First synthesis of [6-15N]-cladribine using ribonucleoside as a starting material

Article abstract of DOI:10.3987/COM-11-12382

We have synthesized two types of [6-15N]-deoxyadenosine analogs: [6-15N]-2'-deoxyadenosine (1) and [6-15N]-2-chloro-2'-deoxyadenosine ([6-15N]-cladribine, 2), which utilized the readily available ribonucleosides inosine and guanosine, respectively, via 3'-benzoyl-5'-trityl- (or tert-butyldimethylsilyl)-substituted intermediates (6, 15).

Full text of DOI:10.3987/COM-11-12382

HETEROCYCLES, Vol. 85, No. 1, 2012
171
HETEROCYCLES, Vol. 85, No. 1, 2012, pp. 171 - 182. © 2012 The Japan Institute of Heterocyclic Chemistry
Received, 31st October, 2011, Accepted, 30th November, 2011, Published online, 2nd December, 2011
DOI: 10.3987/COM-11-12382
FIRST
SYNTHESIS
OF
[6-15N]-CLADRIBINE
USING
RIBONUCLEOSIDE AS A STARTING MATERIAL
Norikazu Sakakibara, Ai Kakoh, and Tokumi Maruyama*
* Faculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima Bunri
University, 1314-1 Shido, Sanuki City, Kagawa, 769-2193, Japan:
maruyama@kph.bunri-u.ac.jp
Abstract – We have synthesized two types of [6-15N]-deoxyadenosine analogs:
[6-15N]-2′-deoxyadenosine
([6-15N]-cladribine, 2), which utilized the readily available ribonucleosides
inosine and guanosine, respectively, via 3′-benzoyl-5′-trityl- (or
tert-butyldimethylsilyl)-substituted intermediates (6, 15).
(1)
and
[6-15N]-2-chloro-2′-deoxyadenosine
15N-labelled adenosine derivatives can be applicable to a variety of intended purposes. Practically
speaking, many of the [15N]-nitrogen-stable isotope-labeled nucleosides are synthetic building blocks of
15N-labelled oligonucleotides, which are more commonly utilized as probes that help clarify the structure
of nucleic acids, the mechanism of drug binding, and the interactions between proteins and nucleosides by
1
15
the use of H-NMR spectra, N-NMR spectra, or both.1-3 In particular, [6-15N]-2′-deoxyadenosine (1)
was synthesized relatively early once this nucleoside analog started to attract attention as a probe material.
Jones et al. reported a synthetic process of this labeled compound in which deoxyadenosine was
enzymatically deaminated to give deoxyinosine, which was then di-acetylated at the 3′- and 5′-hydroxy
groups, and then O6-sulfonylation, followed by [15N]-benzylamination at the 6-position of the adenine
skeleton, following oxidation with ruthenium oxide.1 In addition, Kelly and co-workers have synthesized
[6-15N]-deoxyadenosine (1) in two steps from commercially available starting material, 6-chloropurine
using the enzymatic transglycosylation method.2 Their approach has allowed for the simple and
straightforward construction of large quantities of this material. While Maruyama et al. in our laboratory
synthesized an effectively potent anti-HIV agent, FddA, using a 6-chloropurine riboside analog as a
starting material via the conversion of selective 3′-benzoylation, 2′-fluorination, and 3′-deoxygenation in
the sugar moiety.4,5 In this study, we adapted Maruyama’s approach to develop an alternative synthetic
route to [6-15N]-2′-deoxyadenosine (1) from the readily available and economical ribonucleoside inosine,
using it as the starting substance.

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HETEROCYCLES, Vol. 85, No. 1, 2012
On the other hand, 2-chloro-2′-deoxyadenosine, also known as cladribine, is utilized as a therapeutic
agent of hairy cell leukemia and disseminated sclerosis. However, it remains a mystery as to how to exert
cladribine’s action mechanism against these diseases.6,7 To solve these problems, tracer experiments with
cladribine must be carried out in order to understand the effect administration of cladribine has on the
living system in terms of body distribution, modification, metabolism, and excretion by the use of several
analytical strategies such as metabolome analysis with a Fourier-transform ion cyclotron resonance-mass
spectrometer (FT-ICR-MS)8 and metabolic profiling using nuclear magnetic resonance (NMR).9 In the
organic synthesis, Robins et al. reported an effective synthetic approach for forming unlabeled cladribine
using 2′-deoxyguanosine as the starting material, which is, however, very expensive among the
commercially available nucleic acid reagents.10 Therefore, by the application of the synthetic process for
the above-described [6-15N]-2′-deoxyadenosine (1), we succeeded for the first time in synthesizing a
nitrogen-stable isotope-labeled novel compound [6-15N]-cladribine (2) from the commercially available
and inexpensive ribonucleoside, guanosine.
