Solid-phase synthesis of N-9-substituted 2,8-diaminopurines

Article abstract of DOI:10.1016/j.tetlet.2006.10.050

A general and efficient solid-phase synthesis of N-9-substituted 2,8-diaminopurines from 5-nitrouracil is described. The key synthetic transformation employs a carbodiimide-mediated cyclization of a thiourea. Thiourea formation on solid phase is performed

Full text of DOI:10.1016/j.tetlet.2006.10.050

Tetrahedron Letters 47 (2006) 8897–8900
Solid-phase synthesis of N-9-substituted 2,8-diaminopurines
Andrew G. Cole,* Axel Metzger, Gulzar Ahmed, Marc-Raleigh Brescia, Ray J. Chan,
James Wen, Linda O’Brien, Lan-Ying Qin and Ian Henderson
Pharmacopeia Drug Discovery, Inc., PO Box 5350, Princeton, NJ 08543, USA
Received 4 June 2006; revised 8 October 2006; accepted 10 October 2006
Available online 30 October 2006
Abstract—A general and efficient solid-phase synthesis of N-9-substituted 2,8-diaminopurines from 5-nitrouracil is described. The
key synthetic transformation employs a carbodiimide-mediated cyclization of a thiourea. Thiourea formation on solid phase is per-
formed using both thermal and microwave reaction conditions. Regiospecific solution-phase synthesis of key building blocks allows
the incorporation of desired substituents at N-9 of the purine nucleus.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Purines represent an important class of compounds
from a biochemical perspective with a myriad of physio-
logical processes being controlled or supported by
agents based on the purine core. The diversity of the
purine nucleus within the complex science of nature
ranges from inclusion in nucleic acids to molecules con-
trolling key neurological functions.1 In addition, the
natural diversity present within the ATP binding sites
of kinases provides the opportunity to develop selective
purine-based kinase inhibitors with enormous therapeu-
tic potential.2,3 The importance of purines and purine
mimics in pharmaceutical research necessitates the
development of efficient and versatile syntheses of such
molecules.4
5-Nitrouracil (1) was selected as the basis for construc-
tion of the purine ring system, fixing the C-6 substituent
of the generated purine as hydrogen. This also allows
structural diversity to be incorporated at the 2-, 8- and
9-positions of the purine core. The initial conversion
of 1 to the common intermediate 2,4-dichloro-5-
nitropyrimidine (2) was performed using phosphorus
oxychloride. Solution-phase amination of 2 allows regio-
selective nucleophilic displacement to be conducted
at C-4 based on the increased reactivity of the 4-chloro
position. The reaction of 2 with a selection of primary
amines and primary anilines in THF at ꢀ78 ꢁC resulted
in the formation of 3 and 4 with the ratios of regioisom-
ers between 10:1 and 20:1 (Scheme 1). The desired major
regioisomers 3 were readily isolated by flash chromato-
graphy to provide the key intermediates for incorpora-
tion into the solid-phase synthesis.
In recent years, there have been significant advances in
solid-phase chemistry, facilitating the synthesis of com-
binatorial and parallel compound collections.5–7 A num-
ber of solid-phase syntheses of purine-based compounds
have been reported, using either a scaffold-based
approach8 or an on-resin strategy for the formation of
these purine derivatives.9 The aim of this project was
to develop a general and efficient solid-phase synthesis
of N-9-substituted 2,8-diaminopurines that allows diver-
sity elements to be incorporated at C-2, C-8 and N-9.
The development of a robust solid-phase synthesis
would subsequently be used to generate parallel and
combinatorial N-9-substituted 2,8-diaminopurine com-
pound collections.10
The development of a viable solid-phase synthesis neces-
sitates a connection point to the solid support. Using
intermediates 3, solid-phase attachment can efficiently
be achieved through C-2 of the pyrimidine via chlorine
displacement with a resin-bound secondary amine 8
NO2
Cl
NO2
O
NO2
NO2
Cl
b
a
N
HN
N
N
+
HN
R2
N
O
N
H
Cl
N
NH
R2
Cl
N
3 - Major
4 - Minor
1
2
*
Scheme 1. Reagents and conditions: (a) POCl3, i-Pr2NEt, 0–25 ꢁC,
70%; (b) R2NH2, i-Pr2NEt, THF, ꢀ78ꢁC.
Corresponding author. Tel.: +1 609 452 3653; fax: +1 609 655
4187; e-mail: acole@pcop.com
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.10.050

