Microwave-assisted one step synthesis of 8-arylmethyl-9H-purin-6-amines

Article abstract of DOI:10.1016/j.bmcl.2008.11.057

Molecular chaperone heat shock protein 90 (Hsp90) is an important target in cancer and neurodegenerative diseases, and has rapidly become the focus of several drug discovery efforts. Among small molecule Hsp90 inhibitors with clinical applicability are de

Full text of DOI:10.1016/j.bmcl.2008.11.057

Bioorganic & Medicinal Chemistry Letters 19 (2009) 415–417
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Microwave-assisted one step synthesis of 8-arylmethyl-9H-purin-6-amines
*
Hui Tao, Yanlong Kang, Tony Taldone, Gabriela Chiosis
Program in Molecular Pharmacology and Chemistry and Department of Medicine, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, Box 482, New York, NY 10021, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Molecular chaperone heat shock protein 90 (Hsp90) is an important target in cancer and neurodegener-
ative diseases, and has rapidly become the focus of several drug discovery efforts. Among small molecule
Hsp90 inhibitors with clinical applicability are derivatives of 8-arylmethyl-9H-purin-6-amine class. Here
we report the use of microwave-assisted chemistry for the successful one-pot delivery of 8-arylmethyl-
9H-purin-6-amines. We discuss the applicability as well as the limitations of this method towards the
creation of a large chemical diversity in the 8-arylmethyl-9H-purin-6-amine series.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 23 October 2008
Accepted 17 November 2008
Available online 20 November 2008
Keywords:
Hsp90
Microwave-assisted organic synthesis
Purines
Cancer
Neurodegenerative diseases
Molecular chaperone heat shock protein 90 (Hsp90) is an
important target in cancer, and has rapidly become the focus of
several drug discovery efforts.1 Indeed, Hsp90 has emerged as a
cancer target because of its key participation in regulating clini-
cally validated oncogenic proteins such as Her-2, ER, and Bcr-
Ab1, as well as other signaling proteins that play a role in malig-
nancy, such as Raf-1, p-Akt, Cdk4, mutant p53 and Flt-3.2
Purine-scaffold Hsp90 inhibitors are the first synthetic small
molecule Hsp90 inhibitors to be discovered and to be translated
to clinic in patients with advanced cancers.3 Among purines with
Hsp90 inhibitory activities are the 8-arylmethyl-9H-purin-6-
amine derivatives, such as PU3, PU24FCl and PU-DZ8 (Fig. 1).3a
PU3 is the first discovered synthetic Hsp90 inhibitor, and was iden-
tified through empirical structure-based design.3b PU24FCl, a
derivative of PU3, is the first synthetic Hsp90 inhibitor reported
to exhibit anti-tumor activity and biological activity in a wide-
range of tumor types.3c
consisting of generating the acid fluoride of the corresponding phe-
nyl acetic acid, coupling it with pyrimidine-4,5,6-triamine or
pyrimidine-2,4,5,6-tetraamine in the presence of K2CO3, followed
by condensation of the intermediate amide product (Scheme 1).4
These condensation reactions require harsh dehydration condi-
tions and/or long reaction times, such as refluxing overnight in
ethanol in the presence of 15 equivalents of NaOMe,4a or refluxing
PU-DZ8 has recently been reported to reduce aberrant neuronal
proteins and diminish toxic tau aggregates in transgenic mouse
models of dementias caused by tau protein toxicity, such as Alzhei-
mer’s disease and frontotemporal dementias.3d Considering their
biological effects in both cancer and neurodegenerative diseases,
there is an increased interest in their development and optimiza-
tion towards a clinical candidate.3
Attempts to result in higher chemical diversity in the 8-aryl-
methyl-9H-purin-6-amine series remain limited by the somewhat
cumbersome synthesis on which building of the 8-benzyl purine
core relies.4 Its synthesis continues to employ a three-step protocol
* Corresponding author.
E-mail address: chiosisg@mskcc.org (G. Chiosis).
