A novel method for the introduction of fluorine into the purine 2-position: Synthesis of 2-fluoroadenosine and a formal synthesis of the antileukemic drug fludarabine

Article abstract of DOI:10.1055/s-2006-942544

A novel method for the introduction of fluorine in the purine 2-position is described and employed in the synthesis of a potential antimycobacterial compound. Also 2-fluoroadenosine has been synthesized for the first time from adenosine with perbenzoylated 2-nitroadenosine as a key intermediate. Mild reaction conditions are employed and few synthetic steps are required. The novel synthesis of fluoroadenosine can be regarded as a formal synthesis of the antileukemic drug fludarabine phosphate. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-942544

SHORT PAPER
2993
A Novel Method for the Introduction of Fluorine into the Purine 2-Position:
Synthesis of 2-Fluoroadenosine and a Formal Synthesis of the Antileukemic
Drug Fludarabine
I
ntroduction of Fluorin
o
e
into the Pu
r
rine 2-P
t
ositionen Brændvang, Lise-Lotte Gundersen*
Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, 0315 Oslo, Norway
Fax +47(22)855507; E-mail: l.l.gundersen@kjemi.uio.no
Received 23 April 2006; revised 6 June 2006
The synthesis of 2-fluoroadenosine may also be regarded
Abstract: A novel method for the introduction of fluorine in the pu-
rine 2-position is described and employed in the synthesis of a po-
tential antimycobacterial compound. Also 2-fluoroadenosine has
been synthesized for the first time from adenosine with perbenzo-
as the first formal synthesis of fludarabine phosphate (2)
from adenosine.8 Fludarabine phosphate (2) is a purine
antimetabolite which has revolutionized therapeutic strat-
ylated 2-nitroadenosine as a key intermediate. Mild reaction condi- egies for patients with chronic lymphocytic leukemia.9
tions are employed and few synthetic steps are required. The novel
Other 2-fluoropurines are also associated with interesting
biological activities10 including antiviral and antitumor
synthesis of fluoroadenosine can be regarded as a formal synthesis
of the antileukemic drug fludarabine phosphate.
activity,10a inhibition of heat shock protein 90,10b antima-
larial activity,10c inhibition of adenylyl cyclase,10d and
Key words: anticancer agents, fluorination, nitration, nucleosides,
agonistic activity for adenosine reseptors.10e Moreover, 2-
purines
fluoropurines are useful synthetic intermediates due to
their reactivity in nucleophilic aromatic substitutions.2
Hence, methodology for introduction of fluorine in the
purine 2-position under mild conditions employing readi-
ly available starting materials will be valuable.
Purine is the most widely distributed N-containing hetero-
cycle in nature and both natural and synthetic purine de-
rivatives are associated with
a wide variety of
bioactivities.1 Hence numerous methods have been devel-
oped for functionalization of purines. C-2 substituted de-
rivatives are traditionally synthesized from guanine
derivatives by diazotation followed by halogenation or by
chlorination of xanthine derivatives. Both routes lead to 2-
halopurines, which can be further transformed by nucleo-
philic substitutions and transition-metal-catalyzed cou-
pling reactions, but relatively harsh conditions are
required for introduction of the halide.2
NH2
NH2
N
N
N
N
N
N
F
N
F
N
HO
H2O3P
O
O
O
OH
1
2
HO
OH
HO
Recently, it has been demonstrated that certain 6-substi-
tuted purines can be regioselectively nitrated in the purine
2-position under mild conditions by treatment with tet-
rabutylammonium nitrate and trifluoroacetic anhydride
(TBAN-TFAA).3 The nitro group is readily displaced by
nucleophilic species such as amines and alcohols,4 and
catalytic hydrogenation of the nitro group leads to 2-hy-
droxyaminopurines which can be reacted further to a va-
riety of 2-substituted purines.5
Figure 1 Structure of 2-fluoroadenosine (1), and fludarabine phos-
phate (2).
