Convenient synthesis of 2,9-disubstituted guanines mediated by solid- supported reagents

Article abstract of DOI:10.1016/S0040-4039(00)00466-4

A novel application has been developed to separate regioisomers chemoselectively using a solid-supported reagent. An N7/N9 purine regioisomer mixture was purified using aluminum oxide/H⁺ to provide the N9 isomer selectively as a parallel or high-throughput format (chemoselective high- throughput purification). 2,9-Disubstituted guanines were conveniently prepared by this new method in conjunction with solid-supported reagents. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00466-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 3573–3576
Convenient synthesis of 2,9-disubstituted guanines mediated by
solid-supported reagents
Warren McComas, Li Chen and Kyungjin Kim ∗
Roche Research Center, Hoffmann La-Roche, Inc., Nutley, NJ 07110-1199, USA
Received 17 January 2000; revised 28 February 2000; accepted 3 March 2000
Abstract
A novel application has been developed to separate regioisomers chemoselectively using a solid-supported
reagent. An N7/N9 purine regioisomer mixture was purified using aluminum oxide/H+ to provide the N9 isomer
selectively as a parallel or high-throughput format (chemoselective high-throughput purification). 2,9-Disubstituted
guanines were conveniently prepared by this new method in conjunction with solid-supported reagents. © 2000
Elsevier Science Ltd. All rights reserved.
Keywords: combinatorial chemistry; purines; supported reagents.
The guanine scaffold 1 has been of considerable interest because of its potent antiviral therapeutic
activity.1 Some analogs such as acyclovir, buciclovir, and gancyclovir have demonstrated antiherpes
activity.2 Several communications regarding high-throughput synthesis of 2,6-diaminopurines 2 or ade-
nines 3 have been published;3 however, no synthetic approach to the guanine scaffold using a combi-
natorial application has as yet been reported. During the course of exploring new synthetic strategies
directed toward the purine scaffolds described in Fig. 1, we developed an effective high-throughput
purification method to separate the N7/N9 regioisomers by solid-supported reagents (chemoselective
high-throughput purification).4a This novel application using solid-supported reagents avoids laborious
regioisomer separation and work-up steps and facilitates the library synthesis of diverse purines. Herein,
we describe a convenient high-throughput purification method using solid-supported reagents to separate
N7/N9 regioisomers of purines and its application to 2,9-disubstituted guanine synthesis.4b
Fig. 1. The purine class and the numbering of the core structure
∗
Corresponding author. Fax: (+1) 973-235-6084; e-mail: kyungjin.kim@roche.com (K. Kim)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00466-4
tetl 16746

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The guanine synthesis outlined in Scheme 1 started with the simple and direct alkylation of
O6-benzyl-2-aminopurine 4 with polymeric BEMP (2-tert-butylimino-2-diethylamino-1,3-dimethyl-
perhydro-1,3,2-diazaphosphorine).5,6 This convenient alkylation approach using polymeric base was
attempted in order to avoid the laborious Mitsunobu reaction introducing N9 substituents and tedious
aqueous work-up steps. The alkylations of O6-benzyl-2-aminopurine 4 proceeded smoothly in aceto-
nitrile at room temperature to yield a mixture of N7 and N9 regioisomers 5. The regioisomer ratio varying
between 1:4 and 1:9 was determined by the integration of the C8 1H NMR signal.7 Several inorganic-solid
filtration pads and ion exchange resins were examined to explore an appropriate and convenient high-
throughput method that could isolate the N9 isomer selectively from the mixture. Fortunately, aluminum
oxide/H+ proved to be the best filtration medium to obtain the N9 regioisomer 6 in moderate yields
(30–75%) and high purity (>99%) without any N7 isomer contamination. The moderate yield of 6d
(R1=cyclohexylmethyl) is partially attributed to incomplete N-alkylation, however, the unreactive 4 was
also scavenged while the reaction mixture was passed through the alumina pad. The N7 isomer started to
elute by washing the alumina pad with a 2 M solution of NH3/MeOH.8
Scheme 1. Synthetic approach to purine analogs mediated by solid-supported reagents: isolated yield of the N9 isomers 6 after
chemoselective purification employing an alumina/H+ pad; 6a (R1=CH3, 75%), 6b (R1=CH2CH2CH3, 48%), 6c (R1=CH2Ph,
56%), 6d (R1=cyclohexylmethyl, 30%)
Following the precedent method developed in our laboratory, subsequent acylation of 6 possessing
a weakly nucleophilic amine at C2 proceeded successfully with polymeric BEMP (Scheme 2).9 After
completion of the reaction, polymer-bound trisamine treatment to remove the excess acyl chloride
(scavenging pathway A) was followed by the deprotection of O6-benzyl with 80% TFA/CH2Cl2 to
provide several guanine core structures 8.10 Although the purities of final compounds determined by
LC/MS (ELSD) are good overall, isolated yields are not satisfactory (Table 1).11 To improve the
isolated yield, amino-functionalized silica was tested to sequester the excess acyl chloride instead of
the polystyrene-based trisamine resins.12 The intrinsic advantages of silica such as no swelling and
compatibility with various solvents were envisioned to affect the isolation of products in solution phase
synthesis. After completing N-acylation, ultrapure amino-silica (Si–NH2) was added and the mixture was
shaken overnight (scavenging pathway B). Interestingly, the isolated yields increased from 10 to 40%
with high purity. The increased yields were presumably attributed to the intrinsic physical properties
of silica as described above. General synthetic procedures of 6,9-disubstituted guanine analogues 8
are described as follows: a solution of O6-benzyl-2-aminopurine 4 (1.0 equiv.), polymer-bound BEMP
(1.2 equiv.) and various alkylating reagents (1.2 equiv.), and anhydrous acetonitrile (0.2 M) was stirred
overnight at room temperature. The mixture was passed through a pad of aluminum oxide/H+ with a
weight ratio of 1 g alumina per 1.0 mmol of 4 and then washed with anhydrous acetonitrile to provide
diverse N9 alkylated purines 6. For large-scale reactions, after passing through the alumina pad, the
combined organic filtrates were evaporated in vacuo and the residue was triturated with 1:1 diethyl
ether/hexane to provide highly pure N9 regioisomer (>99%). Subsequent acylation of 6 with polymer-
supported BEMP (3.0 equiv.) and diverse acyl chlorides (5.0 equiv.) in THF was carried out overnight at

