Facile and efficient synthesis of 6-(hydroxymethyl)purines

Article abstract of DOI:10.1021/ol049059r

(Chemical Equation Presented) A facile and efficient methodology of the synthesis of 6-(hydroxymethyl)purine derivatives (bases and nucleosides) was developed based on Pd-catalyzed cross-coupling reactions of 6-halopurines with acyloxymethylzinc iodides f

Full text of DOI:10.1021/ol049059r

ORGANIC
LETTERS
2004
Vol. 6, No. 19
3225-3228
Facile and Efficient Synthesis of
6-(Hydroxymethyl)purines
Peter Sˇilha´r, Radek Pohl, Ivan Votruba, and Michal Hocek*
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the
Czech Republic, FlemingoVo nam. 2, CZ-16610 Prague 6, Czech Republic
hocek@uochb.cas.cz
Received May 21, 2004
ABSTRACT
A facile and efficient methodology of the synthesis of 6-(hydroxymethyl)purine derivatives (bases and nucleosides) was developed based on
Pd-catalyzed cross-coupling reactions of 6-halopurines with acyloxymethylzinc iodides followed by deprotection. Several title compounds are
inhibitors of adenosine deaminase and exert cytostatic activity.
Purines bearing carbon substituents in position 6 possess a
broad spectrum of biological activities. 6-(Arylalkynyl)-,
6-(arylalkenyl)-, and 6-(arylalkyl)purines show cytokinin1 and
antioxidant2 activity, and 9-benzyl-6-arylpurines exhibit
antimycobacterial, antibacterial, and cytotoxic effects.3 6-Aryl
and 6-(arylakyl)purine ribonucleosides,4 as well as some
arylalkynyl-9-benzylpurines,5 display significant cytostatic
activity. However, little is known about the biological activity
of purines bearing functionalized carbon substituents.
6-(Hydroxymethyl)purine can be considered a homologue
of hypoxanthine and a potential transition-state analogue for
adenosine deaminase.6 6-Hydroxymethyl-9-(â-D-ribofurano-
syl)purine isolated from Collybia maculata was reported to
possess antifungal, cytotoxic, and antiviral (vesicular sto-
matitis virus) properties.7 Furthermore, Wolfenden et al.
reported this compound as an adenosine deaminase inhibitor.8
Synthetic approaches to 6-(hydroxymethyl)purines published
so far are based on the radical photoaddition of methanol to
unsubstituted purine in aerobic conditions7a,9 or on the
recently reported reaction of 6-magnesiated purine with
paraformaldehyde.10 Unfortunately, the photoaddition in
aerobic condition proceeds with low chemoselectivity and a
very complex reaction profile,8b whereas in an argon
atmosphere, the main product is 1,6-dihydro-6-(hydroxy-
methyl)purine, which can be oxidatively converted on the
6-hydroxymethyl purine.8b The yield of nucleophilic addition
of 6-magnesiated purine to formaldehyde is low because of
low reactivity of the purinylmagnesium reagent.10 On the
other hand, 6-lithiated purine is much more reactive but is
stable only for a few minutes at -130 °C.11 Indirect routes
(1) Brathe, A.; Gundersen, L.-L.; Rise, F.; Eriksen, A. B.; Vollsnes, A.
V.; Wang, L. Tetrahedron 1999, 55, 211-228.
(2) Brathe, A.; Andresen, G.; Gundersen, L.-L.; Malterud, K. E.; Rise,
F. Bioorg. Med. Chem. 2002, 10, 1581-1586.
(3) (a) Bakkestuen, A. K.; Gundersen, L.-L.; Langli, G.; Liu, F.; Nolsøe,
J. M. J. Bioorg. Med. Chem. Lett. 2000, 10, 1207-1210. (b) Andresen, G.;
Gundersen, L.-L.; Nissen-Meyer, J.; Rise, F.; Spilsberg, B. Bioorg. Med.
Chem. Lett. 2002, 12, 567-569. (c) Gundersen, L.-L.; Nissen-Meyer, J.;
Spilsberg, D. J. Med. Chem. 2002, 45, 1383-1386.