6-Chloropurine 37c was nitrated with TBAN-TFAA, and
the resulting 2-nitropurine 4 was subjected to Stille cou-
pling in order to introduce the 2-furyl substituent, impor-
tant for antimycobacterial activity.7 Treatment of the 6-(2-
furyl)-2-nitropurine 5 with TBAF gave the corresponding
2-fluoro compound 6 in 78% yield (Scheme 1). Com-
Pyridines carrying a nitro group in the activated a- and g- pound 5 did not react with tetrabutylammonium chloride
positions, can easily be converted to the corresponding or bromide even at elevated temperatures.
fluoropyridines by treatment with tetrabutylammonium
Compounds 5 and 6 are closely related to the potent anti-
mycobacterial purine 2-chloro-6-(2-furyl)-9-(4-methoxy-
fluoride (TBAF).6 In connection with our study of antimy-
cobacterial 6-aryl-9-benzylpurines,7 we herein report that
the same methodology can be applied for the conversion
of a 2-nitropurine to the corresponding 2-fluoro deriva-
phenylmethyl)-9H-purine (MIC against M. tuberculosis
0.39 mg/mL).7c The antibacterial data for compounds 5
and 6 will be published elsewhere.
tive. Furthermore, we present the first synthesis of 2-fluo-
Adenosine (7) was perbenzoylated and nitrated to give the
roadenosine (1; Figure 1) from adenosine.
key intermediate 8 as described before (Scheme 2).11 Re-
action of the 2-nitroadenosine 8 with TBAF gave the cor-
responding 2-fluoroadenosine derivative which was
deprotected directly by treatment with ammonia in meth-
SYNTHESIS 2006, No. 18, pp 2993–2995
x
x.
x
x
.
2
0
0
6
Advanced online publication: 02.08.2006
DOI: 10.1055/s-2006-942544; Art ID: T06806SS
© Georg Thieme Verlag Stuttgart · New York

2994
M. Brændvang, L.-L. Gundersen
SHORT PAPER
tic anhydride (6.94 g, 4.59 mL, 33.0 mmol) over 2 min to a solution
of tetrabutylammonium nitrate (10.05 g, 33.0 mmol) in anhyd
CH2Cl2 (50 mL) at 0 °C. After 20 min the solution was added to 6-
chloro-9-(4-methoxyphenylmethyl)-9H-purine (3;7c 5.65 g, 20.6
mmol) in anhyd CH2Cl2 (50 mL) at 0 °C. After 3 h at 0 °C, the re-
action mixture was poured into a cold mixture of H2O (100 mL), aq
NaHCO3 (100 mL) and CH2Cl2-Et2O (1:2, 200 mL). The aqueous
phase was extracted with CH2Cl2-Et2O (1:2; 2 × 200 mL) and
CH2Cl2 (200 mL). The combined organic extracts were washed with
brine (2 × 200 mL), dried (MgSO4) and evaporated in vacuo. The
crude product was recrystallized from CH2Cl2-hexane; yield: 2.55
g. Additional product (1.07 g) was isolated from the filtrate by flash
chromatography, eluting with CH2Cl2-acetone (19:1); total yield:
3.62 (55%); yellow crystalline compound; mp 181–182 °C.
1H NMR (300 MHz, CDCl3): d = 3.78 (s, 3 H, OCH3), 5.45 (s, 2 H,
CH2), 6.89 (d, J = 8.7 Hz, 2 H, Ar), 7.31 (d, J = 8.7 Hz, 2 H, Ar),
8.28 (s, 1 H, H-8).
13C NMR (75 MHz, CDCl3): d = 48.4 (CH2), 55.4 (OCH3), 114.8
(CH in Ar), 125.1 (C-1 in Ar), 130.0 (CH in Ar), 134.3 (C-5), 148.6
(C-8), 151.9 (C-4), 152.2 (C-6, C-2), 153.4 (C-2, C-6), 160.3 (C-4
in Ar).
O
N
Cl
N
Cl
N
N
b
N
a
N
83%
N
R
55%
N
R
N
R
N
O2N
N
O2N
N
3
4
5
c
O
N
78%
R: p-MeOC6H4CH2-
N
N
R
F
N
6
Scheme 1 Reagents and conditions: (a) TBAN, TFAA, CH2Cl2, 0
°C, 3 h; (b) 2-furylSnBu3, (Ph3P)2PdCl2, DMF, 90 °C, 2 h; (c) TBAF,
THF, DMF, r.t., 1 h.
NH2
N(Bz)2
NH2
MS (EI): m/z (% rel. intensity) = 321, 319 (8, 26) [M+], 302 (11),
273 (2), 272 (6), 237 (2), 122 (9), 121 (100), 91 (3), 78 (6), 77 (5).
N
N
N
N
N
N
N
N
N
Anal. Calcd for C13H10ClN5O3: C, 48.84; H, 3.15; N, 21.91. Found:
C, 48.85; H, 3.17; N, 21.97.
