A practical synthesis of 2-chloro-2′-deoxyadenosine (Cladribine) from 2′-deoxyadenosine

Article abstract of DOI:10.3184/174751913X13618878756705

A practical synthesis of 2-chloro-2′-deoxyadenosine (Cladribine) from 2′-deoxyadenosine is reported. Treatment of fully protected 2′-deoxyadenosine with 2,2,2-trifluoroacetic anhydride and tetrabutylammonium nitrate gave protected 2-nitro-2′-deoxyadenosine with high yield. 2-Chloro-2′-deoxyadenosine was synthesised in four steps and 44.8% yield after substitution by chloride and deprotection steps.

Full text of DOI:10.3184/174751913X13618878756705

JOURNAL OF CHEMICAL RESEARCH 2013
RESEARCH PAPER 213
APRIL, 213–215
A practical synthesis of 2-chloro-2′-deoxyadenosine (Cladribine) from
2′-deoxyadenosine
Yao Peng*
Department of Chemistry and Chemical Engineering, Xinxiang University, Xinxiang 453003, P. R. China
A practical synthesis of 2-chloro-2′-deoxyadenosine (Cladribine) from 2′-deoxyadenosine is reported. Treatment
of fully protected 2′-deoxyadenosine with 2,2,2-trifluoroacetic anhydride and tetrabutylammonium nitrate gave
protected 2-nitro-2′-deoxyadenosine with high yield. 2-Chloro-2′-deoxyadenosine was synthesised in four steps and
44.8% yield after substitution by chloride and deprotection steps.
Keywords: 2-chloro-2′-deoxyadenosine (Cladribine), practical synthesis, nitration, 2′-deoxyadenosine
2-Chloro-2′-deoxyadenosine (1, Cladribine), an analogue of
2′-deoxyadenosine, is a deaminase inhibitor used for the treat-
ment of hairy cell leukaemia1 and acute leukaemia2. It also
shows significant activity against chronic lymphocytic leukae-
mia3, lymphoid neoplasms4 and non-Hodgkin’s lymphoma5.
The first route to 2-chloro-2′-deoxyadenosine is through
purine-sugar coupling (route a). Ikehara and Tada6 firstly
synthesised 2-chloro-2′-deoxyadenosine as an intermediate
by this method. Robins7 reported direct glycosylation with a
2-deoxy-chloro sugar of the sodium salt of 6-imidazolylpurine
followed by ammonolysis, avoiding of anomeric mixtures
and N7/N9 isomers successfully. The second route is through
a modification of available nucleosides. Chen8 synthesised
2-chloro-2′-deoxyadenosine from guanosine (2.8% overall
yield) in eight steps (route b). Robins9 reported a concise syn-
thesis of 2-chloro-2′-deoxyadenosine from 2′-deoxyguanosine
with 75% yield (route c). Xu10 reported an improving method
to synthesise 2-chloro-2′-deoxyadenosine from 2-chloro-
adenosine in five steps and 31.0% yield (route d). The third
route is through an enzymatic glycosyl transfer catalysed by
trans-N-deoxyribosylase from available 2′-deoxy nucleosides
Sigma-Aldrich) or 2′-deoxyguanosine (99% purity, 416.7$/g,
Sigma-Aldrich). It would be a practical route to synthesise
2-chloro-2′-deoxyadenosine from 2′-deoxyadenosine. We now
describe the efficient and practical synthesis of 2-chloro-2′-
deoxyadenosine from 2′-deoxyadenosine.