(4) (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) Hocek, M.;
Holy´, A.; Dvorˇa´kova´, H. Collect. Czech. Chem. Commun. 2002, 67, 325-
335.
(7) Leonhardt, K.; Anke, T.; Hillen-Maske, E.; Steglich, W. Z. Natur-
forsch. 1987, 42c, 420-424.
(8) (a) Evans, B.; Wolfenden, R. J. Am. Chem. Soc. 1970, 92, 4751-
4752. (b) Wolfenden, R.; Wentworth, D. F.; Mitchell, G. N. Biochemistry
1977, 16, 5071-5077.
(5) (a) Hocek, M.; Votruba, I. Bioorg. Med. Chem. Lett. 2002, 12, 1055-
1057. (b) Hocek, M.; Dvorˇa´kova´, H.; C´ısaˇrova´, I. Collect. Czech. Chem.
Commun. 2002, 67, 1560-1578
(6) Erion, M. D.; Reddy, M. R. J. Am. Chem. Soc. 1998, 120, 3295-
3304.
(9) (a) Linschitz, H.; Connolly, J. S. J. Am. Chem. Soc. 1968, 90, 2979-
2980. (b) Buffel, D. K.; McGuigan, C.; Robins, M. J. J. Org. Chem. 1985,
50, 2664-2667.
(10) Tobrman, T.; Dvoˇra´k; D. Org. Lett. 2003, 5, 4289-4291.
(11) Leonard, N. J.; Bryant, J. D. J. Org. Chem. 1979, 44, 4612-4616.
10.1021/ol049059r CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/20/2004

from 6-methylpurine: oxidation to aldehyde followed by
reduction12 or rearrangement of its N-oxide with Ac2O to
6-(acetoxymethyl)purine13 gave low yields.
transmetalated to zinc-cuprate, and used for conjugate
additions or for uncatalyzed couplings with vinyl or allyl
halides, as well as acyl chlorides. To the best of our
knowledge, it has not yet been used for Pd-catalyzed cross-
coupling reactions.
The Negishi reaction of this organozinc reagent with
6-halopurines 2 or 3 proceeded21 smoothly at room temper-
ature in about 6-8 h to give the 6-(pivaloyloxymethyl)-
purines 4xa in excellent yields (Table 1, entries 1-8). The
Cross-coupling reactions of halopurines with various
organometallics is an efficient approach for the preparation
of purines bearing carbon substituents in the position 2, 6,
or 8.14 However, the use of cross-coupling reactions for the
introduction of highly functionalized alkyl substituents is so
far underdeveloped and still remains a synthetic challenge.
The reason is the incompatibility of most of the functional
groups with the highly reactive organozinc,15 -magnesium,16
-copper,17 or -aluminum18 groups necessary for an effective
transmetalation of sp3-hybridized substituents (tolerant and
mild Suzuki-Miyaura or Stille couplings are usually not
efficient for such substituents). Therefore, suitable protective
or masking groups must be used.
Table 1. Cross-Couplings of Acyloxymethylzinc Iodides with
6-Halopurines
entry
halopurine
1 (eq.)
t (h)
product (yield %)a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
2a
2a
3a
2b
3b
2c
2d
3d
2a
3a
2b
3b
2c
2d
3d
2a
3a
2b
3b
2c
2d
3d
1a (2.5)
1a (4)
1a (3)
1a (4)
1a (3)
1a (3)
1a (3)
1a (3)
1b (3)
1b (3)
1b (3)
1b (3)
1b (3)
1b (3)
1b (3)
1c (3)
1c (3)
1c (3)
1c (3)
1c (3)
1c (3)
1c (3)
8
8
6
8
6
8
8
6
8
6
8
6
8
8
6
8
6
8
6
8
8
6
4a a (83)
4a a (95)
4a a (88)
4ba (90)
4ba (94)
4ca (95)
4d a (92)
4d a (95)
4a b (81)
4a b (96)
4bb (72)
4bb (95)
4cb (94)
4d b (91)
4d b (84)/4d d (9)
4a c (76)/4a d (15)
4a c (60)/4a d (35)
4bc (35)/4bd (33)
4bc (76)/4bd (20)
4cc (68)/4cd (25)
4d c (42)/4d d (50)
4d c (64)/4d d (30)
Here we wish to report a novel efficient method for
preparation 6-hydroxymethyl-9-substituted purines by the
Negishi cross-coupling reactions of O-acyl-protected hy-
droxymethylzinc iodides 1 with 6-halo-9-substituted purines
2 or 3 (Scheme 1). As the first protected organozinc reagent
Scheme 1a
a Isolated yield.