Figure. 1. Hsp90 inhibitors containing the 8-arylmethyl-9H-purin-6-amine core.
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.11.057

416
H. Tao et al. / Bioorg. Med. Chem. Lett. 19 (2009) 415–417
Table 1
Evaluation of carboxylic acids and pyrimidine-4,5,6-triamine8
Entry
3
R1
R2
R3
X
Yielda
1
3a
3b
3c
OCH2O
H
OMe
—
C
C
N
87
98
93
2
OMe
H
OMe
H
3b
a
Isolated yields by preparative TLC.
3-Pyridylacetic acid hydrochloride was used in this reaction.
b
Scheme 1. Three-step synthesis of 8-arylmethyl-9H-purin-6-amines.
for 5 h in a 25% solution of NaOMe in methanol/isobutanol,4b
which limit the spectrum of suitable starting materials. Reliance
on the multi-step protocol results in lower overall yields, with a
20% isolated yield reported for the precursor of PU-DZ8.4b Dymock
et al. utilized a two-step protocol in which the carboxylic acid is di-
rectly coupled with the tri- or tetra-aminopyrimidine in the pres-
ence of i-Pr2NEt and HBTU in DMF, followed by condensation of
the intermediate amide product by refluxing overnight with
NaOMe (15 equivalents) in EtOH or by microwave irradiation for
30 min at 130 °C.4a Unfortunately, yields were not reported so that
a satisfactory comparison between each of these methods cannot
be made.
To limit the number and length of the synthetic steps, and to
increase the deliverable chemical variability in the 8-arylmethyl-
9H-purin-6-amine series, we investigated the use of microwave
irradiation. Microwave-assisted organic synthesis continues to
grow in importance in synthetic chemistry by enabling rapid
chemistry development and numerous reactions, including con-
densations and heterocycle formations have been explored under
these conditions.5 Herein, we report the use of microwave-assisted
chemistry for the successful one-pot delivery of 8-arylmethyl-9H-
purin-6-amines. In addition, we briefly discuss cases in which the
reaction fails to deliver the desired product.
Previously, Lin et al. have reported a microwave-assisted synthe-
sis of benzimidazoles and other heterocycles from condensation of a
diamine with a carboxylic acid.6 We probed the feasibility of adapt-
ing this method7 to more functionally complex amino-pyrimidines
in an attempt to produce the desired condensation product in a sin-
gle step. In an initial effort to prepare several PU-DZ8 derivatives,
2-(benzo[d][1,3]dioxol-5-yl)acetic acid (1.0 equiv) and pyrimidine-
4,5,6-triamine (1.2 equiv) were irradiated at 220 °C for 15 min
in the presence of P(OPh)3 (1.2 equiv) in pyridine, to result in
8-(benzo[d][1,3]dioxol-5-ylmethyl)-9H-purin-6-amine (3a) in 87%
isolated yield. High recovered yields were also noted when the phe-
nyl ring was replaced with a pyridine moiety (Entry 3, Table 1).
When the triamine-pyrimidine substrate (1a) was replaced
with its tetraamine-equivalent (1b), which is required for the
building of PU-DZ8 derivatives, the recorded yields decreased un-
der the Lin conditions (87% yield, Entry 1, Table 1 versus 62% yield,
Entry 1, Table 2). The relatively lower solubility of 1b in pyridine,
when compared to 1a, could be partly accountable for poorer
yields. Increasing the reaction time to 20 min did not significantly
improve the yield (65% yield, Entry 2, Table 2). Addition of DMSO
or DMF, solvents which in our hands usually increase the solubility
of purines and pyrimidines, had an adverse effect on the yield (En-
tries 3 and 4, Table 2). As expected, addition of a strong base, NaOt-
Bu, augmented the yield to 78% of recovered material, possibly by a
two pronged beneficial effect, on both solubility and coupling and
condensation efficacy (Entry 5, Table 2).