An alternative synthetic process of [6-15N]-2′-deoxyadenosine (1)
Compound 3 had previously been synthesized in five steps by Maruyama et al.4,5,11 from the starting
material inosine (Scheme 1). First, the deoxygenation of compound 3 was conducted at the 2′-position of
the hydroxyl group by the Barton-McCombie method using silicon.12 Compound 3 was then esterified
with phenyl chlorothionoformate13 to give product 4 in 91% yield. Then, the resulting compound 4 was
treated with diphenylsilane4,5 in the presence of 2,2′-azobis(isobutyronitrile) (AIBN) under a nitrogen
atmosphere, providing the corresponding deoxidized product 5 in 85% yield. Next, we attempted the
general reaction for the amination at the 6-position in the purine skeleton under the methanol solvent
condition.14 The 6-chloropurine riboside analog 5 was treated with [15N]-NH4Cl and KHCO3 in methanol
solution in the sealed tube at 100 C, furnishing the [6-15N]-amino-3′-debenzoylated product 7 in 82%
yield. Detritylation was then performed with 5% trifluoroacetic acid in chloroform solution,15 which
unfortunately resulted in the decomposition into adenine due to cleavage of the glycoside bond. To
overcome this challenge, we changed of our point of view and assumed that the free 3′-hydroxyl group in
the pentose system of compound 7 accelerated the nucleoside degradation under the acidic condition,
hence, 5′-detritylation should be carried out before the 3′-debenzoylation. That is, during the substitution
reaction to form the [15N]-amino group at the 6-position in the purine moiety of 5, we examined the
feasibility of carrying out the reaction condition without undergoing the 3ꞌ-debenzoylation: compound 5
was treated with 2.0 and 3.0 equivalents of [15N]-NH4Cl and KHCO3 in the DMSO solution in a sealed
tube at 80 C.15 As an interesting result, [6-15N]-aminated product 6 was obtained in 99% yield without
undergoing simultaneous 3′-debenzoylation. This consequence suggested that in a DMSO solvent,

HETEROCYCLES, Vol. 85, No. 1, 2012
173
Cl
Cl
N
N
N
N
N
Ph
O
5 steps
N
N
N
4
i
Ph3CO
Ph3CO
inosine
O
O
3
BzO
OH
BzO
O
O
15NH2
Cl
S
N
N
N
N
N
iii
ii
N
N
N
5
Ph3CO
Ph3CO
O
vi
15NH2
6
BzO
BzO
N
N
N
iv
N
Ph3CO
O
7
15NH2
15NH2
HO
v
N
N
N
N
N
N
iv
6
N
N
HO
HO
O
O
[6-15N]-deoxy-
adenosine (1)
8
BzO
HO
Reagents and conditions: i, ClC(S)(OPh), DMAP, CH2Cl2, rt, 0.5 h; ii, Ph2SiH2, AIBN, toluene, 100 C, 13 h;
iii, 15NH4Cl, KHCO3, DMSO, 100 C, 2 d; iv, 5% TFA in CHCl3, rt, 5 min; v, NH3 in MeOH, 100 C, 18 h;
vi, 15NH4Cl, KHCO3, MeOH, 100 C, 2 d
Scheme 1. Synthesis of [6-15N]-deoxyadenosine (1)
[15N]-ammonia, which is generated by [15N]-NH4Cl and KHCO3 in the reactive system, essentially failed
to conduct a nucleophilic attack on the carbonyl carbon at the 3′-benzoyl group. It is noteworthy that this
is the first reported conversion of amination at the 6-position of a 6-chloropurine riboside analog whose
sugar alcohol is protected by a benzoyl group without undergoing deprotection of the benzoyl group at
the sugar hydroxyl moiety. There are two advantages to this approach: i) It enables the minimum usage of
a stable-labeled compound, [15N]-NH4Cl, which is a relatively expensive reagent, without consuming the
debenzoylation, ii) It allows the selective deprotection at the 3′ or 5′ protecting group of compound 6
because of negligible debenzoylation. The resulting compound 6 was treated with 5% trifluoroacetic acid
in chloroform to give predictably the detritylated product 8 in 94% yield without any decomposition.16
This result demonstrated that the deoxyadenosine derivative 6, whose pentose alcohol at the 3′-position is
protected by a benzoyl group, is less likely to experience cleavage of the glycoside linkage in comparison
to the case of the detritylation of 7. Thus, we were able to synthesize [6-15N]-2′-deoxyadenosine (1) by
carrying out the 3′-debenzoylation according to the previous method that utilized a methanolic ammonia
solution.17 Meanwhile, the spectrum data of 1 were found to be in identical with the corresponding
previous data.1

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HETEROCYCLES, Vol. 85, No. 1, 2012
First synthesis of [6-15N]-2-chloro-2′-deoxyadenosine ([6-15N]-cladribine, 2)
[6-15N]-cladribine (2) synthesis began with guanosine as a starting material, which led to the 6-chloro
analog 9 in three steps according to a previous report (Scheme 2).18 The resulting 9 was added to
dibutyltin oxide (IV), then we carried out the monobenzoylation11 with benzoyl chloride, followed by
separation on silica-gel chromatography, providing the desired 3′-O-benzoate 10a and its regioisomer
2′-O-benzoate 10b in 61% and 9% yields respectively. Next, formation of the desired product 10a was
carried out by the 5′-selective silylation using one equivalent of tert-butyldimethylsilyl chloride (TBSCl),
imidazole, and DMF, to give the corresponding 5′-silylated inseparable mixture of 11a and 11b in 94%
