Cyclin-dependent kinase (CDK) inhibitors: Development of a general strategy for the construction of 2,6,9-trisubstituted purine libraries. Part 2

Article abstract of DOI:10.1016/S0040-4039(01)01752-X

Purine bound resins 1a-c were obtained by the reaction of 6-thiopurines 2 or 6-chloropurines 3-5 with Merrifield-Cl or -SH resins (DMF, 70°C, base). S-Oxidation of resins 1c and reaction of the desired sulfone with p-methoxybenzylamine to give 6, proved e

Full text of DOI:10.1016/S0040-4039(01)01752-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 8165–8167
Cyclin-dependent kinase (CDK) inhibitors: development of a
general strategy for the construction of 2,6,9-trisubstituted purine
libraries. Part 2
Virginie Brun, Michel Legraverend* and David S. Grierson
Laboratoire de Pharmacochimie, UMR 176 CNRS-Institut Curie, Baˆt 110 Centre Universitaire, 91405, Orsay, France
Received 2 August 2001; accepted 13 September 2001
Abstract—Purine bound resins 1a–c were obtained by the reaction of 6-thiopurines 2 or 6-chloropurines 3–5 with Merrifield-Cl
or -SH resins (DMF, 70°C, base). S-Oxidation of resins 1c and reaction of the desired sulfone with p-methoxybenzylamine to give
6, proved effective for release of the purine from the resin and simultaneous C-6 substitution. Reaction of resin 1c with pyrrolidine
and pyrrolidine-2-methanol prior to S-oxidation led to the C-2 amine substituted resin bound purines 8 and 9. Activation of sulfur
in these intermediates, followed by reaction with Ar–CH2–NH2 gave the 2,6,9-trisubstituted purines 10–14 in 42–60% yields.
© 2001 Elsevier Science Ltd. All rights reserved.
The development of potent and specific inhibitors of
cyclin dependent kinases (CDKs) is a current challenge
with potentially important applications for such
molecules as biochemical probes for the study of the
different phases of the cell cycle and as therapeutic
agents.1 In the preceding communication we outlined
the principle behind a solid-phase synthesis strategy for
the construction of 2,6,9-trisubstituted purine libraries
based upon the reactivity of a 6-thio substituted purine
intermediate connected to the resin support via the
sulfur atom.2–8 The basic tenet is that the low reactivity
of the purine C6ꢀS bond will provide the possibility to
introduce a wide variety of functionality at the N-9 and
C-2 positions prior to reaction at C-6. Subsequent
activation of the sulfur atom (oxidation, alkylation,
etc.) then opens the way to concomitant introduction of
a substituent (nucleophile) at C-6 and cleavage of the
trisubstituted purine product from the resin. In this
way, full advantage is taken of solid-phase synthesis to
introduce functionality at all three centers of interest in
the purine ring.
Having demonstrated the efficacy of this strategy in
solution, it remained to determine whether the sequence
of operations could be effected on solid support. In this
communication we describe the results of experiments
to optimize both the loading of purine derivatives onto
a Merrifield type resin, as well as the cleavage of the
corresponding 6-sulfone linked purines from this resin
through reaction with amine nucleophiles. We further
describe the construction of a small, but representative
2,6-diaminopurine library of potential CDK inhibitors.
To prepare the resin bound purine 1a, the reaction of
6-thiopurine 2 with Merrifield-Cl resin at 70°C in DMF
was examined in initial experiments using either KOtBu
or NaH as base (Scheme 1). Under these mild condi-
tions it was found that the loading yield (determined by
Scheme 1. See preceding paper for reaction conditions of 3–5, Ref. 10 for 2 and Ref. 11 for 3.
* Corresponding author. Fax: 33 (0) 169 86 30 85; e-mail: michel.legraverend@curie.u-psud.fr
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01752-X