8898
A. G. Cole et al. / Tetrahedron Letters 47 (2006) 8897–8900
O
borohydride in 1,2-dichloroethane (Scheme 3). The
resin-bound 5-nitro-pyrimidine 9 was then formed by
N-arylation of 8 with 3.
4
c
a, b
NH2
O
N
NH2
H
NH2
5
6
The determination of optimal conditions for the reduc-
tion of the nitro group was subsequently examined. A
number of methods have been used to conduct such
reductions on the solid phase, including tin(II) chlo-
ride,11 chromium(II) chloride12 and lithium aluminum
hydride/aluminum chloride.9 However, the reduction
conditions that generate an acidic medium were
excluded due to the potential for undesired premature
cleavage of the pyrimidine intermediate from the acid-
cleavable linker. Also, reduction conditions compatible
with a variety of protecting groups that would be
included within the R1 and R2 substituents were also
required. The ability of polyethyleneglycolated resins
such as Argogelꢂ to swell in alcoholic and aqueous sol-
vents provided the opportunity to employ a sodium
hydrosulfite-based reduction under mildly basic condi-
tions in aqueous media13 (Scheme 3). A variety of
hydrosulfite concentrations was investigated, in con-
junction with different bases and solvent systems. The
use of 0.5 M sodium hydrosulfite and 0.5 M ammonium
hydroxide in 2:1 v/v water/p-dioxane provided good
results, allowing a clean conversion of 9 to triamino-
pyrimidine 10.
CHO
OMe
3
H
N
H
N
H
O
L
O
O
=
HN
4
O
OMe
CHO
7
O
3
7
Scheme 2. Reagents and conditions: (a) N-a-N-e-bis-Fmoc-Lys, HOBt
monohydrate, DIC, CH2Cl2, DMF, 25 ꢁC; (b) piperidine, DMF,
25 ꢁC; (c) 4-(40-formyl-30-methoxy)phenoxybutyric acid, HOBt mono-
hydrate, DIC, CH2Cl2, DMF, 25 ꢁC.
(vide infra). Solid-phase synthesis was initiated by acyl-
ating aminomethyl-terminated Argogelꢂ (5) with N-a-
N-e-bis-Fmoc-lysine followed by Fmoc deprotection to
generate 6 (Scheme 2). This increases the loading capa-
city of the resin by doubling the number of amino
groups for further functionalization. This was followed
by acylation with 4-(40-formyl-30-methoxy)phenoxy-
butyric acid. This linker allows acid-mediated cleavage
of the final compounds from solid phase, and is compat-
ible with the chemistry required to conduct the purine
synthesis. The generation of a resin-bound secondary
amine 8 was achieved by the reductive alkylation of a
primary amine (R1NH2) with 7 using sodium triacetoxy-
Cyclization to the diaminopurine derivative was envi-
saged via a two-step process involving reaction with an
isothiocyanate, followed by carbodiimide activation of
the resulting thiourea with subsequent cyclization to
the purine (Scheme 4).14 The functionalization of triami-
nopyrimidine 10 with an isothiocyanate required forcing
conditions due to its poor nucleophilicity. Alkyl thio-
urea 11 formation with alkyl isothiocyanates (AlkNCS)
occurred under thermal conditions via reaction of 10
with a 0.5 M solution of an alkyl isothiocyanate in
DMF/EtOH at 80 ꢁC. However, heating 10 with aryl
isothiocyanates (ArNCS) showed incomplete aryl thio-
urea formation. Microwave conditions were then inves-
tigated to perform this reaction. Microwave irradiation
of 10 with 0.5 M aryl isothiocyanate in CH2Cl2/DMF
resulted in the complete conversion of 10 to aryl thio-
urea15 12. The conditions to facilitate purine formation
H
a
b
L
O
L
NH
R1
7
8
NO2
NH2
N
c
N
L
N
R1
N
NH
R2
L
N
R1
N
NH
R2
9
10
Scheme 3. Reagents and conditions: (a) R1–NH2, Na(OAc)3BH, 1,2-
dichloroethane, 25 ꢁC; (b) 3, i-Pr2NEt, DMF, 25 ꢁC; (c) Na2S2O4,
NH4OH, p-dioxane, H2O, 25 ꢁC.
H
S
N
AlkR3
AlkR3
NH
AlkR3
NH
N
N
R2
N
N
R2
NH
N
N
N
e
c
HN
R1
N
L
N
R1
N
L
N
R1
N
NH
R2
a
b
15
13
11
NH2
H
N
S
N
Ar3
Ar3
NH
Ar3
NH
L
N
R1
N
NH
R2
N
N
N
N
NH2
N
e
d
N
R2
N
R2
HN
R1
N
10
L
N
R1
N
L
N
R1
N
NH
R2
16
14
12
Scheme 4. Reagents and conditions: (a) AlkNCS, DMF, EtOH, 80 ꢁC; (b) ArNCS, DMF, CH2Cl2, microwave; (c) DIC, DMF, EtOH, 80 ꢁC;
(d) DIC, CH2Cl2, DMF, EtOH, 25 ꢁC; (e) TFA, CH3CN, 25 ꢁC.