N
O2N
N
F
N
a, b
c, d
HO
BzO
HO
84%
67%
O
O
O
8
7
(ref. 11)
6-(2-Furyl)-9-(4-methoxyphenylmethyl)-2-nitro-9H-purine (5)
The title compound was prepared by Stille coupling between 4 (2.55
g, 7.97 mmol) and 2-(tributylstannyl)furan (3.41 g, 3.01 mL, 9.56
mmol) as described in the literature.7c The reaction time was 3 h.
The product was purified by flash chromatography, eluting with
EtOAc-hexane (1:2) followed by CH2Cl2-acetone (29:1) and
CH2Cl2-acetone (19:1); yield: 2.32 g (83%); yellow crystalline
compound; mp 182-183 °C.
1H NMR (300 MHz, CDCl3): d = 3.77 (s, 3 H, OCH3), 5.43 (s, 2 H,
CH2), 6.67 (dd, J = 1.7, 3.6 Hz, 1 H, H-4 in furyl), 6.87 (d, J = 8.7
Hz, 2 H, Ar), 7.30 (d, J = 8.7 Hz, 2 H, Ar), 7.80 (dd, J = 0.7, 1.7 Hz,
1 H, H-5 in furyl), 7.94 (dd, J = 0.7, 3.6 Hz, 1 H, H-3 in furyl), 8.22
(s, 1 H, H-8).
1
HO
OH
BzO
OBz
HO
OH
(ref. 8)
fludarabine
phosphate (2)
Scheme 2 Reagents and conditions: (a) BzCl, pyridine 65 °C, 4 h;
(b) TBAN, TFAA, CH2Cl2, 0 °C to r.t., 14 h; (c) TBAF, THF, DMF,
r.t., 1 h; (d) NH3, MeOH, r.t., 21 h.
anol, and the desired 2-fluoroadenosine (1) was isolated in
67% yield from compound 8. This novel synthesis of 2-
fluoroadenosine (1) can be regarded as a formal synthesis
of the antileukemic drug fludarabine phosphate.8
13C NMR (75 MHz, CDCl3): d = 47.7 (CH2), 55.3 (OCH3), 113.2
(C-4 in furyl), 114.7 (CH in Ar), 120.1 (C-3 in furyl), 125.9 (C-1 in
Ar), 129.9 (CH in Ar), 130.2 (C-5), 146.6 (C-6), 147.5 (C-5 in fur-
yl), 147.6 (C-8), 148.4 (C-2 in furyl), 152.5 (C-4), 155.3 (C-2),
160.0 (C-4 in Ar).
1
The H NMR spectra were recorded at 300 MHz with a Bruker
MS (EI): m/z (% rel. intensity) = 351 (25) [M+], 334 (1), 322 (2),
Avance DPX 300 instrument, or at 200 MHz with a Bruker Avance
DPX 200 instrument. The 1H decoupled 13C NMR spectra were re-
corded at 75 MHz using the Bruker Avance DPX 300 instrument
and 19F NMR spectra were recorded at 188 MHz with the Avance
DPX 200 instrument using CCl3F in CDCl3 as reference (d = 0
ppm). Mass spectra under electron impact conditions (EI) were re-
corded at 70 eV ionizing voltage with a VG Prospec instrument, and
are presented as m/z (% relative intensity). Elemental analyses were
performed by Ilse Beetz Mikroanalytisches Laboratorium, Kro-
nach, Germany. Melting points were determined with a C. Reichert
melting point apparatus or a Büchi Melting Point B-545 apparatus
and are uncorrected. DMF was distilled from BaO and stored over
4 Å molecular sieves. THF was distilled from Na/benzophenone
and CH2Cl2 and pyridine from CaH2. A solution of TBAF in THF
was prepared from Bu4NF·3H2O. The salt was dissolved in benzene
and evaporated in vacuo (5 ×) and the anhydrous TBAF was dis-
solved in anhydrous tetrahydrofuran.
321 (6), 122 (9), 121 (100), 91 (3), 78 (3), 77 (4).
Anal. Calcd for C17H13N5O4: C, 58.12; H, 3.73; N, 19.93. Found: C,
58.05; H, 3.61; N, 19.84.