combined yield (ratio 11a/11b: 78/22).19 Since 10a was identified as a pure single compound by 1H-NMR
analysis, we assumed that the acyl migration of 10a occurred during the 5′-silylated reaction.4,5 Following
deoxygenation by the above-described Barton-McCombie method,12 phenoxythiocarbonylation of the
hydroxyl group at the 2′- or 3′-position was carried out to obtain the corresponding ester 12a and 12b as
the inseparable regioisomeric mixture in 89% combined yield (ratio 12a/12b: 79/21).13 Then, the mixture
of 12a and 12b was treated with diphenylsilane in the presence of AIBN under the nitrogen atmosphere,4,5
Cl
N
Cl
N
N
N
N
N
N
N
3 steps
NH2
NH2
i
HO
HO
guanosine
O
O
10a
+
HO
9
O
BzO
OH
OH OH
Cl
10b
Cl
HO
OBz
N
N
N
N
N
ii
iii
iv
N
N
NH2
N
NH2
HO
TBSO
12a
TBSO
11a
O
O
+
+
TBSO
12b
TBSO
11b
O
O
BzO
O(CS)OPh
BzO
OH
Cl
PhO(SC)O
OBz
vi
OBz
Cl
N
N
N
N
N
v
N
NH2
TBSO
N
O
N
Cl
TBSO
TBSO
O
O
13a
+
14
BzO
13b
BzO
OBz
vii
15NH2
15NH2
15NH2
N
N
N
N
N
N
N
N
viii
N
N
Cl
N
Cl
N
Cl
TBSO
BzO
HO
HO
O
O
O
[6-15N]-cladribine
(2)
15
16
BzO
HO
Reagents and conditions: i, (n-Bu)2Sn=O, MeOH then BzCl, Et3N, rt, 0.5 h; ii, TBSCl, imidazole, DMF, rt, 20 h; iii,
ClC(S)(OPh), DMAP, CH2Cl2, rt, 1 h; iv, Ph2SiH2, AIBN, toluene, 100 C, 7 h; v, BTEA-Cl, SbCl3, NaNO2,
Cl2CHCO2H, CH2Cl2, rt, 17 h; vi, 15NH4Cl, KHCO3, DMSO, 100 C, 1 d; vii, TBAF in THF, rt, 5 min; viii, 2M NH3 in
MeOH, rt, 12 h
Scheme 2. Synthesis of [6-15N]-cladribine (1)

HETEROCYCLES, Vol. 85, No. 1, 2012
175
followed by the separation of silica-gel, providing 3′-O-benzoate 13a as the isolated product and the
corresponding 2′-O-benzoate 13b in 69% and 21% yields, respectively. The resulting 13a was reacted
with benzyltriethylammonium chloride, dichloroacetic acid, antimony trichloride, and sodium nitrite in
dichloromethane solution to provide the 2,6-dichloropurine riboside analog 14 in 90% yield, reported by
Robins et al.10 In the same manner as the [6-15N]-aminated reaction in Scheme 1, compound 14 was
converted into the [6-15N]-6-amino-nucleoside 15 without undergoing the debenzoylation at the 3′-positon
1
by the use of [15N]-NH4Cl and KHCO3 in DMSO solution. The H-NMR spectrum of 15 revealed an
amino proton signal at  7.82 (2H, d, J = 90.8 Hz), supporting evidence of the presence of a [15N]-labeled
amino group at the 6-positon in the purine skeleton in 15. Finally, deprotective reactions of TBS group at
the 5′-position and benzoyl group at the 3′-positon were carried out in high yields, providing the objective
15
compound, [6-15N]-cladribine (2). The molecular formula of 2 was determined to be C10H12CIN4 NNaO3
from the HRMS (ESI) at m/z 309.04885 [M+Na]+ (Calcd for 309.04912). In its 1H-NMR spectrum, there
15
were significant differences in the chemical shifts for the N-labelled amino group at the 6-position of
the purine system,  7.75 (2H, d, J = 90.8 Hz), in comparison to the previous report for unlabelled
1
cladribine about H, 13C-NMR, and HRMS data,20 suggesting the introduction of [6-15N]-aminated
cladribine, namely 2.
In summary [6-15N]-cladribine (2) has been first synthesized from the readily available ribonucleoside,
guanosine, to further medical and pharmaceutical progress, and further research is necessary using these
labeled compounds.
EXPERIMENTAL
Instrumentation
13
15
1H-, C- and N-NMR spectra were taken with a UltrashieldTM 400 Plus FT NMR System (BRUKER).
Chemical shifts and coupling constants (J) were given in d and Hz, respectively. High-resolution mass
spectrometry (HRMS) was performed on a APEX IV mass spectrometer (BRUKER) with electrospray
ionization mass spectroscopy (ESI-MS). Melting points (mp) were determined using a YANACO
micro-melting point apparatus (MP-500D) and are uncorrected.
9-(3-O-Benzoyl-2-O-phenoxythiocarbonyl-5-O-trityl--D-ribofuranosyl)-6-chloropurine (4)
Compound 3 (633.1 mg, 1.00 mmol) was dissolved in dry CH2Cl2 (10.0 mL), under nitrogen atmosphere.
To this stirred solution was carefully added ClC(S)(OPh) (220.0 L, 1.63 mmol) and DMAP (273.0 mg,
2.33 mmol). Stirring was continued at room temperature for 0.5 h and the mixture was extracted with
AcOEt. The organic extracts were washed with water, saturated aqueous sodium chloride solution, and
dried with sodium sulfate, and then evaporated. The residue was purified by silica gel column

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HETEROCYCLES, Vol. 85, No. 1, 2012
1
chromatography (33% AcOEt in hexane) to give 4 as oil (700.0 mg, 0.91 mmol, 91%). H-NMR
(400MHz, CDCl3):  8.72 (1H, s, H-8), 8.39 (1H, s, H-2), 8.14 (2H, m, 3′O-Bz), 7.66 (1H, m, 3′O-Bz),
6.96−7.55 (22H, m, 3′O-Bz, 2′O-Ph, 5′O-Tr), 6.79 (1H, dd, J = 6.0 and 5.6, H-2′), 6.63 (1H, d, J = 6.4,
H-1′), 6.27 (1H, dd, J = 5.6 and 3.2, H-3′), 4.63 (1H, m, H-4′), 3.66 (1H, dd, J = 10.8 and 3.2 Hz, H-5′a),
3.61 (1H, dd, J = 10.4 and 3.2 Hz, H-5′b); HRMS (ESI) Calcd for C43H33ClN4NaO6S [M+Na]+:
791.17015. Found 791.170100.
9-(3-O-Benzoyl-5-O-trityl--D-2-deoxyribofuranosyl)-6-chloropurine (5)
Compound 4 (689.0 mg, 0.89 mmol) was dissolved in toluene (4.0 mL), and AIBN (60.0 mg, 0.37 mmol)
and diphenylsilane (0.6 mL, 1.95 mmol) was added to the solution, and then stirred for 13 h at 100 C
under nitrogen atmosphere. The mixture was then extracted with AcOEt, and the organic extracts were
washed with saturated aqueous sodium chloride solution, and dried with sodium sulfate, and then
evaporated. The residue was purified by silica gel column chromatography (50% AcOEt in hexane) to
give crystals 5 (392.7 mg, 0.63 mmol, 71%). 1H-NMR (400MHz, CDCl3):  8.67 (1H, s, H-8), 8.33 (1H, s,
H-2), 8.08 (2H, m, 3′O-Bz), 7.66 (1H, m, 3′O-Bz), 7.48 (2H, m, 3′O-Bz), 7.20-7.44 (15H, m, 5′O-Tr),
6.59 (1H, dd, J = 8.4 and 6.0 Hz, H-1′), 5.81 (1H, m, H-3′), 4.50 (1H, m, H-4′), 3.58 (1H, dd, J = 10.4 and
4.0 Hz, H-5a′), 3.49 (1H, dd, J = 10.4 and 4.0 Hz, H-5b′), 3.66 (1H, ddd, J = 14.0, 8.0 and 6.0 Hz, H-2′a),
2.85 (1H, ddd, J = 14.0, 6.0 and 2.0 Hz, H-2′b); HRMS (ESI) Calcd for C36H29ClN4NaO4 [M+Na]+:
639.17695; Found 639.17513; mp 80.1−84.5 C.
[6-15N]-3′-Benzoyl-5′-trityldeoxyadenosine (6)
Compound 5 (124.0 mg, 0.2 mmol) was dissolved in DMSO (3.0 mL), and 15NH4Cl (21.7 mg, 0.4 mmol),
KHCO3 (60.0 mg, 0.6 mmol) was added to the solution, and then sealed and stirred for 2 d at 100 C. The
mixture was extracted with AcOEt, and washed with saturated aqueous sodium chloride solution, and
dried with sodium sulfate, and then evaporated. The residue was purified by silica gel column
1
chromatography (10% MeOH in AcOEt) to give crystals 6 (118.8 mg, 0.20 mmol, 99%). H-NMR
(400MHz, CDCl3): 8.31 (1H, s, H-8), 8.07 (2H, m, 3′O-Bz), 8.01 (1H, s, H-2), 7.62 (1H, m, 3′O-Bz),
7.44 (2H, m, 3′O-Bz), 7.20-7.41 (15H, m, 5′O-Tr), 6.55 (1H, dd, J = 8.4 and 5.6 Hz, H-1′), 5.77 (1H, m,
15
H-3′), 5.77 (1H, d, J = 90.0 Hz, NH2), 4.45 (1H, m, H-4′), 3.50-3.65 (2H, m, H-5′ab), 2.78 (1H, m,
15
H-2′a), 2.75 (1H, m, H-2′b); HRMS (ESI) Calcd for C36H31N4 NNaO4 [M+Na]+: 621.22386. Found
621.21765; mp 99.9−101.8 C.
[6-15N]-5′-trityl-2′-deoxyadenosine (7)
15
Compound 5 (617.1 mg, 1.0 mmol) was dissolved in MeOH (15.0 mL), and NH4Cl (1,000 mg, 18.4
mmol), KHCO3 (1.84 g, 18.4 mmol) was added to the solution, and then sealed and stirred for 1 d at 100
C. The mixture was extracted with AcOEt, and washed with saturated aqueous sodium chloride solution,

HETEROCYCLES, Vol. 85, No. 1, 2012
177
and dried with sodium sulfate, and then evaporated. The residue was purified by silica gel column
1
chromatography (25% MeOH in CH2Cl2) to give crystals 7 (405.7 mg, 0.82 mmol, 82%). H-NMR
(400MHz, DMSO-d6):  8.23 (1H, s, H-8), 8.07 (1H, s, H-2), 7.20-7.28 (15H, m, 3′O-Tr), 7.26 (1H, d, J
= 90.0 Hz, 15NH2), 6.37 (1H, m, H-1′), 5.37 (1H, d, J = 4.4 Hz, 3′=OH), 4.49 (1H, m, H-3′), 3.39 (1H, m,
H-4′), 3.15-3.22 (2H, m, H-5′ab), 2.88 (1H, m, H-2′a), 2.32 (1H, m, H-2′b); HRMS (ESI) Calcd for
15
C29H27N4 NNaO3 [M+Na]+: 517.19765. Found 517.19832; mp 193.2−195.0 C.