effect of the leaving group was not crucial: 6-iodopurines
3x usually gave just slightly better yields than 6-chloropu-
rines 2x. The reaction was applied on protected (Bn or THP)
halopurine bases 2a/3a and 2b/3b, as well as on protected
a (i) 1y, Pd(PPh3)4, THF, rt, 4-12 h.
we used (pivaloyloxymethyl)zinc iodide 1a prepared by a
known procedure19 from iodomethyl pivalate (easily available
from commercial chloromethyl pivalate by Finkelstein reac-
tion20). The reagent 1a has been previously19 prepared,
(20) Finkelstein, H. Ber. Dtsch. Chem. Ges. 1910, 43, 1528-1532.
(21) Typical procedure for cross-coupling of pivaloyloxymethylzinc
iodide (1a) and 6-chloro-9-benzylpurine (2a). A solution of iodomethyl
pivalate (538 mg, 2.22 mmol) in THF (3 mL) was added at 15 °C to a
suspension of zinc dust (288 mg, 4.4 mmol) in THF (1 mL) that was
preactivated with dibromoethane (20 µL) and trimethylsilyl chloride (18
µL). After 1 h, the solution of 1a (4 mL, 0.5 mmol/mL) in THF was added
at room temperature to the solution of 2a (122 mg, 0.5 mmol) and
Pd(PPh3)4 (29 mg, 5%) in THF (1 mL) and stirred at room temperature for
8 h. The reaction was quenched with 1 M NH4Cl (30 mL) and extracted
with ethyl acetate (3 × 25 mL). Collected organic phases were dried over
MgSO4, filtered, and evaporated. Crude oil was purified by chromatography
on silica gel (hexanes/ethyl acetate 2/1-3/2), affording a yellowish oil (154
mg, 95%), which was crystallized from ethyl acetate-hexanes to give 123
mg (80%) of white crystals (mp 68-71 °C). Anal. Calcd for C18H20N4O2:
C, 66.65; H, 6.21; N, 17.27. Found: C, 66.43; H, 6.12; N, 16.90. FAB MS
m/z (%): 325 (M+, 35); 241 (7); 224 (6); 91 (100); 57 (100). 1H NMR
(CDCl3, 400 MHz): 1.29 (s, 9H, t-Bu); 5.44 (s, 2H, N-CH2); 5.62 (s, 2H,
(12) Giner-Sorolla, A. Chem. Ber. 1968, 101, 611-618.
(13) Stevens, M. A.; Giner-Sorolla, A.; Smith, H.; Brown, G. B. J. Org.
Chem. 1962, 27, 567-572.
(14) Reviews: (a) Hocek, M. Eur. J. Org. Chem. 2003, 245-254. (b)
Agrofoglio, L. A.; Gillaizeau, I.; Saito, Y. Chem. ReV. 2003, 103, 1875-
1916.
(15) Hassan, A. E. A.; Abou-Elkair, R. A. I.; Montgomery, J. A.; Secrist
III, J. A. Nucleosides, Nucleotides, Nucleic Acids 2000, 19, 1123-1134.
(16) (a) Hocek, M.; Dvoˇra´kova´, H. J. Org. Chem. 2003, 68, 5773-
5776. (b) Hocek, M.; Hockova´, D.; Dvorˇa´kova´, H. Synthesis 2004, 889-
894.
(17) Dvorˇa´kova´, H.; Dvoˇra´k, D.; Holy´, A. Tetrahedron Lett. 1996, 37,
1285-1288.