2-Fluoro-6-(2-furyl)-9-(4-methoxyphenylmethyl)-9H-purine (6)
TBAF (570 mL, 1.16 M in THF, 0.66 mmol) was added to a solution
of 5 (116 mg, 0.33 mmol) in DMF (3 mL) and the mixture was
stirred for 1 h at ambient temperature under N2. Sat. aq NH4Cl (10
mL) was added and the mixture was extracted with CH2Cl2 (2 × 25
mL). The combined organic extracts were washed with H2O (25
mL) and brine (2 × 25 mL), dried (MgSO4) and evaporated in vacuo.
The product was purified by flash chromatography on silica gel,
eluting with CH2Cl2-acetone (19:1); yield: 84 mg (78%); colorless
crystals; mp 217–218 °C.
1H NMR (200 MHz, CDCl3): d = 3.77 (s, 3 H, OCH3), 5.30 (s, 2 H,
CH2), 6.65 (dd, J = 1.7, 3.6 Hz, 1 H, H-4 in furyl), 6.87 (d, J = 8.7
Hz, 2 H, Ar), 7.26 (d, J = 8.7 Hz, 2 H, Ar), 7.76 (dd, J = 0.7, 1.7 Hz,
1 H, H-5 in furyl), 7.83 (dd, J = 0.7, 3.5 Hz, 1 H, H-3 in furyl), 7.98
(s, 1 H, H-8).
6-Chloro-9-(4-methoxyphenylmethyl)-2-nitro-9H-purine (4)
A nitrating mixture was prepared by adding cold 2,2,2-trifluoroace-
Synthesis 2006, No. 18, 2993–2995 © Thieme Stuttgart · New York

SHORT PAPER
Introduction of Fluorine into the Purine 2-Position
2995
13C NMR (75 MHz, CDCl3): d = 47.0 (CH2), 55.3 (OCH3), 112.8
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121 (100), 91 (3), 78 (5), 77 (5), 65 (1).
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2-Fluoroadenosine (1)
TBAF (1.72 mL, 1.16 M in THF, 2.00 mmol) was added dropwise
over 1 min to a solution of the protected 2-nitroadenosine 811 (833
mg, 1.00 mmol) in DMF (8 mL). The mixture was stirred for 1 h at
ambient temperature under N2 before the solution was eluted quick-
ly through a plug of silica (10 g) using CH2Cl2-acetone (9:1; 100
mL) as eluent. The solution was evaporated in vacuo (water bath <
30 °C) and dried under high vacuum. Saturated methanolic NH3 (70
mL) was added and the solution was stirred for 21 h at ambient tem-
perature and evaporated in vacuo. The product was purified by flash
chromatography, eluting with EtOAc, followed by EtOAc-MeOH
mixtures gradually increasing to EtOAc-MeOH (9:1) and later with
EtOAc-MeOH (5:1); yield: 190 mg (67%); colorless crystalline
compound; mp 232 °C (dec.) [Lit.12 mp 200 °C (dec.)].
1H NMR (300 MHz, DMSO-d6): d = 3.51-3.70 (m, 2 H, H-5¢),
3.92-3.96 (m, 1 H, H-4¢), 4.11-4.15 (m, 1 H, H-3¢), 4.49-4.55 (m,
1 H, H-2¢), 5.05 (t, J = 5.6 Hz, 1 H, OH-5¢), 5.20 (d, J = 4.7 Hz, 1
H, OH-3¢), 5.47 (d, J = 5.9 Hz, 1 H, OH-2¢), 5.79 (d, J = 5.9 Hz, 1
H, H-1¢), 7.88 (br s, 2 H, NH2), 8.35 (s, 1 H, H-2).
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13C NMR (75 MHz, DMSO-d6): d = 61.4 (H-5¢), 70.4 (C-3¢), 73.6
(C-2¢), 85.6 (C-4¢), 87.5 (C-1¢), 117.5 (d, JCF = 4.1 Hz, C-5), 139.9
(d, JCF = 2.5 Hz, C-8), 150.6 (d, JCF = 20.2 Hz, C-4), 157.7 (d, JCF
21.3 Hz, C-6), 158.5 (d, JCF = 204.0 Hz, C-2).
=
19F NMR (188 MHz, DMSO-d6): d = -52.6.
MS (EI): m/z (% rel. intensity) = 285 (2) [M+], 255 (2), 1 96 (9), 182
(16), 166 (7), 153 (100), 134 (10), 133 (19), 106 (6), 108 (3).
Acknowledgment
The Norwegian Research Council is greatly acknowledged for par-
tial financing of the Bruker Avance instruments used in this study.
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Synthesis 2006, No. 18, 2993–2995 © Thieme Stuttgart · New York