Synthesis of Nucleoside Libraries on Solid Support. II. 2,6,8-Trisubstituted Purine Nucleosides Using 8-Bromoguanosine as Precursor

Article abstract of DOI:10.1081/NCN-120028343

A series of 2,6,8-trisubstituted purine nucleoside libraries was prepared by parallel solid-phase synthesis using 8-bromoguanosine as a common synthetic precursor. Polystyrene-methoxytrityl chloride resin was linked to the N ² or O5′ position o

Full text of DOI:10.1081/NCN-120028343

NUCLEOSIDES, NUCLEOTIDES & NUCLEIC ACIDS
Vol. 23, Nos. 1 & 2, pp. 501–507, 2004
Synthesis of Nucleoside Libraries on Solid Support.
II. 2,6,8-Trisubstituted Purine Nucleosides
Using 8-Bromoguanosine as Precursory
Yung-hyo Koh, Michael B. Landesman, Roberto Amador, Frank Rong,
*
Haoyun An, Zhi Hong, and Jean-Luc Girardet
Ribapharm, Inc., Costa Mesa, California, USA
ABSTRACT
A series of 2,6,8-trisubstituted purine nucleoside libraries was prepared by parallel
solid-phase synthesis using 8-bromoguanosine as a common synthetic precursor.
Polystyrene-methoxytrityl chloride resin was linked to the N2 or O5’ position of the
guanosine analogues. 8-Bromoguanosine was derivatized at the C8 position via
carbon-carbon bond formation. Nucleophilic aromatic substitution at C2 and/or C6
positions with various amines produced two series of purine nucleoside libraries with
very diverse substitution.
Key Words: Combinatorial library; Nucleoside; Purine.
yIn honor and celebration of the 70th birthday of Professor Leroy B. Townsend.
*
Correspondence: Jean-Luc Girardet, Ribapharm, Inc., 3300 Hyland Ave., Costa Mesa, CA 92626,
USA; Fax: (714) 641-7222; E-mail: jlgirardet@ribapharm.com.
501
DOI: 10.1081/NCN-120028343
1525-7770 (Print); 1532-2335 (Online)
www.dekker.com
Copyright D 2004 by Marcel Dekker, Inc.

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INTRODUCTION
Nucleoside analogues are an important class of chemotherapeutic agents and many
of them have been approved for treating both cancer and viral infections.[1,2] Testing a
large number of diverse sets of nucleoside analogues, using high throughput screening
techniques, would greatly increase the chance to find more efficient drug candidates.
Therefore, there is a strong interest in synthesizing a large number of nucleoside
analogues with a large variety of substituents. Recently, solid-phase synthesis has
become an important tool for generating diverse compound libraries of pharmaceutical
interest.[3,4] However, it has not been applied to the synthesis of nucleoside analogues
until recently when our group reported the synthesis of a triazine nucleoside library.[5]
We have reported in this journal the synthesis of a N2,N6-diaminopurine nucleoside
library,[6] and in this present article, we report a solid-phase parallel synthesis of two
2,6,8-trisubstituted purine nucleoside libraries using the same approach.
RESULTS AND DISCUSSION
Scheme 1 illustrates the synthesis of 2,8-disubstituted guanosine libraries.
Derivatization at the C8 position is interesting because this position is known to have an
effect on adenosine A1 receptors[7] and to strongly influence the syn/anti conformation
of nucleosides.[8] Both alkylation and arylation of the C8 position of 8-bromo-2’,3’,5’-
O-triacetylguanosine (1) with organostannane reagents through palladium-catalyzed
coupling reactions were carried out using the reported procedure.[9] Compounds 2a–e
were obtained in moderate to good yields (60–85%) while compound 2f was obtained
from commercial sources. To derivatize the C2 position, it was necessary to protect the
O6 position with a base-labile group.[10] Under Mitsunobu conditions, compounds 2a–f
were protected with 2-(4-nitrophenyl)ethyl (Npe) group to give the corresponding
orthogonally protected compounds, and the subsequent removal of the acetyl groups
with methanolic ammonia gave 3a–f. These reaction conditions did not affect the Npe
protection on O6. Diazotization and fluoride displacement of 3a–f gave 2-fluoroinosine
derivatives 4a–f. The scaffold 4 was linked to polystyrene methoxytrityl chloride (PS-
MMTrCl) resin in pyridine through the O5’ position of the sugar moiety to afford resins
5a–f. Displacement of the fluoro atom at C2 position of 5a–f with various primary and
secondary amines was achieved at 80°C in 1-methyl-2-pyrrolidone (NMP). We
discovered during the development research that most aliphatic amines gave high
yields, while most substituted anilines were unreactive, and therefore they were
excluded from our library synthesis. The guanosine libraries 6a–f were obtained from
5a–f by deprotection of the Npe group with DBU, followed by cleavage of the
nucleosides from the resin. As expected, the glycosidic bond of the purine nucleosides
6a–f was extremely susceptible to various acidic conditions, such as trichloroacetic or
dichloroacetic acid in dichloromethane. Successful cleavage of the nucleoside libraries
from our PS-MMTr resin without deglycosylation was achieved by using a 30%
solution of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) in dichloroethane, which was
originally developed for peptide synthesis.[11] During our first experiments, we noticed
that our compounds were contaminated by a noticeable amount of DBU, which
indicated that a standard washing method for the resin (dimethylformamide, methanol,