Table 2
Evaluation of methodology for the synthesis of PU-DZ8
Entry
1b
Solvent
Co-solvent (50%)
Base (2 eq.)
t (min)
Yielda
1
2
3
4
5
6
7
8
Base
Base
Base
Base
Py
Py
Py
Py
Py
Py
Py
Py
N/Ab
N/A
DMF
DMSO
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
NaOtBu
N/A
NaOH
NaOtBu
15
20
15
15
15
15
15
15
62
65
18
11
78
5
Base
Sulfate
Sulfate
Sulfate
25
31
a
Isolated yields by preparative TLC.
None used.
b
The free base form of pyrimidine-2,4,5,6-tetraamine is commer-
cially unavailable due to its propensity to oxidize and decompose,
and therefore short shelf life. The compound is instead sold as a sul-
fate salt of very poor solubility in organic solvents. The base is usu-
ally generated by recrystallization from 10% NaOH aqueous solution
before its use in the coupling with carboxylic acids 2. To eliminate
the extra step, we probed the in situ conversion of the sulfate into
free base. Surprisingly, pyrimidine-2,4,5,6-tetraamine sulfate re-
mained poorly soluble in pyridine even when heated at 220 °C. Addi-
tion of NaOH (4 equiv), rendered pyrimidine-2,4,5,6-tetraamine
sulfate completely soluble in pyridine, however, increased the yield
to only 25% (Entry 7, Table 2). We assumed that one reason could be
the in situ generated water. This however, can only partly be true,
because replacement of NaOH with NaOtBu had only a marginal
benefit on the recovered yield (Entry 8, Table 2).
To evaluate the limitations of this procedure towards the for-
mation of 8-arylmethyl-9H-purin-6-amines, we chose to investi-
gate the addition of functionalities at positions more likely to
result in interference with the condensation reaction (Table 3).
These were estimated to be functionalities either in the ortho posi-
tion of the aryl ring (R2 Table 3) or adjacent to the carboxyl func-
tionality (R1 Table 3). We found that the reaction was sensitive
to the nature of the ortho functionality, as demonstrated by lower
yields when R2 = H was exchanged with a trifluoromethyl or cyano
functionality (23% recovered yield for 3e and traces detectable by
mass spectrum for 3f). This outcome could be due to steric effects,
because attempts to increase the solubility of the pyrimidine 1b
and to facilitate coupling and condensation by addition of a
base, NaOtBu (2 equiv), failed to result in improved yields (Entry

H. Tao et al. / Bioorg. Med. Chem. Lett. 19 (2009) 415–417
417
Table 3
References and notes
Evaluation of carboxylic acids on the reaction yield9
1. Chiosis, G.; Caldas Lopes, E.; Solit, D. Curr. Opin. Invest. D 2006, 6, 534.
2. (a) Kamal, A.; Boehm, M. F.; Burrows, F. J. Trends Mol. Med. 2004, 10, 283; (b)
Neckers, L.; Neckers, K. Expert Opin. Emerg. Drugs 2005, 10, 137; (c) Powers, M.
V.; Workman, P. Endocr. Relat. Cancer 2006, 125. Suppl. 1; (d) Chiosis, G. Expert
Opin. Ther. Targets 2006, 10, 37.
3. (a) Chiosis, G.; Kang, Y.; Sun, W. Expert Opin. Drug Disc. 2008, 3, 99; (b) Chiosis,
G.; Timaul, M. N.; Lucas, B.; Munster, P. N.; Zheng, F. F.; Sepp-Lorenzino, L.;
Rosen, N. Chem. Biol. 2001, 8, 289; (c) Vilenchik, M.; Solit, D.; Basso, A.; Huezo,
H.; Lucas, B.; He, H.; Rosen, N.; Spampinato, C.; Modrich, P.; Chosis, G. Chem. Biol.
2004, 11, 787; (d) Luo, W.; Dou, F.; Rodina, A.; Chip, S.; Kim, J.; Zhao, Q.; Moulick,
K.; Aguirre, J.; Wu, N.; Greengard, P.; Chiosis, G. Proc. Natl. Acad. Sci. USA 2007,
104, 9511.