[6-15N]-3′-Benzoyl-2′-deoxyadenosine (8)
Compound 6 (1180.0mg, 1.97 mmol) was dissolved in dry CHCl3 (60.0 mL) and CF3CO2H (3.0 mL) was
added to the solution, and then stirred for 5 min at room temperature. The reaction was quenched with
NaHCO3 (6.7g, 80.0 mmol), and the mixture was evaporated in vacuo and the residue was purified by
silica gel column chromatography (14% MeOH in CHCl3) to give a crystal 8 (660.0 mg, 1.85 mmol,
94%). 1H-NMR (400MHz, DMSO-d6):  8.38 (1H, s, H-8), 8.31 (1H, s, H-2), 8.06 (2H, m, 3′O-Bz), 7.71
(1H, m, 3′O-Bz), 7.58 (2H, m, 3′O-Bz), 7.29 (2H, d, J = 90.0 Hz, 15NH2), 6.48 (1 H, dd, J = 8.8 and 5.6
Hz, H-1′), 5.64 (1H, m, H-3′), 5.57 (1H, dd, J 6.8 and 4.8 Hz, 5′6OH), 4.27 (1H, m, H-4′), 3.65-3.68 (2H,
15
m, H-5′ab), 3.08 (1H, m, H-2′a), 2.63 (1H, m, H-2′b); HRMS (ESI) Calcd for C17H17N4 NNaO4
[M+Na]+: 379.11431. Found 379.11191; mp 248.0−249.5 C.
[6-15N]-2′-Deoxyadenosine (1)
Compound 8 (997.2 mg, 2.80 mmol) was dissolved in 2N NH3/MeOH (50.0 mL), and then sealed and
stirred for 18 h at 100 C. The mixture was evaporated, and the residue was purified by silica gel column
1
chromatography (25% MeOH in CHCl3) to give crystals 1 (505.3 mg, 2.00 mmol, 72%). H-NMR
(400MHz, DMSO-d6):  8.31 (1H, s, H-8), 8.11 (1H, s, H-2), 7.29 (2H, d, J = 90.0 Hz, 15NH2), 6.32 (1H,
dd, J = 7.6 and 6.0 Hz, H-1′), 5.29 (1H, d, J = 4.0 Hz, 3′=OH), 5.22 (1H, dd, J = 6.8 and 4.8 Hz, 3′=OH),
4.39 (1H, m, H-3′), 3.86 (1H, m, H-4′), 3.60 (1H, ddd, J = 12.0, 9.2 and 4.8 Hz, H-5′a), 3.50 (1H, ddd, J =
11.6, 6.8, and 4.4 Hz, H-5′b), 2.71 (1H, ddd, J = 13.2, 7.6 and 5.6 Hz, H-2′a), 2.23 (1H, ddd, J = 13.2, 6.0
and 2.8 Hz, H-2′b); 13C-NMR (100MHz, CD3OD):  156.1 (d, J = 21.0 Hz, C6), 152.1 (d, J = 2.0 Hz, C2),
148.5, 140.1, 119.4 (d, J = 3.0 Hz, C5), 88.5, 85.7, 71.6, 62.2, 40.1; 15N-NMR (40MHz, CD3OD):  83.8
15
(s, 15NH2); HRMS (ESI) Calcd for C10H13N4 NNaO3 [M+Na]+: 275.08810. Found 275.08800; mp
185.1−185.8 C.
2-Amino-9-(3-O-benzoyl--D-ribofuranosyl)-6-chloropurine (10a) and 2-Amino-9-(2-O-benzoyl-
-D-ribofuranosyl)-6-chloropurine (10b)
2-Amino-6-chloro-9-(-D-ribofuranosyl)purine (9) (286.8 mg, 1.00 mmol) and di-n-butyltin (IV) oxide
(249.2 mg, 1.00 mmol) was dissolved in MeOH (10.0 mL) was refluxed for 1 h, then added triethylamine
(0.70 mL, 5.00 mmol) and benzoyl chloride (0.58 mL, 5.00 mmol) and was stirred for 0.5 h at room

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HETEROCYCLES, Vol. 85, No. 1, 2012
temperature. The mixture was dissolved in AcOEt, and then extracted with AcOEt. The organic extracts
were washed with saturated aqueous sodium chloride solution, and dried with sodium sulfate. After
removal of the organic solvent, the residue was purified by silica gel column chromatography (AcOEt) to
give 3ive ied by si analogue (10a) as white crystals (182.9 mg, 0.48 mmol, 61%). Evaporation of the
second fraction gave 2vaporation of analogue (10b) as by-product and white crystals (22.4 mg, 0.05
mmol, 9%). 10a: 1H-NMR (400MHz, DMSO-d6):  8.45 (1H, s, H-8), 8.05 (2H, m, 3′O-Bz), 7.72 (1H, m,
3′O-Bz), 7.58 (2H, m, 3′O-Bz), 7.03 (2H, brs, NH2), 5.92-5.97 (2H, m, H-1′, 2′-OH), 5.53 (1H, dd, J = 5.6
and 2.4 Hz, H-3′), 5.30 (1H, t, J = 5.2, 5′-OH), 4.93 (1H, m, H-2′), 4.29 (1H, m, H-4′), 3.66-3.78 (2H, m,
H-5′ab); HRMS (ESI) Calcd for C17H16ClN5NaO5 [M+Na]+: 428.07322. Found 428.07436; mp 130.2−
132.0 C. 10b: 1H-NMR (400MHz, DMSO-d6):  8.46 (1H, s, H-8), 7.98 (2H, m, 2′O-Bz), 7.68 (1H, m,
2′O-Bz), 7.56 (2H, m, 2′O-Bz), 7.02 (2H, brs, NH2), 6.24 (1H, d, J = 5.2 Hz, H-1′), 5.69-5.77 (2H, m,
H-2′ and 3′-OH), 5.19 (1H, t, J = 5.2 Hz, 5′-OH), 4.59 (1H, m, H-3′), 4.09 (1H, m, H-4′), 3.62-3.81 (2H,
m, H-5′ab).