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Scheme 1. Reagents and conditions: (i) (R1)4Sn (for a and b), R1SnBu3 (for c and d), R1H (for e),
Cl2Pd(PPh3)2, DMF; (ii) p-NO2PhCH2CH2OH, DEAD, PPh3 dioxane; NH3, MeOH; (iii) HF-
pyridine, t-butyl nitrite; (iv) PS-MMTrCl, pyridine; (v) R2R3NH, NMP, 6 h at 60°C, then 24 h at
80°C; DBU, NMP; (vi) HFIP, DCE, 24 h at 50°C.
followed by methylene chloride) was not sufficient to wash away the excess of base.
An additional washing step with a 10% solution of acetic acid in dimethylformamide
was needed to completely remove DBU, and this extra step was implemented before
the cleavage by HFIP. Six 96-well plates allowed the production of more than 570
compounds based on structure 6a–f by a Vanguard automated synthesizer.
Characterization by LC-MS showed that 87% of the compounds synthesized had a
purity over 60%.
Scheme 2 illustrates the synthetic strategy for 2-amino-6,8-disubstituted purine
nucleoside libraries 10a,f starting from 8-bromoguanosine. Chlorination of compounds
2a,f with POCl3 produced their corresponding 6-chloro derivatives 7a,f.[12] The amino
group at the C2 position of 7a,f was reacted with PS-MMTrCl resin using 2,6-lutidine
as a base in tetrahydrofuran. The resulting resins 8a,f were subjected to nucleophilic
aromatic substitution with various amines in NMP, followed by subsequent deacetylation
with methylamine in methanol. The reactivity of this halogen toward various amines was
comparable to that of the fluoro atom of the scaffold 5. Cleavage of the nucleoside libraries

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Scheme 2. Reagents and conditions: (i) PS-MMTrCl, 2,6-lutidine, THF; (ii) R2R3NH, NMP, 6 h at
60°C, then 24 h at 80°C; MeNH2, MeOH, 24 h at 50°C; (iii) HFIP, DCE, 24 h at 50°C.
from the resin without deglycosylation was achieved by using the same method described
earlier (HFIP, DCE). More than 190 compounds in this series of nucleoside were
synthesized based on the structure 10.
In conclusion, parallel solid-phase strategies for the synthesis of nucleoside libraries
have been developed and successfully applied to the preparation of more than 760 2,6,8-
trisubstituted purine nucleoside analogues for a wide range of biological screenings.
EXPERIMENTAL
General methods. NMR spectra were recorded at 300 MHz and the chemical
shifts are expressed relative to TMS. The libraries were enumerated by Afferent
TeamWorks 3.0, labeled and weighted by Label Automador, and synthesized on ACT
Vanguard semi-automated synthesizers. Libraries were analyzed on a LC-MS system,
consisting of Waters 2790 HPLC, Waters 996 photodiode array (PDA) detector, and
micromass/Waters ZQ mass spectrometer. Luna C18 columns from Phenomenex were
used for compound separation. Mass spectra from 100 to 1000 were acquired using
electrospray ionization with positive and negative ion detections. UV spectra were
recorded at 200–400 nm by the PDA, and the compound purity was monitored based
on UV absorbency at 220 nm. The LC/MS operation was controlled by MassLynx
software, and the LC/MS data were processed by OpenLynx software. Polystyrene
methoxytrityl chloride resin was purchased from Novabiochem. Other reagents were
purchased from Aldrich, Fluka, Acros and other companies, and used directly.
2-Amino-6-[2-(4-nitrophenyl)ethoxy]-8-phenyl-9-( -D-ribofuranosyl)purine
(3d). To a stirred solution of 2d[9] (4.4 g, 9.1 mmol), 4-nitrophenethyl alcohol (1.7 g,
10 mmol) and triphenylphosphine (2.9 g, 11 mmol) in anhydrous dioxane (150 mL)
under argon was added a solution of diethyl azodicarboxylate (1.7 mL, 11 mmol) in

