Zhong et al.
mmol (5.8 equiv)] was added. The mixture was stirred overnight
at 65 °C (reaction complete, TLC). The solution was concentrated
to 100 mL and cooled. The solid precipitate was filtered, and the
filter cake was washed with HOAc and then CH2Cl2 to give 3a‚
HCl (1.78 g, 80%).
in acetonitrile or TMSOTf in DCE at ambient temperature can
produce the desired N9 isomers exclusively,13 and more recent
results have confirmed the applicability of this approach for
regiospecific alkylation at N9. Substrates with 6-(1,2,4-triazol-
1-yl)- and 6-(substituted-imidazol-1-yl)purines can be prepared
from both purine base and nucleoside precursors. We have
prepared two types of substrates with small (1.4-6.5°) and
moderate (∼60°) projection angles between the planes of the
linked azole and purine rings. Studies on alkylation and
glycosylation reactions as well as biological evaluations are
presently under investigation.
(A) Recrystallization of this material (MeOH) gave 3a‚HCl: mp
1
296-297 °C (dec); H NMR (DMSO-d6) δ 10.01 (d, J ) 1.8 Hz,
1H), 8.96, 8.88, 8.73, 7.85 (4 × s, 4 × 1H); 13C NMR (DMSO-d6)
δ 156.0, 152.3, 147.5, 143.3, 137.1, 124.1, 122.7, 119.8; HRMS
m/z 186.0673 (M+ [C8H6N6] ) 186.0654).
(B) The precipitated 3a‚HCl was dissolved in 0.5 M NaOH/
H2O, and needlelike crystals precipitated from the solution upon
standing at ambient temperature. This material was filtered and
recrystallized (MeOH/Et2O) to give the sodium salt of 3a as a white
solid: 1H NMR (DMSO-d6) δ 9.15 (t, J ) 1.1 Hz, 1H), 8.44 (t, J
) 1.3 Hz, 1H), 8.40, 8.04 (2 × s, 2 × 1H), 7.13-7.14 (m, 1H).
(C) The white solid from (B) was washed with CO2/H2O
(saturated) to give 3a: UV max 281, 290 nm (ꢀ 15 100, 11 900),
Experimental Section
The starting materials 6-(1,2,4-triazol-4-yl)purine11 (B), 2′,3′,5′-
tri-O-acetylinosine24 (9), 9-(2,3,5-tri-O-acetyl-â-D-ribofuranosyl)-
6-(imidazol-1-yl)purine8a (10a), 9-(2,3,5-tri-O-acetyl-â-D-ribofura-
nosyl)-2-amino-6-chloropurine22 (11), 9-(2,3,5-tri-O-acetyl-â-D-
ribofuranosyl)-2,6-dichloropurine23 (12), and 2′,3′,5′-tri-O-acetyl-
guanosine22 (14) were prepared as described, and their purities were
1
min 234, 288 nm (ꢀ 3300, 11 800); H NMR (DMSO-d6) δ 13.90
(br s, 1H), 9.09-9.10 (m, 1H), 8.82, 8.72 (2 × s, 2 × 1H), 8.42-
8.44 (m, 1H), 7.26-7.27 (m, 1H).
1
confirmed by H and 13C NMR.
See the Supporting Information for analogous preparations of
3b-3d.
I. 9-Tritylhypoxanthine (2). A suspension of hypoxanthine (1)
(2.0 g, 14.7 mmol) and (NH4)2SO4 (0.46 g) was stirred in HMDS
(300 mL) for 15 h at reflux. Volatiles were evaporated in vacuo
from the clear solution, and the residue was dissolved in dried CH3-
CN (150 mL). Trityl chloride (8.81 g, 30 mmol) was added, and
the clear solution was stirred for 48 h at reflux. Volatiles were
evaporated in vacuo, and the residue was dissolved in CH2Cl2 (50
mL). NH3/H2O (28-30% w/w, 80 mL) was added, and precipitation
occurred immediately. The mixture was stirred overnight at ambient
temperature, and volatiles were evaporated in vacuo. The residue
was washed (H2O, CH2Cl2) to give solid 2 (3.02 g). Volatiles were
evaporated from the combined organic layers, and the residue was
chromatographed (MeOH/CH2Cl2, 1:60 f 1:30). The solid was
washed (CH2Cl2) to give additional 2 (0.59 g) (65% total yield):
1H NMR (DMSO-d6) δ 12.23, 7.72, 7.65 (3 × s, 3 × 1H), 7.30-
7.37 (m, 9H), 7.13 (d, J ) 7.0 Hz, 6H); 13C NMR (DMSO-d6) δ
157.5, 150.2, 145.0, 141.9, 140.7, 130.0, 128.8, 128.4, 126.6, 75.8;
HRMS m/z 401.1390 (MNa+ [C24H18N4ONa] ) 401.1378).
