PAPER
Purine Nucleoside Analogues; 9: Benzylation of N2-Acetyl-8-bromoguanine
777
corresponded to the same signals in 4 and 5 thus excluding case, particularly taking into account the fact that hydro-
N1-benzylation. An extra carbonyl group (ν = 1677 cm–1) gen bromide, liberated in the benzylation reaction, could
was present in the IR spectrum. N-Deacetylation of 3–5 catalyze the hydrolysis of bromine atom. On the other
with aqueous methylamine afforded 7,9-dibenzyl-7,8-di- hand, the hydrolysis of the C-8 halogen might be initiated
hydro, 8-oxo-, 7-benzyl-8-bromo- and 9-benzyl-8-bromo- by the formation of 7,9-dialkylated quaternary intermedi-
guanines (6–8).
ate. Although charged structures of this type have been
demonstrated with related purines20 we have failed so far
to synthesize such 7,9-disubstituted 8-bromo derivatives
by alkylation of 8-bromoguanosine or 9-alkoxyalkyl-8-
bromoguanines under neutral reaction conditions.
The reaction of 2 with benzyl bromide in the presence of
anhydrous potassium carbonate followed a different
course. In this case the aralkylation proceeded at room
temperature, affording four benzylated products, i.e. two
bis- and two mono-alkylated ones. The rise in the reaction To sum up, this investigation allowed us to develop a pre-
temperature up to 100 °C did not substantially influence parative one-step route to N9-benzyl-8-bromoguanines as
either the overall yield or regioselectivity of benzylation. well as to a series of new N- and O-benzylated derivatives
The mono-benzylated products were identical to com- of 8-bromo, 8-chloro and 8-oxoguanine. The inhibitory
pounds 4 and 5. Bis-substituted derivatives 9, 10 were effect of these compounds on MOLT-4 leukemia cells is
identified as N2-acetyl-O6,7-dibenzyl-8-bromoguanine under investigation and the results will be published else-
and N2-acetyl-1,7-dibenzyl-8-bromoguanine, respective- where.
ly, due to the chemical shifts of NCH2-group protons and
C-5 carbon in their 1H and 13C NMR spectra (Table). We
All reagents were of commercial grade. Reagent grade solvents
found that the benzyl group at the N1-position of the het-
were used without further purification. The solvent mixtures are in
erocycle in compound 10 influenced the adjacent exocy-
volumes. Mps were determined on a Boetius hot-stage microscope
clic acetamido group that was reflected both in 1H and 13C
1
and are uncorrected. H NMR spectra were recorded on a Bruker
spectra, i.e. considerable downfield shifts of NH-proton
singlet (d = 13.45) and the carbon peak (d = 184.16) in
comparison to N1-unsubstituted derivatives.
WH-90/DS or on a Bruker AC 400 spectrometer. The latter instru-
ment was also used for 13C NMR spectra. IR spectra were recorded
on a Perkin-Elmer IFS28 spectrometer in KBr. Column chromatog-
raphy was performed on silica gel L 100/400 (Czech Republic). Re-
actions were monitored by TLC on Merck DC-Alufolien Kieselgel
60 F254 plates, which were visualized under UV radiation.
Treatment of substrate 2 with benzyl chloride in dimeth-
ylformamide (100 °C) containing potassium carbonate
gave N2-acetyl-7-benzyl-8-bromo- and N2-acetyl-9-ben-
zyl-8-bromoguanines (4, 5) in poor yield. If this reaction
was carried out without the base, N2-acetyl-7-benzyl-8-
chloro- and N2-acetyl-9-benzyl-8-chloroguanines (11, 12)
were isolated. Such halogen exchange to produce 8-chloro
derivatives from 8-bromo ones has been observed by us in
all the cases when the alkylating agents with chlorine as a
leaving group were used for the alkylation of 8-bromo-
guanine in the absence of base.20
8-Bromo-N2-acetylguanine (2)
A mixture of 1 (12.60 g; 25.8 mmol), Ac2O (130 mL) and H3PO4
(0.5 mL) was stirred at 100 °C for 1 h and cooled to r.t. The precip-
itate was separated by filtration and washed thoroughly with CHCl3
to yield 6.53 g (93%) of product 2, which was used further without
any additional purification. An analytical sample was obtained by
recrystallization from EtOH/H2O 3:1, mp > 300 °C (dec).
1H NMR (90 MHz, DMSO-d6) δ = 2.13 (s, 3 H, CH3), 11.56 (s, 1 H,
NH), 12.00 (s, 1 H, NH), 13.76 (s, 1 H, NH).
Our results show that the introduction of a bromine atom
in position 8 of guanine ring substantially increases the
yield of alkylation as compared with guanine. At room or
elevated temperature in the presence of base the prefera-
ble site of aralkylation is N7, although the reaction may
also proceed at the ring nitrogens N1 or N9 and at the exo-
cyclic oxygen atom. No N3-substituted products men-
tioned in References 13–15 were traced in our
experiments. If the reaction was carried out at 100 °C
without added potassium carbonate the formation of an
unanticipated product, i.e. N2-acetyl-7,9-dibenzyl-7,8-di-
hydro-8-oxoguanine, was observed. A similar compound,
namely, 6-chloro-7,8-dihydro-7,9-dimethyl-8-oxopurine,
was isolated in the methylation of 6,8-dichloropurine with
methyl iodide.21 Two reaction pathways have been pro-
posed to explain this phenomenon.21 On the one hand, the
Benzylation of 2 in the Absence of K2CO3
(a) with Benzyl Bromide: A solution of 2 (1.69 g, 6.2 mmol) and
benzyl bromide (2.12 g, 1.47 mL, 12.4 mmol) in DMF (50 mL) was
stirred at 100 °C for 20 h. The mixture was cooled to r. t., adjusted
to pH 8 with concd aq ammonia and evaporated to dryness in vacuo.
The residue was suspended in CHCl3 (200 mL), the insoluble mate-
rial was filtered off, the filtrate was concentrated and loaded onto a
column with silica gel (20 î 350 mm). Elution was performed with
CHCl3 (for compounds 3 and 4) followed by CHCl3/H2O (40:1) (for
compound 5) . Chromatographically identical fractions were pooled
and evaporated to dryness. Recrystallization from EtOH afforded
0.76 g of compound 3. Products 4 and 5 were recrystallized from
EtOH/H2O (5:1) to give 0.40 g of 4 and 0.45 g of 5.
(b) with Benzyl Chloride: A solution of 2 (1.00 g, 3.7 mmol) and
benzyl chloride (0.93 g, 0.85 mL, 7.4 mmol) in DMF (20 mL) was
stirred at 100 °C for 24 h. Further workup of the reaction mixture
was carried out as described above affording 0.24 g of compound
initial alkylation at N7 or N9 may increase the susceptibil- 11 and 0.19 g of compound 12.
ity of the halogen atom at the 8 position of the heterocycle
Benzylation of 2 with Benzyl Bromide in the Presence of K2CO3
towards nucleophiles and facilitate its hydrolysis to 8-ox-
opurine. As a result the remaining imidazole nitrogen at-
om, now protonated, also becomes available for
alkylation. This scheme seems feasible enough in our
A mixture of compound 2 (2.01 g, 7.4 mmol), benzyl bromide (2.51
g, 1.74 mL, 14.7 mmol) and K2CO3 (2.03 g, 14.7 mmol) in DMF (50
mL) was stirred at r.t. for 24 h. The mixture was filtered, the filtrate
Synthesis 1999, No. 5, 775–778 ISSN 0039-7881 © Thieme Stuttgart · New York