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anhydrous dioxane (10 mL) dropwise for 1 h. After stirring at room temperature for
22 h, the mixture was concentrated to dryness. Silica gel chromatography
(CH2Cl2:EtOAc = 75:25) yielded 2’,3’,5’-tri-O-acetyl-2-amino-6-[2-(4-nitrophenyl)-
ethoxy]-8-phenyl-purine-riboside as yellow solids. The solids were dissolved in
methanolic ammonia (50 mL, saturated at 0°C) and the solution was stirred at room
temperature in a sealed bomb for 15 h. The bomb was cooled to 0°C before opening to
an air and the mixture was concentrated to dryness. Silica gel chromatography
1
(CH2Cl2:MeOH = 90:10) yielded 3.1 g of 3d (66% for 2 steps) as a pale foam. H
NMR (DMSO-d6): d 8.17 (d, J = 8.7 Hz, 2H), 7.69–7.59 (m, 7H), 6.43 (bs, 2H), 5.65
(d, J = 6.6 Hz, 1H), 5.38 (d, J = 6.0 Hz, 1H), 5.22 (m, 1H), 5.14 (m, 1H), 5.04 (d,
J = 4.8 Hz, 1H), 4.69 (t, J = 6.9 Hz, 2H), 4.08(m, 1H), 3.85 (m, 1H), 3.70–3.50 (m,
2H), 3.26 (t, J = 6.9 Hz, 2H).
2-Fluoro-6-[2-(4-nitrophenyl)ethoxy]-8-phenyl-9-( -D-ribofuranosyl)purine
(4d). In a polypropylene flask, 3d (1.6 g, 3.2 mmol) was dissolved in 60% HF/
pyridine (130mL) at ꢀ50°C under argon. To the solution was added tert-butyl nitrite
(0.56 mL, 4.7 mmol) via syringe over 10 min, while the temperature was maintained at
ꢀ50°C. After stirring at ꢀ40°C for additional 30 min, the mixture was diluted with
CHCl3 (100 mL) and poured into K2CO3 (30 g). To the mixture was added H2O
(100 mL) carefully. The aqueous layer was extracted with CHCl3 (2 ꢁ 200 mL) and
the combined organic solution was washed with brine (1 ꢁ 100 mL), dried with
Na2SO4, and concentrated to dryness. Silica gel chromatography (CH2Cl2:
1
MeOH = 95:5) yielded 1.28 g of 4d (79%) as a yellow solid. H NMR (DMSO-d6):
d 8.17 (d, J = 8.7 Hz, 2H), 7.76–7.59 (m, 7H), 5.71 (d, J = 6.6 Hz, 1H), 5.44 (d,
J = 6.0 Hz, 1H), 5.23 (d, J = 5.1 Hz, 1H), 5.10 (m, 1H), 4.85 (m, 3H), 4.18(m, 1H),
3.88 (m, 1H), 3.72–3.49 (m, 2H). 19F NMR (DMSO-d6): d ꢀ 47.5. MS(ES) m/e 512.5
(M + H).
5’-O-PSMMTr-2-fluoro-6-[2-(4-nitrophenyl)ethoxy]-8-phenyl-9-( -D-ribofura-
nosyl)purine (5d). A solution of 4d (7.0 g, 14 mmol) in anhydrous pyridine (45 mL)
was added to a reaction vessel containing MMTrCl resin (5.6 g, 1.73 mmol/g, 9.7
mmol). The reaction mixture was shaken at room temperature for 3 days. The mixture
was quenched by the addition of methanol (6 mL), followed by shaking for 30 min.
The resin was filtered, and washed with DMF (3 ꢁ 15 mL), MeOH (3 ꢁ 15 mL), and
CH2Cl2 (3 ꢁ 15 mL). The washed resin was dried in vacuo at 45°C overnight to yield
9.8 g of 5d (91%).
2-(N-Alkyl)-8-phenyl-guanosine library (6d). To each reaction vessel contain-
ing 70 mg of resin nucleoside 5d was added a 0.5 M solution of various amines in
anhydrous 1-methyl-2-pyrrolidinone (1.6 mL). The vessels were shaken at 60°C for
4 h and then shaken at 80°C for 20 h to make amination complete. The vessels were
cooled down, filtered and washed with DMF (3 ꢁ 1 mL), MeOH (3 ꢁ 1 mL), and
CH2Cl2 (3 ꢁ 1 mL). To each reaction vessel was added a 0.2 M solution of 1,8-
diazabicyclo[5,4,0]undec-7-ene in anhydrous pyridine (1.5 mL). The vessels were
shaken at room temperature for 16 h, filtered and washed with DMF (3 ꢁ 1 mL), 10%
AcOH in DMF (3 ꢁ 1 mL), MeOH (3 ꢁ 1 mL), and CH2Cl2 (3 ꢁ 1 mL). For cleavage
of nucleosides from the resin, a 30% solution of 1,1,1,3,3,3-hexafluoro-2-propanol in