Results and discussion
Purines can be regioselectively nitrated at the 2 position under
mild conditions with tetrabutylammonium nitrate and trifluo-
roacetic anhydride.13,14 The nitro group was readily replaced
by nucleophilic halogens (Cl, F).15–16 This nitration/SNAr
method was successfully applied to synthesise Fludarabine
and 2-fluoroadenosine.14
The amino group and hydroxyl groups of 2′-deoxyadenosine
were protected with a benzoyl group following the reported
method.17 The substrates with activated NH or OH gave
side reactions during the nitration step. N6,N6-Dibenzoyl-3′,5′-
O-dibenzoyl-2′-deoxyadenosine (3) was obtained with 95%
yield. We also tried the acetyl group as protecting group, but
the product was a syrup which was difficult to purify. Further-
more, the acetyl group was more easily displaced than the
benzoyl group and it was not stable in the following nitration
step. The nitration was realised with tetrabutylammonium
nitrate and trifluoroacetic anhydride in dichloromethane
which produced F3COO−NO2+ as an in situ nitration reagent.
The NO2+ reacted at the 2-position of the purine selectively.
Under the optimised conditions, 2-nitro-N6,N6-dibenzoyl-3′,5′-
O-dibenzoyl-2′-deoxyadenosine (4) was obtained in 76%
and 2-chloroadenine in low yields (< 27%)11,12
.
The disadvantages associated with these methods are the
involvement of expensive starting materials, too many syn-
thetic operations and low yields. There is a demand for a more
practical method to synthesise 2-chloro-2′-deoxyadenosine.
2′-Deoxyadenosine (99% purity, 56.6$/g, Sigma-Aldrich) is
much cheaper than 2-chloroadenosine (97% purity, 223$/50 mg,
Scheme 1 The routes to 2-chloro-2′-deoxyadenosine.
* Correspondent. E-mail: yaopeng874@163.com

214 JOURNAL OF CHEMICAL RESEARCH 2013
Scheme 2 Synthesis of 2-chloro-2′-deoxyadenosine from 2′-deoxyadenosine.
20 min the nitrating mixture was added to 3 (0.668 g, 1 mmol) in
anhydrous CH2Cl2 (1 mL) at 0 °C. After 3 h at 0 °C, the reaction mix-
ture was poured into a cold mixture of H2O (10 mL), aqueous NaHCO3
(10 mL) and CH2Cl2 (5 mL). The aqueous phase was extracted with
CH2Cl2 (2 × 5 mL). The combined organic extracts were washed
with brine (2 × 10 mL), dried (MgSO4) and evaporated under reduced
pressure. The product was purified by chromatography on silica gel,
eluting with CH2Cl2–MeOH (19:1), 0.542 g, 76%. Yellow oil, 1H
NMR (400 MHz, CDCl3): δ 8.52 (s, 1H, 8-H), 8.13 (d, J = 7.1 Hz, 2H,
Ar), 8.06 (d, J = 7.1 Hz, 2H, Ar), 7.82 (d, J = 7.1 Hz, 4H, Ar), 7.60
(t, J = 7.4 Hz, 1H, Ar), 7.57 (t, J = 7.4 Hz, 1H, Ar), 7.47–7.39 (m, 6H,
Ar), 7.39 (t, J = 7.4 Hz, 4H, Ar), 6.64 (t, J = 6.6 Hz, 1H, 1′-H), 5.79
(d, J = 2.2 Hz, 1H, 3′-H), 4.84 (dd, J = 4.5, 1.4 Hz, 2H, 5′-H), 4.65–
4.59 (m, 1H, 4′-H), 3.22–3.18 (m, 1H, 2′-H), 2.88–2.84 (m, 1H, 2′-H).
13C NMR (100 MHz, CDCl3): δ 175.3, 169.9, 153.1, 152.4, 149.8,
140.3, 134.2, 133.0, 132.1, 130.1, 129.9, 128.9, 128.8, 128.6, 127.8,
127.5, 126.4, 92.6, 84.5, 74.6, 64.1, 36.9. HRMS calcd for C38H29N6O9
[M + H]+ 713.1996, found 713.1992.
yield. When we replaced tetrabutylammonium nitrate by the
equivalent tetramethylammonium nitrate or ammonium nitrate,
the yield decreased to 42% and 36% respectively. The main
reason for a lower yield was the poor solubility of tetramethyl-
ammonium nitrate or ammonium nitrate in dichloromethane.