2-Amino-9-(3-O-benzoyl-5-O-tert-butyldimetylsilyl--D-ribofuranosyl)-6-chloropurine (11a) and
2-Amino-9-(2-O-benzoyl-5-O-tert-butyldimetylsilyl--D-ribofuranosyl)-6-chloropurine (11b)
Compound 10a (387.3 mg, 0.95 mmol) was dissolved in dry DMF (4.0 mL) and imidazole (152.4 mg,
2.24 mmol) and tert-butyldimetylchlorosilane (143.9 mg, 0.95 mmol) were added to the solution, and
then stirred for 20 h at room temperature. The mixture was extracted with AcOEt. The organic extracts
were washed with water, saturated aqueous sodium chloride solution, and dried with sodium sulfate, and
then evaporated. The residue was purified by silica gel column chromatography (33% AcOEt in hexane)
to give the inseparable mixture of 11a and 11b as a crystal (463.6 mg, 0.89 mmol, 94% combined yield),
ratio 78:22 according to 1H-NMR spectrum. 1H-NMR (400MHz, CDCl3):  8.10 (0.2H, s, H-8, 11b), 8.06
(0.8H, s, H-8, 11a), 7.99-8.04 (1.6H, m, 3′O-Bz, 11a), 7.90-7.95 (0.4H, m, 2′O-Bz, 11b), 7.44-7.53 (1H,
m, 2′O-Bz of 11b and 3′O-Bz of 11a), 7.31-7.39 (2H, m, 2′O-Bz of 11b and 3′O-Bz of 11a), 6.23 (0.2H,
d, J = 5.2 Hz, H-1′, 11b), 5.98 (0.8H, d, J = 6.4 Hz, H-1′, 11a), 5.67 (0.2H, m, H-2′, 11b), 5.53 (0.8H, dd,
J = 5.6 and 2.4 Hz, H-3′, 11a), 4.78 (0.8H, m, H-2′, 11a), 4.71 (0.2H, m, H-3′, 11b), 4.39 (0.8H, m, H-4′,
11a), 4.22 (0.2H, m, H-4′, 11b), 3.88-3.96 (0.4H, m, H-5′ab, 11b), 3.86 (1.6H, m, H-5′ab, 11a), 0.85
(1.8H, s, 5′O-TBS, 11b), 0.80 (7.2H, s, 5′O-TBS, 11a), 0.06 (0.6H, s, 5′O-TBS, 11b), 0.05 (0.6H, s,
5′O-TBS, 11b), 0.03 (2.4H, s, 5′O-TBS, 11a), 0.00 (2.4H, s, 5′O-TBS, 11a); HRMS (ESI) Calcd for
C23H30ClN5NaO5 [M+Na]+: 542.15969. Found 542.16198.
2-Amino-9-(3-O-benzoyl-2-O-phenoxythiocarbonyl-5-O-tert-butyldimetylsilyl--D-ribofuranosyl)-6-
chloropurine (12) and 2-Amino-9-(2-O-benzoyl-3-O-phenoxythicarbonyl-5-O-tert-butyldimetylsilyl-
-D-ribofuranosyl)-6-chloropurine (12b)

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The mixture of 11a and 11b (463.6 mg, 0.89 mmol) was dissolved in dry CH2Cl2 (7.26 mL), under
nitrogen atmosphere. To this stirred solution was carefully added ClC(S)(OPh) (254.1 L, 1.42 mmol)
and DMAP (261.4 mg, 2.14 mmol). Stirring was continued at room temperature for 1 h and the mixture
was extracted with AcOEt. The organic extracts were washed with water, saturated aqueous sodium
chloride solution, and dried with sodium sulfate, and then evaporated. The residue was purified by silica
gel column chromatography (50% AcOEt in hexane) to give the inseparable mixture of 12a and 12b as a
1
crystal (411.6 mg, 0.79 mmol, 89% combined yield), ratio 79:21 according to H-NMR spectrum.
1H-NMR (400MHz, CDCl3):  8.23 (0.8H, s, H-8, 12a), 8.20 (0.2H, s, H-8, 12b), 8.15 (1.6H, m, 3′O-Bz,
12a), 8.07 (0.4H, m, 2′O-Bz, 12b), 7.66 (0.8H, m, 3′O-Bz, 12a), 7.61 (0.2H, m, 2′O-Bz, 12b), 7.53 (1.6H,
m, 3′O-Bz, 12a), 7.46 (0.4H, m, 2′O-Bz, 12b), 6.35−7.41 (5H, m, Ph of 12a and 12b), 6.51 (0.8H, d, J =
7.2 Hz, H-1′, 12a), 6.45 (0.2H, d, J = 6.4 Hz, H-1′, 12b), 6.38 (0.8H, dd, J = 7.2 and 5.6 Hz, H-2′, 12a),
6.18 (0.2H, dd, J = 5.2 and 2.4 Hz, H-3′, 12b), 6.11 (0.2H, dd, J = 6.4 and 5.2 Hz, H-2′, 12b), 6.02 (0.8H,
dd, J = 5.6 and 2.0 Hz, H-3′, 12a), 5.12 (1.6H, brs, NH2, 12a), 5.05 (0.4H, brs, NH2, 12b), 4.64 (0.2H, m,
H-4′, 12b), 4.53 (0.8H, m, H-4′, 12a), 4.04−4.06 (0.4H, m, H-5′ab, 12b), 4.00−4.04 (1.6H, m, H-5′ab,
12a), 0.97 (1.8H, s, 5′O-TBS, 12b), 0.95 (7.2H, s, 5′O-TBS, 12a), 0.19 (0.6H, s, 5′O-TBS, 12b), 0.17
(2.4H, s, 5′O-TBS, 12a), 0.16 (2.4H, s, 5′O-TBS, 12a), 0.14 (0.6H, s, 5′O-TBS, 12b); HRMS (ESI) Calcd
for C30H34ClN5NaO6SSi [M+Na]+: 678.15798. Found 678.15932.