4. (a) Dymock, B.; Barril, X.; Beswick, M.; Colliere, A.; Davies, N.; Drysdale, M.; Fink,
A.; Fromont, C.; Hubbard, R.; Massey, A.; Surgenor, A.; Wright, L. Bioorg. Med.
Chem. Lett. 2004, 14, 325; (b) He, H.; Zatorska, D.; Kim, J.; Aguirre, J.; Llauger, L.;
She, Y.; Wu, N.; Immormino, R. M.; Gewirth, D. T.; Chiosis, G. J. Med. Chem. 2006,
49, 381.
Entry
3
R1
R2
R3, R4
R5
Base
Yielda
1
2
3
4
5
6
7
8
3d
3e
3f
H
H
H
H
OH
OMe
Me
H
H
OCH2O
OCH2O
OCH2O
OCH2O
OCH2O
OCH2O
H, H
H
H
H
H
H
H
H
OMe
N/Ab
N/A
N/A
NaOtBu
N/A
N/A
62
23
<5
<5
NR
NR
48
53
CF3
CN
CN
H
H
H
3f
3g
3h
3i
N/A
N/A
5. (a) Kappe, C. O. Comprehensive Med. Chem. II 2006, 3, 837; (b) Kappe, C. O. Angew.
Chem. Int. Ed. 2004, 43, 6250.
3j
H
OMe, OMe
6. Lin, S.; Isome, Y.; Stewart, E.; Liu, J.; Yohannes, D.; Yu, L. Tetrahedron Lett. 2006,
47, 2883.
7. General reaction procedure for the synthesis of 8-arylmethyl-9H-purin-6-amines 3:
a
Isolated yields by preparative TLC.
None used.
b
In
a
conical-bottomed Smith process vial, the mixture of aryl acetic acid
L,
(0.2 mmol), amino pyrimidine (0.24 mmol) and triphenyl phosphite (63
l
0.24 mmol) in 1 mL anhydrous pyridine were charged. The sealed vial was
irradiated in the microwave for 15 min at 220 °C. The pressure reading at this
temperature was around 8 bar. After cooling, the reaction mixture was
concentrated under vacuum and the residue purified by preparative TLC
(CHCl3/ammonia MeOH = 25:1) to give the desired product 3.
4, Table 3). We found that insertion of a substituent at the
a
-posi-
tion of the acetyl carboxylic acid (R1) also played an important role
in this reaction. The transformation tolerated the addition of a
methyl (Entry 7, Table 3), however, no product was detected when
hydroxyl or methoxy derivatives were employed (Entries 5 and 6,
Table 3). It is unclear whether a lack of product in the case of hy-
droxyl and methoxy derivatives 2 is due to decomposition of the
carboxylic acid 2, or to electronic effects which reduce the electro-
philicity of the carboxyl group.
8. 8-(Benzo[d][1,3]dioxol-5-ylmethyl)-9H-purin-6-amine (3a): 1H NMR: 12.61 (br
s, 1H), 7.94 (s, 1H), 6.87 (br s, 2H), 6.83 (s, 1H), 6.74 (d, J = 7.9 Hz, 1H), 6.65 (d,
J = 7.9 Hz, 1H), 5.86 (s, 2 H), 3.91 (s, 2H); 13C NMR: 155.0, 151.9, 151.1, 150.5,
147.3, 145.9, 130.9, 121.7, 109.1, 108.2, 100.8, 34.6; MS m/z 269.9 (M+H)+.
8-(3,4,5-Trimethoxybenzyl)-9H-purin-6-amine (3b): 1H NMR: 12.50 (br s, 1H),
7.82 (s, 1H), 6.76 (s, 2H), 6.42 (s, 2H), 3.80 (s, 2H), 3.79 (s, 6H), 3.67 (s, 3H); 13C
NMR: 157.3, 152.8, 151.9, 136.2, 132.7, 129.3, 118.8, 115.2, 106.1, 59.9, 55.8,
35.3; MS m/z 316.08 (M+H)+.