O-CH2); 7.33-7.36 (m, 5H, Ph); 8.03 (s, 1H, H-8); 8.97 (s, 1H, H-2). 13
C
(18) Hirota, K.; Kitade, Y.; Kanbe, Y.; Maki, Y. J. Org. Chem. 1992,
57, 5268-5270.
NMR (CDCl3, 100 MHz): 27.2 (3 × CH3); 38.9 (CMe3); 47.3 (N-CH2);
62.4 (O-CH2); 127.9; 128.7; 129.2 (CH-phenyl); 131.7 (C-5); 134.9
(C-1-phenyl); 144.4 (C-8); 151.6 (C-4); 152.5 (C-2); 155.7 (C-6); 178.3
(19) Knochel, P.; Chou, T.-S.; Jubert, C.; Rajagopal, D. J. Org. Chem.
1993, 58, 588-599.
(CdO). IR (CCl4): ν ) 2934, 1739, 1594, 1500, 1480, 1332, 1146 cm-1
.
3226
Org. Lett., Vol. 6, No. 19, 2004

(acylated) ribo- or 2-deoxyribonucleosides 2c and 2d/3d,
respectively.
Table 2. Cleavage of Ester Protecting Groups in Purines or
Nucleosides 4xy
Analogously, (benzoyloxymethyl)zinc iodide (1b) was
prepared by insertion of zinc into the iodomethyl benzoate22
at 5-10 °C. The yields of cross-couplings of 6-halopurines
with 1b were comparable or somewhat lower than with 1a
(entries 9-15), and in the case of synthesis of protected
2-deoxyribonucleoside 4db (entry 15), partial hydrolysis to
4dd was observed.
(Acetyloxymethyl)zinc iodide 1c (prepared from iodo-
methyl acetate20b and activated zinc) was also used as reagent
in the cross-coupling reactions with 6-halopurines. Unfor-
tunately, the resulting 6-(acetyloxymethyl)purines appeared
to be less stable and suffered from facile de-O-acetylation
during the workup and purification steps. Thus, besides the
desired 6-(acetyloxymethyl)purines 4xc, significant amounts
of deacetylated byproducts 4xd were isolated by column
chromatography. From the yields of products 4xc and 4xd,
overall conversions of these cross-coupling reactions in the
range of 90-96% are obtained (Table 1, entries 16-22).
Standard deprotection of 6-(acyloxymethyl)purine inter-
mediates 4xy was used to prepare the final free 6-(hy-
droxymethyl)purine bases and nucleosides 5a-5e (Scheme
2, Table 2). The ester groups were cleaved by NaOMe in
entry
4xy
equiv of NaOMe
t (h)
product (yield, %)a
1
2
3
4
5
6
7
8
9
10
11
12
13
4a a
4ba
4ba
4ca
4d a
4a b
4bb
4cb
4d b
4a c
4bc
4cc
4d c
1
36
48
24
48
48
20
20
20
20
12
12
12
12
5a (22)
5b (36)
5b (42)
5c (38)/6ca (22)
5d (35)/6d a (12)
5a (84)
5b (85)
5c (77)
5d (82)
5a (96)
5b (88)
5c (81)
0.2
1.25
0.5
0.5
0.2
0.1
0.1
0.2
0.1
0.1
0.1
0.1
5d (86)
a Isolated yield.
The THP protective group in position 9 of purine is easily
cleavable under mild acidic conditions. Thus compounds 5b
and 4by were deprotected using Dowex 50X8 (H+ form)24
in ethanol to afford 6-substituted-9H-purines 5e and 6ey,
respectively (Scheme 2, Table 3).
Scheme 2 a
Table 3. Cleavage of THP Protecting Groups
entry
THP derivative
product (yield, %)a
1
2
3
4
5b
5e (78)
4ba
4bb
4bc
6ea (89)
6eb (55)
6ec (56)
a Isolated yield.
