An efficient synthesis of 2-substituted 6-methylpurine bases and nucleosides by Fe- or Pd-catalyzed cross-coupling reactions of 2,6-dichloropurines

Article abstract of DOI:10.1021/jo034351i

Fe-catalyzed cross-coupling reactions of 9-substituted or protected 2,6-dichloropurines with 1 equiv of methylmagnesium chloride gave regioselectively 2-chloro-6-methylpurines in good yields. The same reactions with 3 equiv of methylmagnesium chloride or

Full text of DOI:10.1021/jo034351i

proaches consisting in chlorination of 6-methyluracil fol-
lowed by amination, nitration, reduction, and finally
cyclization of the intermediate 4,5-diamino-6-methylpy-
rimidine with orthoformate. In 1974, the coupling of
6-chloropurines with ylides from alkyltriphenylphospho-
nium halides offered9 the first straightforward method
for the synthesis of 6-alkylpurines. In the last two
decades, the development of cross-coupling reactions led
to general and efficient methodology for the synthesis of
6-alkylpurines.10 Thus, 6-methylpurine derivatives were
prepared by Pd-catalyzed cross-couplings of 6-chloropu-
rines with methylzinc bromide11 or trimethylaluminum.12
Other 6-alkylpurines were also prepared by Ni-13 or Cu-
catalyzed14 couplings of 6-halo- or 6-methylsulfanylpu-
rines with Grignard reagents. Very recently, Fe-catalyzed
cross-couplings of Grignard reagents with aryl halides
(including 6-chloropurines) have been described15 as an
efficient and general methodology.
This paper reports on the general synthesis of 2-sub-
stituted 6-methylpurines by regioselective cross-coupling
methylations of 2,6-dichloropurines. In the past, only
several compounds of this class have been prepared by
cyclization7,8 or by methylation of protected 2-methyl-6-
chloropurine ribonucleoside with a Wittig reagent.16 The
only example of regioselective methylation of 9-benzyl-
2,6-dichloropurine with methylzinc bromide leading to
9-benzyl-2-chloro-6-methylpurine was reported in a pre-
liminary communication17 without details. Our aim was
to study and compare the regioselectivity of methylation
of 2,6-dichloropurines with diverse methylorganometal
reagents and to develop a practical and general meth-
odology for the synthesis of 2-chloro-6-methylpurines as
well as of some 2-C-substituted 6-methylpurines ap-
plicable both to purine bases and nucleosides.
An Efficien t Syn th esis of 2-Su bstitu ted
6-Meth ylp u r in e Ba ses a n d Nu cleosid es by
F e- or P d -Ca ta lyzed Cr oss-Cou p lin g
Rea ction s of 2,6-Dich lor op u r in es
Michal Hocek*,† and Hana Dvorˇa´kova´‡
Institute of Organic Chemistry and Biochemistry,
Academy of Sciences of the Czech Republic, CZ-16610
Prague 6, Czech Republic. Central NMR Laboratory,
Prague Institute of Chemical Technology,
CZ-16628 Prague 6, Czech Republic
hocek@uochb.cas.cz
Received March 18, 2003
Abstr a ct: Fe-catalyzed cross-coupling reactions of 9-sub-
stituted or protected 2,6-dichloropurines with 1 equiv of
methylmagnesium chloride gave regioselectively 2-chloro-
6-methylpurines in good yields. The same reactions with 3
equiv of methylmagnesium chloride or Pd-catalyzed reac-
tions with trimethylaluminum afforded 2,6-dimethylpurines.
The 2-chloro-6-methylpurines underwent another coupling
with phenylboronic acid to give 6-methyl-2-phenylpurines.
All reactions were perfomed for Bn- and THP-protected
purine bases as well as for acyl-protected ribosides and
2-deoxyribosides. After deprotection, free purine bases and
nucleosides were obtained.