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V. Brun et al. / Tetrahedron Letters 42 (2001) 8165–8167
microanalysis; % nitrogen) varied little (87–95%) with
mesh size (400–500 mm“35–75 mm) and loading capac-
ity (1 or 2 mmol/g). The best results were obtained
using the smaller 35–75 mm resin at 1 mmol/g charge
capacity, and KOtBu base. Identical loading yields
were obtained under these conditions in the reaction of
6-chloropurine 3 with Merrifield-SH resin. Somewhat
lower loadings (75%) were observed for the attachment
of compounds 4, and 5 onto the Merrifield-SH resin.7
matography) was in the 70–90% range (approx. 50%
overall yield for loading and release operations).
The 2-iodo-9-(iso-propyl)purine containing resin 1c was
used to subsequently measure the efficiency with which
amine substituents could be introduced at C-2 of the
purine ring while it is bound to the support. As heating
to 100–150°C for extended periods is often required to
effect exchange of the halogen atom in 2-chloro or
2-iodopurine derivatives by an amine nucleophile there
was some concern that the reaction of purine 1c with
amines may be accompanied by significant decomposi-
tion of the resin. To get an idea as to what conditions
would be required, a series of different amines were first
reacted with the 6-benzylthiol-2-iodopurine 79 (Scheme
2). It was found that it was necessary to heat at 120°C
The efficiency of the release of the purine from resin 1c
was then evaluated following the two-step protocol
developed in solution (S-oxidation/nucleophilic dis-
placement). Note that longer reaction times (16–24 h)
were employed for each step to ensure complete trans-
formation. The yield for product 6 (after column chro-
Scheme 2.
Scheme 3.

V. Brun et al. / Tetrahedron Letters 42 (2001) 8165–8167
8167
Acknowledgements
for only short time periods for the cyclic amines and
ethanolamine, whereas heating up to 17 and 12 h,
respectively, was needed for N-ethylethanolamine and
m-methoxybenzylamine. Most interesting, it was
found that the reaction with pyrrolidine as solvent
occurred readily at room temperature!
This work has been generously financed by ARC
(Association pour la Recherche sur le Cancer, Grant
no. 5713), and by an MENRT scholarship for V.B.
Pyrrolidine-2-methanol and pyrrolidine were selected
for reaction with resin 1c. In separate experiments the
resin was heated at 80°C for 24 h in dimethyl-
acetamide (solvent) containing 5 equiv. of the respec-
tive amine and 3 equiv. of tri-n-propylamine. After
washing, the resin bound purine was treated with 2.2
equiv. of m-CPBA in CH2Cl2 at room temperature
for 24 h, and the derived sulfone was then reacted
with ArCH2NH2 (5 equiv.) in THF at 65°C for 24 h.
The released trisubstituted purine products 10–14
were isolated pure by silica-gel column chromatogra-
phy. As can be seen, the overall yields for the four
operations are good (45–60%), indicating that the I“
amine exchange reaction was efficient, and that little
if any competing oxidation (N-oxide formation) of
the pyrrolidine nitrogen in intermediates 8 and 9
occurred. It is probable that the isolated product
yields could be further improved by treatment of the
crude product mixture containing the excess amine
with a formyl scavenger resin before (or instead of)
chromatography (Scheme 3).
References
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It was thus shown that a small library of 2,6,9-trisub-
stituted purines could be prepared via solid-phase
organic synthesis using the 6-thiopurine bound Mer-
rifield resin. Current efforts are being directed to
increase the scope of amine substituents that can be
introduced at C-2. Modern Pd(0) coupling methodol-
ogy is playing an important role in this work, and as
described in the following communication, the nature
of the linker joining the purine to the resin is also a
crucial factor.9
1
10. Compound 2: mp >250°C. H NMR (DMSO-d6) l 1.3–
2.3 (m, 6H, H2%,3%,4%); 3.5–3.8 (m, 1H, H5%); 3.8–4.0 (m,
1H, H5%); 5.3–5.5 (m, 1H, H1%); 7.96 (s, 1H, H8); 10.7 (s
broad, 1H, NH or SH). MS (CI/NH3) m/z 279 (M-
THP)+; m/z 385 (MH+NH3)+; m/z 386 and 387.
1
11. Compound 3: mp 120°C; H NMR (DMSO-d6) l 1.55–
2.2 m, 6H, H2,3,4); 3.6–3.85 (dt, 1H, J=3.61 and 10.29
Hz, H5%); 4.05–4.2 (m, 1H, H5%); 5.6–5.8 (dd, 1H, J=
2.06 and 9.81, H1%); 8.21 (s, 1H, H8). MS (CI/NH3) m/z
365 (MH)+; m/z 364 (M)+; m/z 366 (M+2)+.