The substitution of nitro group was carried out in DMF (N,N-
dimethylformamide) with ammonium chloride. Other polar
solvents such as acetonitrile or ethanol gave lower yields. The
soluble chloride salts, such as tetrabutylammonium chloride or
tetramethyl ammonium chloride, gave similar results.
Conclusion
In summary, we have described a practical and efficient method
to synthesise 2-chloro-2′-deoxyadenosine from the commer-
cially available and cheaper 2′-deoxyadenosine. The key step
was the selective nitration of the purine 2 position. The nitro
group was replaced by chloride through a nucleophilic
aromatic substitution. 2-Chloro-2′-deoxyadenosine was syn-
thesised in four steps and 44.8% yield. This method provided
clear cost advantages over the published synthesis. The efforts
to improve the batch process with respect to substrate scope
are ongoing in our laboratory.
2-Chloro-N6,N6-dibenzoyl-3′,5′-O-dibenzoyl-2′-deoxyadenosine
(5): Ammonium chloride (0.053, 1 mmol) was added to a solution of
4 (0.356 g, 0.5 mmol) in DMF (2 mL) and the mixture was stirred for
4 h at ambient temperature. H2O (2 mL) was added and the mixture
was extracted with CH2Cl2 (2 × 2 mL). The combined organic extracts
were washed with H2O (2 mL) and brine (2 × 2 mL), dried (MgSO4)
and evaporated under reduced pressure. The product was purified
by chromatography on silica gel, eluting with CH2Cl2–MeOH
(19:1), 0.218 g, 62%. Light yellow solid, m.p. 190–192 °C. 1H NMR
(400 MHz, CDCl3): δ 8.53 (s, 1H, 8-H), 8.09 (d, J = 7.1 Hz, 2H, Ar),
8.02 (d, J = 7.2 Hz, 2H, Ar), 7.80 (d, J = 7.2 Hz, 4H, Ar), 7.63 (t,
J = 7.2 Hz, 1H, Ar), 7.58 (t, J = 7.2 Hz, 1H, Ar), 7.42–7.40 (m, 6H,
Ar), 7.36 (t, J = 7.2 Hz, 4H, Ar), 6.61 (t, J = 6.4 Hz, 1H, 1′-H), 5.80
(d, J = 2.2 Hz, 1H, 3′-H), 4.81 (dd, J = 4.5, 1.4 Hz, 2H, 5′-H), 4. 56–
4.51 (m, 1H, 4′-H), 3.20–3.17 (m, 1H, 2′-H), 2.82–2.79 (m, 1H, 2′-H).
13C NMR (100 MHz, CDCl3): δ 175.3, 167.0, 152.8, 152.0, 149.3,
140.3, 134.2, 133.0, 132.1, 130.1, 129.9, 128.9, 128.8, 126.9, 127.8,
125.7, 126.4, 92.6, 84.3, 75.1, 63.9, 36.5. HRMS calcd for
C38H29ClN5O7 [M + H]+ 702.1756, found 702.1752.
2-Chloro-2′-deoxyadenosine, Cladribine (1): 5 (0.351 g, 0.5 mmol)
was added to saturated NH3/MeOH solution (10 mL) at 0 °C, the reac-
tion vessel was sealed and stirred at room temperature for 12 h. Vola-
tiles were evaporated. The product was recrystallised from MeOH,
0.133 g, 95%. The m.p. and NMR data were in agreement with
published values9. White solid, m.p. 306–310 °C (dec.), lit.9 > 300 °C.