6-Methylpurine is highly cytotoxic.1 Its liberation from
the 2′-deoxyribonucleoside by purine nucleoside phos-
phorylases is used for detection of mycoplasma in cell
cultures.2 It is highly potent and toxic to nonproliferating
and proliferating tumor cells. Recently, the use of cyto-
toxic 6-methylpurine base liberated by purine nucleoside
phosphorylases from its nontoxic deoxyribonucleoside
was proposed as a novel principle in the gene therapy of
cancer.3 6-Methylpurines are also versatile starting
materials for further modifications of the methyl group
leading to 6-formyl-4 or 6-halomethylpurines,5 purine-6-
carboxamides,6 etc.
Cross-coupling reactions of 2,6- and 6,8-dihalopurines
are chemo- and regioselective.17,18 Thus, the reactions of
(9) Taylor, E. C.; Martin, S. F. J . Am. Chem. Soc. 1974, 96, 8095-
8102.
(10) Review: Hocek, M. Eur. J . Org. Chem. 2003, 245-254.
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H.; Rise, F. Tetrahedron 1994, 50, 9743-9756. (b) Hassan, A. E. A.;
Abou-Elkair, R. A. I.; Montgomery, J . A.; Secrist III, J . A. Nucleosides,
Nucleotides, Nucleic Acids 2000, 19, 1123-1134.
(12) Cˇ esnek, M.; Hocek, M.; Holy´, A. Collect. Czech. Chem. Commun.
2000, 65, 1357-1373.
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4191-4194. (b) Sugimura, H.; Takei, H. Bull. Chem. Soc. J pn. 1985,
58, 664-666. (c) Estep, K. G.; J osef, K. A.; Bacon, E. R.; Carabates, P.
M.; Rumney, S., IV; Pilling, G. M.; Krafte, D. S.; Volberg, W. A.; Dillon,
K.; Dugrenier, N.; Briggs, G. M.; Canniff, P. C.; Gorczyca, W. P.;
Stankus, G. P.; Ezrin, A. M. J . Med. Chem. 1995, 38, 2582-2595.
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37, 1285-1288. (b) Dvorˇa´kova´, H.; Dvorˇa´k, D.; Holy´, A. Collect. Czech.
Chem. Commun. 1998, 63, 2065-2074.
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Soc. 2002, 124, 13856-13864.
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Carbohydr. Nucleosides Nucleotides 1977, 4, 175-188.
(17) Gundersen, L.-L.; Langli, G.; Rise, F. Tetrahedron Lett. 1995,
36, 1945-1948.
(18) (a) Langli, G.; Gundersen L.-L.; Rise. F. Tetrahedron 1996, 52,
5625-5638. (b) Nolsoe, J . M. J .; Gundersen, L.-L.; Rise, F. Acta Chem.
Scand. 1999, 53, 366-372. (c) Havelkova´, M.; Dvorˇa´k, D.; Hocek, M.
Synthesis 2001, 1704-1710. (d) Hocek M., Holy´ A., Dvorˇa´kova´ H.:
Collect. Czech. Chem. Commun. 2002, 67, 325-335. (e) Hocek, M.;
Votruba, I.; Dvorˇa´kova´, H. Tetrahedron 2003, 59, 607-611. (f) Hocek
M., Hockova´, D.; Sˇtambasky´, J . Collect. Czech. Chem. Commun. 2003,
68, 837-848.
Traditionally, the 6-methylpurine bases were pre-
pared7,8 by low-yielding multistep heterocyclization ap-
* To whom correspondence should be addressed. Fax: +420
233331271.
† Institute of Organic Chemistry and Biochemistry.
‡ Prague Institute of Chemical Technology.
(1) Montgomery, J . A.; Hewson, K. J . Med. Chem. 1968, 11, 48-52.
(2) (a) McGarrity, G. J .; Carson, D. A. Exp. Cell Res. 1982, 139, 199-
205. (b) Whitaker, A. M.; Windsor, G. D.; Burnett, C. M.; Taylor, C. H.