2-Amino-6-chloro-9-(3-O-benzoyl-5-O-tert-butyldimetylsilyl--D-ribofuranosyl)purine (13a) and
2-Amino-6-chloro-9-(2-O-benzoyl-5-O-tert-butyldimetylsilyl--D-ribofuranosyl)purine (13b)
The mixture of 12a and 12b (393.0 mg, 0.51 mmol) was dissolved in toluene (2.3 mL), and AIBN (34.1
mg, 0.21 mmol) and diphenylsilane (0.6 mL, 1.95 mmol) was added to the solution, and then stirred for 7
h at 100 C under nitrogen atmosphere. The mixture was then extracted with AcOEt, and the organic
extracts were washed with saturated aqueous sodium chloride solution, and dried with sodium sulfate, and
then evaporated. The residue was purified by silica gel column chromatography (50% AcOEt in hexane) to
give 3vefied by 13a as crystals (209.5 mg, 0.42 mmol, 69%). Evaporation of the second fraction gave
2vaporation 13b as crystals (64.6 mg, 0.13 mmol, 21%). 13a: 1H-NMR (400MHz, CDCl3):  8.20 (1H, s,
H-8), 8.08 (2H, m, 3′O-Bz), 7.62 (1H, m, 3′O-Bz), 7.49 (2H, m, 3′O-Bz), 6.47 (1H, dd, J = 8.4 and 6.0 Hz,
H-1′), 5.66 (1H, m, H-3′), 5.09 (2H, brs, NH2), 4.37 (1H, m, H-4′), 4.01 (1H, dd, J = 11.2 and 2.4 Hz,
H-5′a), 3.95 (1H, dd, J = 11.2 and 2.4 Hz, H-5′b), 2.74−2.78 (2H, m, H-2′ab), 0.93 (9H, s, 5′O-TBS), 0.15
(6H, m, 5′O-TBS); HRMS (ESI) Calcd for C23H30ClN5O4Si [M+Na]+: 526.16478. Found 526.16563; mp
1
67.7−68.4 C. 13b: H-NMR (400MHz, CDCl3):  8.23 (1H, s, H-8), 8.08 (2H, m, 2′O-Bz), 7.62 (1H, m,
2′O-Bz), 7.49 (2H, m, 2′O-Bz), 6.20 (1H, d, J = 1.6 Hz, H-1′), 5.83 (1H, m, H-2′), 5.03 (2H, brs, NH2),
4.58 (1H, m, H-4′), 4.07 (1H, dd, J = 11.2 and 2.8 Hz, H-5′a), 3.79 (1H, dd, J = 11.2 and 2.8 Hz, H-5′b),

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HETEROCYCLES, Vol. 85, No. 1, 2012
2.66 (1H, m, H-3′a), 2.26 (1H, dd, J = 5.6 and 3.6 Hz, H-3′b), 0.92 (9H, s, 5′O-TBS), 0.15 (3H, m,
5′O-TBS), 0.15 (3H, m, 5′O-TBS); HRMS (ESI) Calcd for C23H30ClN5O4Si [M+Na]+:526.16478. Found
526.16619; mp 92.0−95.1 C.
9-(3-O-Benzoyl-5-O-tert-butyldimetylsilyl--D-ribofuranosyl)-2,6-dichloropurine (14)
SbCl3 (684 mg 3 mmol) in CH2Cl2 (7.5 mL) was added to a mixture of compound 13a (756.0 mg, 1.5
mmol), benzyltriethylammonium chloride (340.5 mg 1.5 mmol), and NaNO2 (2.1 g, 30 mmol) in CH2Cl2
(60 mL). Cl2CHCO2H (246 L, 6 mmol) was added, and the flask was flushed with dried N2 and sealed,
and the mixture was stirred at room temperature. The reaction was completed after 17 h, and celite (3 g)
and CHCl3 were added, and filtered. The filtrate was then extracted with CHCl3, and the organic extracts
were washed with saturated aqueous sodium chloride solution, and dried with sodium sulfate, and then
evaporated. The residue was purified by silica gel column chromatography (AcOEt) to give crystals 14
(706.3 mg, 1.35 mmol, 90%). 1H-NMR (400MHz, CDCl3):  8.62 (1H, s, H-8), 8.09 (2H, m, 3′O-Bz), 7.63
(1H, m, 3′O-Bz), 7.50 (2H, m, 3′O-Bz), 6.65 (1H, dd, J = 8.4 and 6.0 Hz, H-1′), 5.66 (1H, m, H-3′), 4.43
(1H, m, H-4′), 3.98−4.07 (2H, m, H-5′ab), 2.88 (1H, m, H-2′a), 2.74 (1H, m, H-2′b), 0.96 (9H, s,
5′O-TBS), 0.16 (6H, m, 5′O-TBS); HRMS (ESI) Calcd for C23H28Cl2N4NaO4Si [M+Na]+: 545.11491.