A. G. Cole et al. / Tetrahedron Letters 47 (2006) 8897–8900
8899
from the thiourea through carbodiimide-mediated cycli-
zation were subsequently investigated. The reaction of
alkyl thiourea intermediate 11 with diisopropylcarbodi-
imide (DIC) at room temperature resulted in minimal
cyclization. However, thermal cyclization to 13 occurred
smoothly in DMF/EtOH at 80 ꢁC. The cyclization of
aryl thiourea 12 to purine 14 was more facile, occurring
at room temperature on treatment with DIC. The final
cleavage of 13 and 14 from the solid support was
conducted using TFA in acetonitrile to provide 8-alkyl
diaminopurines 15 and 8-aryl diaminopurines 16, respec-
tively.
Using the reaction conditions developed for the solid-
phase synthesis, the formation of both 8-alkylamino
diaminopurines 15 and 8-arylamino diaminopurines
16 provided consistently good results with a wide
range of R1, R2 and R3 components. Alkyl-, hetero-
alkyl-, benzyl- and aryl-substitution could efficiently
be incorporated at all three diversity points (Table
1). The functionalities containing acid-labile protect-
ing groups were also included in the solid-phase syn-
thesis, with 15b and 16a incorporating N-Boc and
O-t-Bu groups, respectively. The removal of these
protecting groups was achieved concomitantly with
the acid cleavage of the diaminopurines from the solid
support.
To determine the yield of 15a–16d released from the
solid support upon acid cleavage, the combined eluent
from 20 beads of each compound was quantitatively
analyzed versus an analytically pure sample of the corre-
Figure 1. (A) HPLC profile of purified 16a at 220 nm. (B) HPLC
profile of the crude bead eluent of 16a at 220 nm.
Table 1. N-9-substituted 2,8-diaminopurines synthesized on solid phase
R3
NH
N
N
R1
N
R2
N
N
H
Compound
R1
R2
R3
nmol per beada
2.1
Yieldb (%)
67
Purityc (%)
81
O
F
15a
H2N
HO
15b
15c
2.8
1.7
89
54
88
77
O
O
O
16a
16b
16c
16d
1.7
2.5
1.9
2.3
55
81
61
75
83
89
80
79
O
F
O
O
O
O
O
NC
N
a Observed nmol per bead based on comparison with an analytical reference.
b % Yield based on average loading per bead.
c % Purity based on analytical HPLC analysis at 220 nm.

8900
A. G. Cole et al. / Tetrahedron Letters 47 (2006) 8897–8900
6. Lowrie, J. F.; DeLilse, R. K.; Hobbs, D. W.; Diller, D. J.
Comb. Chem. High Throughput Screening 2004, 7, 495–
510.
7. Ding, S.; Gray, N. S.; Wu, X.; Ding, Q.; Schultz, P. G.
J. Am. Chem. Soc. 2002, 124, 1594–1596.
sponding purine derivative. The released quantities ran-
ged from 1.7 to 2.8 nmol per bead, corresponding to
yields of 54–89% (Table 1). In addition to good yields,
the purity level of the crude N-9-substituted dimino-
purines was exceptionally high.
8. (a) Dorff, P. H.; Garigipati, R. S. Tetrahedron Lett. 2001,
42, 2771–2773; (b) Norman, T. C.; Gray, N. S.; Koh, J. T.;
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This is exemplified by Figure 1, which shows the HPLC
profile at 220 nm of the purified analytical standard of
16a in comparison to the crude bead eluent (Fig. 1).16
In conclusion, a general and efficient solid-phase synthe-
sis of N-9-substituted 2,8-diaminopurines has been
developed. The synthesis performs well with a wide
range of R1, R2 and R3 components, and provides a
high level of both yield and purity. This route provides
an effective method for the construction of both parallel
and combinatorial N-9-substituted 2,8-diaminopurine
libraries.
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15. Microwave reactions were performed using a LAVIS-1000
multiQUANT irradiating at 500 W for 5 · 1 min with
15 min intervals between each irradiation.
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50 · 3 mm C8 column.
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