Dev. Biol. Stand. 1987, 66, 503-509. (c) Ishiguro, K.; Taira, S.; Sasaki,
T.; Nariuchi, H. J . Immunol. Methods. 1988, 108, 39-43.
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A. H.; Gillespie, G. Y.; Sorscher, E. J . Hum. Gene Ther. 1997, 8, 1637-
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10.1021/jo034351i CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/13/2003
J . Org. Chem. 2003, 68, 5773-5776
5773

SCHEME 1
TABLE 1. Cr oss-Cou p lin g Rea ction s of Ch lor op u r in es 1
a n d 2 w ith Or ga n om eta llic Rea gen ts
unreacted starting compound (20%) and hydrolytic prod-
ucts (entry 2). If this reaction was performed with 4 equiv
of Me3Al, the 2,6-dimethylpurine 3a was isolated in an
excellent yield of 95% (entry 3). Finally, the application
of the Fu¨rstner’s Fe-catalyzed coupling15 turned out to
be the most practical for the regioselective methylation
of 2,6-dichloropurines. Thus, the reaction of 1a with 1
equiv of methylmagnesium chloride in the presence of a
catalytic amount of Fe(acac)3 in THF containing N-
methylpyrrolidinone (NMP) at ambient temperature gave
the 2-chloro-6-methylpurine 2a in 72% yield accompanied
by the unreacted starting compound 1a (21%) (entry
4). The presence of NMP was beneficial but not crucial
for this reactionsanalogous reaction in absence of
NMP gave a mixture of 2a (68%) and 1a (25%) (entry 5).
When using 1.2 equiv of the Grignard, the yield of 2a
remained in the same level but the reaction mixture
contained also some 2,6-dimethylpurine 3a (ca. 10%)s
as the unreacted 1a can be easily separated and reused,
it is more practical to use just one equivalent of the
reagent. The Fe-catalyzed reaction of 1a with 3 equiv of
MeMgCl gave the dimethylpurine 3a in an excellent yield
of 96% (entry 6).
yieldb (%)
starting
entry compd
reagent ratio condnsa catalyst
1
2
3
4
1
2
3
4
1a
1a
1a
1a
MeZnBr
Me3Al
Me3Al
1:1.2
1:1
1:4
A
B
B
C
Pd(PPh3)4
Pd(PPh3)4 20
Pd(PPh3)4
71 15
46
95
MeMgCl 1:1
Fe(acac)3 21 72
(+NMP)
C
5
6
1a
1a
MeMgCl 1:1
MeMgCl 1:3
Fe(acac)3 25 68
Fe(acac)3
C
96
(+NMP)
7
1b
MeMgCl 1:1
MeMgCl 1:1
Me3Al
MeMgCl 1:1
C
Fe(acac)3 28 67
(+NMP)
8
9
10
1b
1b
1c
C
B
C
Fe(acac)3 26 57
Pd(PPh3)4
Fe(acac)3 27 60
1:4
86
74
87
(+NMP)
C
B
11
12
13
1c
1c
1d
MeMgCl 1:1
Me3Al
MeMgCl 1:1
Fe(acac)3 41 41
Pd(PPh3)4
Fe(acac)3 23 61
1:4
C
(+NMP)
14
15
16
17
18
19
1d
1d
2a
2b
2c
2d
MeMgCl 1:1
Me3Al
PhB(OH)2 1:2
PhB(OH)2 1:2
PhB(OH)2 1:2
PhB(OH)2 1:2
C
B
D
D
D
D
Fe(acac)3 35 50
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
1:4
98
96
73
89
Pd(PPh3)4
As the Fe-catalyzed coupling of Grignard reagent was
found to be quite practical for the synthesis of 2-chloro-
6-methylpurines and both couplings with excess of tri-
methylaluminum or Grignard were suitable for the
synthesis of 2,6-dimethylpurines, these methods were
further explored in order to prepare the corresponding
2-substituted 6-methylpurine bases and nucleosides. The
starting THP-protected 2,6-dichloropurine 1b,19 benzoyl-
protected 2,6-dichloropurine riboside 1c18d and toluoyl
protected deoxyriboside 1d 20 are known compounds.