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anhydrous dichloroethane (1.5 mL) was added to each reaction vessel. The vessels were
shaken at 50°C for 24 h and the solution was pushed down into the receiving vessels
while keeping the temperature at 50°C. The reaction vessels were washed with a 1:1
mixture of MeOH:CH2Cl2 (1.5 mL). The combined solution (3 mL) was concentrated
to yield 6d.
2’,3’,5’-Tri-O-acetyl-2-N-PSMMTr-6-chloro-8-methyl-9-( -D-ribofuranosyl)-
purine (8a). A solution of 7a (6.8 g, 15 mmol) and 2,6-lutidine (2.2 mL, 19 mmol)
in anhydrous THF (45 mL) was added to a reaction vessel containing MMTrCl resin
(6.16 g, 1.80 mmol/g, 11 mmol). The reaction mixture was shaken at room temperature
for 5 days. The mixture was quenched by the addition of methanol (6 mL), followed by
shaking for 30 min. The resin was filtered, and washed with DMF (3 ꢁ 15 mL), MeOH
(3 ꢁ 15 mL), and CH2Cl2 (3 ꢁ 15 mL). The washed resin was dried in vacuo at 45°C
overnight to yield 9.8 g of 8a (86%).
2-Amino-6-(N-alkyl)-8-methyl-9-( -D-ribofuranosyl)purine (10a). To each re-
action vessel containing resin nucleoside 8a (70 mg) was added a 1 M solution (1.5 mL)
of various amines in anhydrous 1-methyl-2-pyrrolidinone (NMP). The vessels were
shaken at 60°C for 6 h and then shaken at 80°C for 24 h. The vessels were cooled
down and washed with DMF (3 ꢁ 1 mL), MeOH (3 ꢁ 1 mL), and CH2Cl2 (3 ꢁ 1 mL).
To each reaction vessel was added a 2 M solution (1.5 mL) of CH3NH2 in anhydrous
MeOH. The vessels were shaken at 50°C for 24 h and washed with DMF (3 ꢁ 1 mL),
MeOH (3 ꢁ 1 mL), and CH2Cl2 (3 ꢁ 1 mL) to yield 9a. To each reaction vessel
containing the resin 9a was added a 30% solution (1.5 mL) of 1,1,1,3,3,3-hexafluoro-2-
propanol in anhydrous dichloroethane. The vessels were shaken at 50°C for 24 h and
the solution was pushed down into the receiving vessels while keeping the temperature
at 50°C. The reaction vessels were washed with a 1:1 mixture of MeOH:CH2Cl2
(1.5 mL). The combined solution (3 mL) was concentrated to yield 10a.
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Received August 22, 2003
Accepted November 3, 2003

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