz, 1 H, 2Ј-H), 5.59 (t, J2Ј,3Ј = J3Ј,4Ј = 9.4 Hz, 1 H,
ject, and for the postdoctoral research grants to N. M. X. and S. S.
(grants SFRH/BPD/91226/2012 and SFRH/BPD/81065/2011,
3
3
3
3
3Ј-H), 5.41 (t, J3Ј,4Ј = 9.4, J4Ј,5Ј = 9.9 Hz, 1 H, 4Ј-H), 4.27 (d,
3J4Ј,5Ј = 9.9 Hz, 1 H, 5Ј-H), 2.30 (s, 3 H, CH3, NHAc), 2.05, 2.01, respectively).
2778
www.eurjoc.org
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 2770–2779

Synthesis of Purine Nucleosides
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Received: December 23, 2013
Published Online: March 6, 2014
Eur. J. Org. Chem. 2014, 2770–2779
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