Analogously to 1a , all protected 2,6-dichloropurines 1b-d
were subjected to the reactions with 1 equiv of MeMgCl
under Fe(acac)3 catalysis or with 4 equiv of Me3Al under
Pd(PPh3)4 catalysis (entries 7-15). In general, the reac-
tions of 1b-d with 1 equiv of MeMgCl in the presence of
NMP gave the desired 2-chloro-6-methylpurines 2b-d
in about 60% yield, while in absence of NMP the yields
of 2 were somewhat lower. The products 2b-d were
easily separated from the starting compound by column
a
Conditions: (A) THF, 50 °C; (B) THF, 75 °C; (C) THF, rt; (D)
b
toluene, K2CO3, 90 °C. Isolated yields.
2,6- or 6,8-dichloropurines with 1 equiv of an organome-
tallic reagent lead to regioselective substitution in the
position 6. On the other hand, 2- or 8-iodo-6-chloropurines
react chemoselectively in the position of the iodine (2 or
8). These reactions were performed with arylboronic
acids, aryl- and alkenylstannanes, and to certain extent,
aryl- or alkylzinc halides.
The above-mentioned preliminary communication17
reported a regioselective Pd-catalyzed methylation of
9-benzyl-2,6-dichloropurine (1a ) with methylzinc bromide
(formed in situ from methylmagnesium chloride and
ZnBr2) to form the 2-chloro-6-methylpurine in 69% yield.
First of all, we have reproduced this reaction with 1.2
equiv of MeZnBr to confirm the formation of the desired
2-chloro-6-methylpurine 2a in 71% yield (Scheme 1, Table
1, entry 1) accompanied by the 2,6-dimethylpurine 3a
(14%). The next atempt was the cross-coupling of 1a with
1.2 equiv of trimethylaluminum. This reaction gave a
complex mixture containing the 9-benzyl-2,6-dimethylpu-
rine (3a ) as a major product (46%) accompanied by some
(19) Naito, T.; Nakagawa, S.; Okita, T. A.; Yamashita, H.; Yamasaki,
T.; Kamei, H.; Tomatsu, K.; Imanishi, H.; Kawaguchi, H. Chem. Pharm.
Bull. 1982, 30, 2011-2019.
(20) Christensen, L. F.; Broom, A. D.; Robins, M. J .; Bloch, A. J .
Med. Chem. 1972, 15, 735-739.
5774 J . Org. Chem., Vol. 68, No. 14, 2003

TABLE 2. Dep r otection of Com p ou n d s 2b-4d
to 6-methyl-2-phenylpurines. The methods are easily
applied for the synthesis of free purine bases, as well as
for ribonucleosides and 2-deoxyribonucleosides. Prelimi-
nary cytostatic activity screening of compounds 2-4 on
their in vitro inhibition of the cell growth in the following
cell cultures did not show any considerable activity:
mouse leukemia L1210 cells (ATCC CCL 219); human
promyelocytic leukemia HL60 cells (ATCC CCL 240);
human cervix carcinoma HeLa S3 cells (ATCC CCL 2.2);
and human T lymphoblastoid CCRF-CEM cell line
(ATCC CCL 119). However, this practical and versatile
methodology could find much wider applications in the
synthesis of other derivatives of the important class of
6-methylpurines.
entry
starting compd
conditionsa
product
yieldb (%)
1
2
3
4
5
6
7
8
9
2b
3b
4b
2c
3c
4c
2d
3d
4d
E
E
E
F
F
F
F
F
F
2e
3e
4e
2f
3f
4f
2g
3g
4g
79
68
84
92
83
86
98
97
80
a
Conditions: (E) Dowex 50X8 (H+ form), EtOH, H2O, reflux;
b
(F) NaOMe, MeOH. Isolated yields after crystallization.
chromatography and the starting compounds were re-
used. The Pd-catalyzed reactions of 1b-d with trimeth-
ylaluminum gave the 2,6-dimethylpurines 3b-d in good
yields. The 2-chloro-6-methylpurines 2a -d are good
intermediates for further derivatizations in the position
2. It was demonstrated by the Suzuki-Miyaura cross-
coupling reactions21 with phenylboronic acid that fur-
nished a series of protected 6-methyl-2-phenylpurine
bases and nucleosides 4a -d (entries 16-19) in very good
yields.