1H NMR (400 MHz, DMSO-d6): δ 8.36 (s, 1H, 8-H), 7.89 (brs, 2H,
NH2), 6.13 (t, J = 6.4 Hz, 1H, 1′-H), 5.24 (d, J = 4 Hz, 1H, 3′-H), 5.14
(t, J = 4.8 Hz, 1H, 4′-H), 4.24–4.22 (m, 2H, OH), 3.60 (dd, J = 3.2 Hz,
2H, 5′-H), 2.13 (dd, J = 3.4 Hz, 2H, 2′-H); 13C NMR (100 MHz,
d6-DMSO) δ 157.5, 153.6, 150.8, 140.5, 118.8, 88.6, 84.2, 71.4, 62.3,
38.0; HRMS calcd for C10H13ClN5O3 [M + H]+ 286.0707, found
286.0701.
Experimental
1H and 13C NMR spectra were determined on a Bruker AC 400 spec-
trometer (Bruker, Billerica, MA, USA) as DMSO-d6 solution. Chemi-
cal shifts were expressed in parts per million (δ) downfield from the
internal standard tetramethylsilane and were reported as s (singlet),
d (doublet), t (triplet), q (quartet), m (multiplet), and dd (doublet of
doublets) and coupling constants J were given in hertz (Hz). Mass
spectra were obtained on a Waters Q-Tof MicroTM spectrometer
(Waters Synapt).
N6,N6-Dibenzoyl-3′,5′-O-dibenzoyl-2′-deoxyadenosine (3): Benzoyl
chloride (1.15 mL, 10 mmol) was slowly added to a magnetically
stirred suspension of 2′-deoxyadenosine (2, 0.502 g, 2 mmol) in pyri-
dine (5 mL). The mixture was stirred and heated to 65 °C, keeping at
this temperature for 4 h. The solvent was removed under reduced pres-
sure. The residue was purified by recrystallisation from acetone/EtOH
to give the desire product 1.26 g, 95%. White solid, m.p. 172–174 °C.
1H NMR (400 MHz, CDCl3): δ 8.57 (s, 1H, 8-H), 8.24 (s, 1H, 2-H),
8.10 (d, J = 7.2 Hz, 2H, Ar), 8.04 (d, J = 7.2 Hz, 2H, Ar), 7.86 (d,
J = 7.2 Hz, 4H, Ar), 7.65 (t, J = 7.6 Hz, 1H, Ar), 7.59 (t, J = 7.6 Hz,
1H, Ar), 7.51–7.41 (m, 6H, Ar), 7.37 (t, J = 7.6 Hz, 4H, Ar), 6.60 (t,
J = 6.6 Hz, 1H, 1′-H), 5.86 (d, J = 2 Hz, 1H, 3′-H), 4.79 (dd, J = 4.3,
1.3 Hz, 2H, 5′-H), 4.68–4.66 (m, 1H, 4′-H), 3.26–3.20 (m, 1H, 2′-H),
2.87–2.82 (m, 1H, 2′-H). 13C NMR (100 MHz, CDCl3): δ 172.3, 165.9,
153.1, 152.4, 149.8, 140.3, 134.2, 133.0, 132.1, 130.1, 129.9, 128.9,
128.8, 128.6, 127.8, 127.5, 126.4, 92.6, 84.5, 74.6, 63.6, 36.6. HRMS
calcd for C38H30N5O7 [M + H]+ 668.2141, found 668.2141.
We are grateful to the Analysis and Testing Center of
Zhengzhou University for HRMS testing (Henan Province,
Zhengzhou City, Zhengzhou University). We thank Prof. Xu
Shaohong for fruitful discussions and critical comments.
2-Nitro-N6,N6-dibenzoyl-3′,5′-O-dibenzoyl-2′-deoxyadenosine (4):
The nitrating mixture was prepared by adding 2,2,2-trifluoroacetic
anhydride (0.209 mL, 1.5 mmol) to a solution of tetrabutylammonium
nitrate (0.457 g, 1.5 mmol) in anhydrous CH2Cl2 (1 mL) at 0 °C. After

JOURNAL OF CHEMICAL RESEARCH 2013 215
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Paper 1201675 doi: 10.3184/174751913X13618878756705
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