Found 545.11255; mp 141.2−142.6 C.
[6-15N]-9-(3-O-Benzoyl-5-O-tert-butyldimetylsilyl--D-ribofuranosyl)-2-chloropurine (15)
Compound 14 (52.3 mg, 0.10 mmol) was dissolved in DMSO (5.0 mL), and 15NH4Cl (54.4 mg, 1.0 mmol),
KHCO3 (144.0 mg, 1.44 mmol) was added to the solution, and then sealed and stirred for 24 h at 100 C.
The mixture was extracted with AcOEt, and washed with saturated aqueous sodium chloride solution, and
dried with sodium sulfate, and then evaporated. The residue was purified by silica gel column
1
chromatography (50% AcOEt in hexane) to give crystals 15 (37.4 mg, 0.07 mmol, 74%). H-NMR
15
(400MHz, DMSO-d6):  8.31 (1H, s, H-8), 8.02 (2H, m, 3′O-Bz), 7.82 (2H, d, J = 90.8 Hz, NH2), 7.66
(1H, m, 3′O-Bz), 7.53 (2H, m, 3′O-Bz), 6.37 (1H, dd, J = 8.0 and 6.0 Hz, H-1′), 5.58 (1H, m, H-3′), 4.25
(1H, m, H-4′), 3.88 (1H, dd, J = 11.2 and 4.8 Hz, H-5′a), 3.81 (1H, dd, J = 11.2 and 4.8 Hz, H-5′b), 3.02
(1H, m, H-2′a), 2.68 (1H, ddd, J = 14.0, 6.0 and 2.0 Hz, H-2′b), 0.80 (9H, s, 5′O-TBS), 0.01 (6H, m,
15
5′O-TBS); HRMS (ESI) Calcd for C23H30CIN4 NNaO4Si [M+Na]+: 527.16181. Found 527.16227; mp
175.2−176.9 C.
[6-15N]-9-(3-O-Benzoyl--D-ribofuranosyl)-2-chloropurine (16)
Compound 15 (50.5 mg, 0.10 mmol) was dissolved in THF (3.0 mL), and 1.0N tetrabutylammonium
fluoride-THF solution (1.0 mL) was added to the solution, and then stirred for 5 min at room temperature.
The mixture was extracted with AcOEt, and washed with saturated aqueous sodium chloride solution, and
dried with sodium sulfate, and then evaporated. The residue was purified by silica gel column

HETEROCYCLES, Vol. 85, No. 1, 2012
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1
chromatography (14% MeOH in CH2Cl2) to give crystals 16 (39.0 mg, 0.10 mmol, 100%). H-NMR
15
(400MHz, DMSO-d6):  8.07 (2H, m, 3′O-Bz), 7.97 (1H, s, H-8), 7.70-7.90 (2H, d, J = 90.8 Hz, NH2),
7.62 (1H, m, 3′O-Bz), 7.58 (2H, m, 3′O-Bz), 6.37 (1H, dd, J = 9.6 and 5.2 Hz, H-1′), 5.50 (1H, m, H-3′),
4.43 (1H, m, H-4′), 4.03−4.04 (2H, m, H-5′ab), 3.17−3.23 (1H, m, H-2′a), 2.63 (1H, m, H-2′b); HRMS
15
(ESI) Calcd for C17H16CIN4 NNaO4 [M+Na]+: 413.07534. Found 413.07563; mp 192.1−193.6 C.
[6-15N]-2-Chloro-2'-deoxyadenosine ([6-15N]-Cladribine, 2)
Compound 16 (36.0 mg, 0.09 mmol) was dissolved in 2N NH3/MeOH, then kept for 12 h at room
temperature. After condensation of the solution, the residue was purified by silica gel column
1
chromatography (25% MeOH in CH2Cl2) to give crystals 2 (23.7 mg, 0.08 mmol, 91%). H-NMR
(400MHz, DMSO-d6):  8.29 (1H, s, H-8), 7.75 (2H, d, J = 90.8 Hz, NH2), 6.19 (1H, dd, J = 6.4 and 6.4
Hz, H-1′), 5.24 (1H, m, 3′-OH), 4.89 (1H, m, 5′-OH), 4.32 (1H, m, H-3′), 3.79 (1H, m, H-4′), 3.52−3.55
(1H, m, H-5′a), 3.43-3.46 (1H, m, H-5′b), 2.54−2.61 (1H, m, H-2′a), 2.18−2.24 (1H, ddd, J = 9.2, 6.0 and
13
3.2 Hz, H-2′b); C-NMR (100MHz, DMSO-d6):  156.7 (d, J = 21.0 Hz, C6), 152.9 (d, J = 3.0 Hz, C2),
150.0, 139.8, 118.1 (d, J = 3.0 Hz, C5), 87.9, 83.5, 70.7, 61.6, 40.1; 15N-NMR (40MHz, CD3OD):  83.7 (s,
15
15NH2); HRMS (ESI) Calcd for C10H12CIN4 NNaO3 [M+Na]+: 309.04912. Found 309.04885; mp 215.9−
219.0 C.
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