The THP-protected purines 2b-b were easily depro-
tected22 making use of wet cation exchanger Dowex 50X8
(H+ form) in boiling ethanol to give the free purine bases
2e-4e by simple filtering off the ionex, evaporation of
the filtrate and crystallization. The acyl-protected nucleo-
sides 2c-4c and 2d -4d were cleaved by catalytic
amount of sodium methoxide in dry methanol to give
the free ribonucleosides 2f-4f and deoxyribonucleosides
2g-4g. The yields of the deprotection reactions are
summarized in Table 2.
Though the regioselectivity of other cross-coupling
reactions of 2,6-dichloropurines has been proved previ-
ously by means of long-range HETCOR17 or 1H-15N
HMBC18e spectra, an additional independent proof for
these particular methylations was based on 1H-13C
HMBC spectrum of compound 2a . A charactetistic cross-
peak of CH3 to C-5 and C-6 signals has been found
indicating the presence of the methyl group in the
position 6. All other compounds were fully characterized
by 1H and 13C NMR (assignment based on COSY and
1H-13C HMBC and HMQC experiments), MS, and mi-
croanalyses.
In conclusion, the application of the Fu¨rstner Fe-
catalyzed cross-coupling reaction of 2,6-dichloropurines
with 1 equiv of methylmagnesium chloride is a practical
method for the synthesis of 2-chloro-6-methylpurines,
while the use of Pd-catalyzed reaction with trimethyla-
luminum is a good method for the synthesis of 2,6-
dimethylpurines. The Fe-catalyzed cross-couplings are of
comparable efficiency but much cheaper than the Pd-
catalyzed ones. The 2-chloro-6-methylpurines could be
used for further substitutions in the position 2 as
demonstrated by the Suzuki-Miyaura reactions leading
Exp er im en ta l Section
Gen er a l P r oced u r es. Cr oss-Cou p lin g Rea ction of 2,6-
Dich lor op u r in e 1a w ith Meth ylzin c Br om id e (Meth od A).
MeMgCl (3 M solution in THF, 1.2 mL, 3.6 mmol) was added
dropwise to a stirred solution of ZnBr2 (810 mg, 3.6 mmol) in
THF (10 mL) at -78 °C under Ar, the stirring was continued
for 30 min at -78 °C, and then the mixture was allowed to reach
0 °C. Then a solution of 1a (837 mg, 3 mmol) and Pd(PPh3)4
(180 mg, 0.15 mmol) in THF (10 mL) was added, and the
resulting reaction mixture was stirred at 50 °C for 8 h. The
mixture was cooled to rt and poured onto a mixture of ice (∼100
mL) and NH4Cl (1 g), and the products were exctracted with
chloroform (3 × 100 mL). Evaporation of the organic phase
followed by a column chromatography on silica gel (100 g, ethyl
acetate/hexanes 1:1 f ethyl acetate f ethyl acetate/MeOH 9:1)
afforded the products.
Cr oss-Cou p lin g Rea ction s of 2,6-Dich lor op u r in es 1 w ith
Tr im eth yla lu m in u m (Meth od B). Me3Al (2 M solution in
toluene, 0.5 mL, 1 mmol or 2 mL, 4 mmol) was added dropwise
to a stirred solution of 1 (3 mmol) and Pd(PPh3)4 (180 mg, 0.15
mmol) in THF (20 mL) under Ar, and the resulting reaction
mixture was stirred at 75 °C for 8 h. The workup and isolation
was the same as in method A.