8-(pyridin-3-ylmethyl)-9H-purin-6-amine (3c): 1H NMR: 12.77 (br s, 1H), 8.57
(d, J = 1.5 Hz, 1H), 8.46 (dd, J = 4.8, 1.5 Hz, 1H), 8.07 (s, 1H), 7.72–7.70 (m, 1 H),
7.35 (dd, J = 7.5, 4.8 Hz, 1H), 7.03 (s, 2H)4.18 (s, 2H); 13C NMR: 154,5, 152.0,
151.4, 150.2, 149.7, 136.4, 132.9, 123.7, 32.0; MS m/z 226.99 (M+H)+.
9. 8-(Benzo[d][1,3]dioxol-5-ylmethyl)-9H-purine-2,6-diamine (3d): 1H NMR:
11.97 (s, 1H), 6.91 (s, 1H), 6.88 (d, J = 7.9 Hz, 1H), 6.78 (d, J = 7.9 Hz, 1H), 6.55
(s, 2H), 5.62 (s, 2H), 3.94 (s, 2H); 13C NMR: 159.8, 155.2, 153.3, 147.2, 146.7,
145.7, 131.6, 121.5, 112.8, 109.0 108.1, 100.8, 34.5; MS m/z 284.99 (M+H)+.
8-((6-(Trifluoromethyl)benzo[d][1,3]dioxol-5-yl)methyl)-9H-purine-2,6-diamine
(3e): 1H NMR: 11.96 (br s, 1H), 7.26 (s, 1H), 6.95 (s, 1H), 6.43 (s, 2H), 6.14 (s, 2H),
5.96 (s, 2H), 5.56 (s, 2H), 4.11 (s, 2H); 13C NMR: 159.8, 155.2, 153.3, 150.1, 146.2,
145.3, 131.0, 129.3, 115.2, 113.0, 111.7, 106.0, 102.3, 31.2; MS m/z 352.99
(M+H)+.
In summary, we present a simplified one-pot synthesis of 8-
arylmethyl-9H-purin-6-amines which can generate the desired
product with yields comparable or significantly higher, depending
on the reaction set-up, to the previously published route. The pro-
cedure is amenable for structure–activity investigation efforts in
the pursuit of a higher chemical diversity in the 8-arylmethyl-
9H-purin-6-amine Hsp90 inhibitor series.
Acknowledgments
8-(1-Phenylethyl)-9H-purine-2,6-diamine (3i): 1H NMR: 11.95 (br s, 1H), 7.38–
7.33 (m, 1H), 7.34 (s, 2H), 7.27–7.20 (m, 1H), 6.82–6.80 (m, 1H), 6.52 (s, 2H), 5.60 (s,
2H), 4.25 (q, J = 7.3 Hz, 1H), 1.67 (d, J = 7.3 Hz, 3H); 13C NMR: 159.8, 155.2, 150.5,
143.9, 129.3, 128.4, 127.1, 126.4, 118.8, 115.2, 20.2; MS m/z 254.99 (M+H)+.
8-(3,4,5-Trimethoxybenzyl)-9H-purine-2,6-diamine (3j): 1H NMR:11.73(brs, 1H),
6.38 (s, 2H), 6.32 (s, 2H), 5.35 (s, 2H), 3.53 (s, 6H), 3.40 (s, 3H), 2.94 (s, 2H); 13C NMR:
159.8, 157.3, 152.8, 146.6, 136.1, 129.3, 118.8, 115.2, 106.0, 59.9, 55.8, 35.3; MS m/z
331.12 (M+H)+.
This work was supported in part by the Susan G. Komen Breast
Cancer Foundation (G.C. and Y.K.), the SynCure Cancer Research
Foundation (G.C.), the Institute for the Study of Aging (Alzdiscovery
award; G.C.), the Manhasset Women’s Coalition Against Breast Can-
cer (G.C.) and the National Institute of Aging (1R21AG028811; G.C.).