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room temperature. Amino-silica gel (1.5-fold excess over acyl chloride) was added to the slurry in order
to remove the excess acyl chloride from the mixture. After shaking overnight, the mixture was filtered
and washed with THF. The combined filtrates were concentrated in vacuo. Treatment of the residue with
80% TFA/CH2Cl2 provided diverse guanine scaffolds 8 in moderate yield with high purity.13 The isolated
yield and purity of 2,9-disubstituted guanines 8 using two different sequestration methods are displayed
in Table 1.
Scheme 2. The library synthesis of 2,9-disubstituted guanines
Table 1
Isolated yields and purity scores of guanines 8
In summary, we report a novel application of solid-supported reagents that conveniently separates
N7/N9 purine regioisomers in a high-throughput format or on a large scale. Using this method, 2,9-
disubstituted guanines were prepared without any laborious regioisomer separation and tedious aqueous
work-up. To the best of our knowledge, this is the first feasible high-throughput application of a solid-
supported reagent used for regioisomer separation. In addition, the possibility of silica as a different solid-

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supported reagent was examined for sequestration. Progress towards the library synthesis of different
purine classes will be reported in due course utilizing these novel high-throughput purification and solid-
supported reagents.
References
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2. Herdewijin, P.; Pauwels, R.; Baba, M.; Balzarini, J.; De Clercq, E. J. Med. Chem. 1987, 30, 2131–2137.
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P. Tetrahedron Lett. 1997, 38, 1161–1164; (c) Nugiel, D.; Cornelius, L.; Corbett, J. J. Org. Chem. 1997, 62, 201–203; (d)
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5. O6-Benzyl-2-aminopurine 4 was prepared in large scale from commercially available 2-amino-6-chloropurine according to
the precedent: Chae, M. Y.; McDougall, M. G.; Dolan, M. E.; Swenn, K.; Pegg, A. E.; Moschel, R. C. J. Med. Chem. 1994,
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7. The N7/N9 ratio was determined by the integration of the C-8 1H NMR signal, which for the N7 isomer appears slightly
downfield from that of the N9 isomer: Raju, N.; Robins, R. K.; Vaghefi, M. M. J. Chem. Soc., Chem. Commun. 1989,
1769–1770 and references cited therein.
8. The purity of the N7 isomer determined by LC/MS (ELSD) varied between 60 and 96% depending on the alkylating
reagents. Due to space limitations, more details will be provided in a full account of this work.
9. Kim, K.; Le, K. Synlett 1999, 12, 1957–1959.
10. Polymer-bound trisamine: Booth, R. J.; Hodges J. C. J. Am. Chem. Soc. 1997, 119, 4882–4886.
11. Purity was determined by a C18 reverse phase HPLC column, WR-C18 3u 120A (30×3.2 mm) of ES Industries in 10–90%
CH3CN/H2O containing 0.02% TFA and monitored at 215 nm using a UV detector and by a SEDEX 55 evaporative light
scattering detector (ELSD). Purity scores reported herein are based on ELSD.
12. Ultrapure amino silica gels are commercially available from SiliCycle, Inc., Quebec, Canada. Hewlett–Packard Co. now
provides various functionalized silica as CombiZorb™.
13. The structures of four N9-alkylated purines 6 and 2,9-disubstituted guanines 8 were assigned by 1H, 13C NMR, and mass
spectrometry (FAB).