Cr oss-Cou p lin g Rea ction s of 2,6-Dich lor op u r in es 1 w ith
Meth ylm a gn esiu m Ch lor id e (Meth od C). MeMgCl (3 M
solution in THF, 0.33 mL, 1 mmol or 1 mL, 3 mmol) was added
dropwise to a stirred solution of 1 (3 mmol) and Fe(acac)3 (103
mg, 0.29 mmol) in THF (20 mL) either in the presence or in the
absence of NMP (1 mL) under Ar, and the resulting reaction
mixture was stirred at rt for 8 h. The workup and isolation was
the same as in method A.
Cr oss-Cou p lin g Rea ction s of 2-Ch lor o-6-m eth ylp u r in es
2 w ith P h en ylbor on ic Acid (Meth od D). Toluene (10 mL)
was added to an argon-purged flask containing the chloropurine
(1 mmol), K2CO3 (200 mg, 1.5 mmol), phenylboronic acid (220
mg, 2 mmol), and Pd(PPh3)4 (59 mg, 0.05 mmol), and the mixture
was stirred under Ar at 100 °C for 8 h. After the mixture was
cooled to ambient temperature, the solvent was evaporated in
vacuo and the residue was chromatographed as in method A.
Clea va ge of th e THP -P r otected P u r in es (Meth od E). A
mixture of a THP-protected base 2b-4b (0.6-0.8 mmol), Dowex
50X8 (H+) (ca. 300 mg), ethanol (50 mL), and water (1 mL) was
refluxed for 1 h and then filtered while hot, and the resin was
washed with hot ethanol (2 × 50 mL). The combined filtrates
were evaporated, and the residue was codistilled with toluene.
Crystallization of the residue from methanol/toluene with an
addition of heptane afforded the free bases 2e-4e.
Dea cyla tion of Nu cleosid es (Meth od F ). A 1 M methanolic
MeONa (100 µL, 0.1 mmol) was added to a solution of a protected
nucleoside 2c-4c or 2d -4d (0.5-1 mmol) in MeOH (20 mL),
and the mixture was stirred at ambient temperature overnight.
The solvent was evaporated, and the residue was chromato-
graphed on a column (silica gel 50 g, ethyl acetate/MeOH
9:1-8:2). The crude products were recrystallized from EtOH/
toluene/heptane to give free nucleosides.
(21) Recent examples in purine chemistry: (a) Hocek, M.; Holy´, A.;
Votruba, I.; Dvorˇa´kova´, H. J . Med. Chem. 2000, 43, 1817-1825. (b)
Hocek, M.; Holy´, A.; Votruba, I.; Dvorˇa´kova´, H. Collect. Czech. Chem.
Commun. 2001, 66, 483-499. (c) Lakshman, M. K.; Hilmer, J . H.;
Martin, J . Q.; Keeler, J . C.; Dinh, Y. Q. V.; Ngassa, F. N.; Russon, L.
M. J . Am. Chem. Soc. 2001, 123, 7779-7787.
(22) Hocek, M.; Holy´, A. Collect. Czech. Chem. Commun. 1995, 60,
1386-1389.
J . Org. Chem, Vol. 68, No. 14, 2003 5775

Yields of the particular products are summarized in Tables 1
and 2, and the characterization data are in the Supporting
Information.
Havl´ıcˇkova´ for technical assistance and Dr. Ivan Votru-
ba for cytostatic activity screening.
Su p p or tin g In for m a tion Ava ila ble: Complete charac-
terization and spectroscopic data for compounds 2-4. This ma-
terial is available free of charge via the Internet at http://
pubs.acs.org.
Ack n ow led gm en t. This work is a part of research
project Z4 055 905. It was supported by the Grant
Agency of the Academy of Sciences of the Czech Republic
(Grant No. B4055201) and by Sumika Fine Chemicals,
Co. Ltd. (Osaka, J apan). We also thank Mrs. Kamila
J O034351I
5776 J . Org. Chem., Vol. 68, No. 14, 2003