The title 6-(hydroxymethyl)purines and nucleosides 5a and
5c-5e, as well as some of the partially deprotected 6-(acyl-
oxymethyl)purines 6ca, 6ea, 6eb, and 6ec, were subjected
to biological activity screening. In vitro cytostatic activity
tests (inhibition of cell growth) were performed using the
following cell cultures: 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). Adenosine deaminase (ADA)
inhibition was studied25 on calf intestinal adenosine amino-
hydrolase (EC 3.5.4.4.). The results are summarized in
Table 4.
a (i) NaOMe, MeOH, rt; (ii) Dowex (H+ form), EtOH.
methanol at room temperature.23 In case of 6-(pivaloyloxy-
methyl)purines 4xa, the cleavage of the bulky pivaloyl ester
was quite slow, and even after 2 days significant amounts
of partly deprotected compounds 6ca, 6da, and some yet
unidentified byproducts were observed in addition to the
desired fully deprotected purines 5x (Table 2, entries 1-5).
On the other hand, cleavage of benzoates 4xb and acetates
4xc proceeded smoothly to give the 6-(hydroxymethyl)-
purines in good yields (Table 2, entries 6-13).
The most active was the 6-(hydroxymethyl)purine ribo-
nucleoside (5c), which exerted very high antiproliferative
activity against HL-60 and CCRF-CEM (IC50 ) 0.01 and
(23) Zemple´n, G.; Kunz, A. Ber. Dtsch. Chem. Ges. 1923, 56, 1705-
1710.
(24) Hocek, M.; Holy, A. Collect. Czech. Chem. Commun. 1995, 60,
1386-1389.
(25) Agarwal R. P.; Parks, R. E., Jr. Adenosine deaminase from human
erythrocytes. Methods in Enzymology; Hoffe, P. A., Jones, M. E., Eds.;
Academic Press: New York, 1978; pp 502-507.
(22) Iodomethyl benzoate was prepared from chloromethyl benzoate.
Preparation of halogenmethyl acylate has been reported: (a) Ulich, L. H.;
Adams, R. J. Am. Chem. Soc. 1921, 43, 660-667. (b) Bigler, P.; Muhle,
H.; Neuenschwander, M. Synthesis 1978, 593-594.
Org. Lett., Vol. 6, No. 19, 2004
3227

In conclusion, 6-(hydroxymethyl)purine bases and nucleo-
sides can be prepared efficiently by this practical methodol-
ogy (superior to the previously known approaches)8-13 in
two steps in overall yields of 75-87%. Benzoyloxymeth-
ylzinc iodide 1b appears to be the most practical reagent
because the benzoyl group is sufficiently stable during the
metalation and cross-coupling but can be easily cleaved off
at the end. 6-(Hydroxymethyl)purine nucleosides possess
interesting cytostatic and ADA inhibitory effect, and thus
other related derivatives and analogues, including acyclic
nucleosides with side chains from known potent ADA
inhibitors (EHNA, etc.) are a desirable target. Extension of
this methodology to these purine derivatives, as well as to
other related heterocycles, is under way.
Table 4. Cytostatic and ADA Inhibitory Activity of Title
Compounds
IC50 (µmol/l)a
compd
HL60
CCRF-CEM
ADA
5c
5d
5e
0.01 ((0.0011)
NAb
12.0 ((0.96)
0.15 ((0.0011)
NA
∼30
1.70 ((0.136)
10.0 ((0.90)
NA
a Values are means of four experiments; standard deviation is given in
parentheses. b NA ) not active (inhibition of cell growth at 10 µM was
lower than 30%).
0.15 µmol/L, respectively) but no considerable effect against
L1210 and HeLa S3 cell lines. The corresponding purine
base 5e showed only weak cytostatic effects, and the other
tested compounds were entirely inactive. Ribonucleoside 5c
was, in accord with the previous finding,8 also found to exert
considerable inhibitory effect on ADA (IC50 ) 1.7 µmol/
L), the corresponding 2-deoxyriboside 5d was about 1 order
of magnitude less potent, and all of the other compounds
were inactive.
Acknowledgment. This work is a part of the research
project Z4 055 905. It was supported by the Grant Agency
of the Czech Republic (grant no. 203/03/0035) and by
Sumika Fine Chemicals, Co. Ltd. (Osaka, Japan).
Supporting Information Available: Experimental and
spectral data for compounds 4-6. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL049059R
3228
Org. Lett., Vol. 6, No. 19, 2004