Other mono- and ditritylated [MS (FAB) m/z 643 (MNa+
[C43H32N4ONa] ) 643)] byproducts were formed. Treatment of a
mixture of two monotritylated regioisomers by the following Appel
conditions followed by detritylation gave 3a as the only product.
II. 6-(Imidazol-1-yl)purine (3a). II.1. Method 1. A suspension
of 2 (3.0 g, 8 mmol), Ph3P (11.9 g, 45 mmol), I2 (11.3 g, 44 mmol),
and imidazole (2.9 g, 43 mmol) in EtN(iPr)2 (7.1 mL, 5.27 g, 41
mmol) and toluene (240 mL) was stirred overnight at 95 °C.
Volatiles were evaporated, and the residue was extracted with
boiling EtOAc (2 × 100 mL). The combined EtOAc extracts were
evaporated to dryness. The original residue and the solid obtained
from the EtOAc extracts were treated independently with HOAc/
H2O (9:1, 150 mL) and stirred for 20 h at 60 °C. Volatiles were
evaporated in vacuo from the two solutions, and the residues were
dissolved in 0.1 M NaOH/H2O and washed (CH2Cl2). Precipitation
(CO2 was bubbled into the solution) and filtration gave 3a (350
mg and 550 mg, respectively). Volatiles were evaporated from the
combined mother liquors, and the residue was extracted (NaOH/
H2O). Neutralization of the extract (CO2) caused precipitation of
additional 3a (30 mg) (63% total yield).
III. 2-N,9-Ditritylguanine (5). Guanine (4) (450 mg, 3 mmol)
[freshly activated by dissolving in 0.5 M NaOH/H2O, precipitating
by neutralization (HCl/H2O) to pH ∼10, drying, grinding to a fine
powder, and redrying] and (NH4)2SO4 (60 mg) were heated at reflux
with stirring for 24 h in HMDS (50 mL). Volatiles were evaporated
in vacuo from the clear solution, and the residue was dissolved in
dried CH3CN (50 mL). Trityl chloride (3.5 g, 12.6 mmol) was
added, and the solution was stirred for 48 h at reflux. Volatiles
were evaporated in vacuo, and the residue was dissolved in CH2-
Cl2 (10 mL). NH3/H2O (28-30%, 30 mL) was added, and
precipitation occurred immediately. The mixture was stirred at
ambient temperature overnight, and volatiles were evaporated in
vacuo. The residue was washed (H2O, CH2Cl2) to give a solid (1.37
g, 72%). This material was stirred with MeOH/CH2Cl2 (1:15), and
the cloudy solution was filtered. Volatiles were evaporated to give
purified 5: 1H NMR (DMSO-d6) δ 10.75 (s, 1H), 7.35 (s, 1H),
7.08-7.19 (m, 19H), 6.87 (d, J ) 7.4 Hz, 6H), 6.81 (d, J ) 7.3
Hz, 6H); 13C NMR (DMSO-d6) δ 157.3, 151.8, 151.0, 145.3, 142.4,
139.6, 129.6, 128.8, 128.5, 128.3, 127.6, 126.9, 120.3, 75.4, 71.1;
HRMS m/z 635.2675 (M+ [C43H33N5O] ) 635.2685).
IV. 2-Amino-6-(imidazol-1-yl)purine (6). A mixture of 5 (1.90
g, 3 mmol), I2 (3.88 g, 15 mmol), Ph3P (3.99 g, 15 mmol), and
imidazole (1.10 g, 15 mmol) in toluene (150 mL) was stirred for
15 min at 95 °C, and EtN(iPr)2 (2.9 mL, 2.15 g, 16.6 mmol) was
added. The reaction mixture was stirred overnight at 95 °C, volatiles
were evaporated, and the residue was extracted with boiling EtOAc
(3×). The combined extracts were filtered (hot), and volatiles were
evaporated from the filtrate. The residue was dissolved in TFA/
H2O (9:1, 60 mL), and the solution was stirred for 4 h at 0 °C.
Volatiles were evaporated in vacuo, and the residue was partitioned
between 0.1 M NaOH/H2O (100 mL) and CH2Cl2 (100 mL). The
organic layer was extracted with 0.1 M NaOH/H2O (50 mL × 2),
and the combined aqueous layers were washed [CH2Cl2 (2 × 50
mL)]. Neutralization (CO2 was bubbled into the basic solution) and
evaporation of volatiles in vacuo gave a residue that was washed
(H2O, CH2Cl2) to give 6 (400 mg, 66%): UV (MeOH) max 222,
320 nm (ꢀ 29 800, 8700), min 280 nm (ꢀ 1500); 1H NMR (DMSO-
d6) δ 12.89, 8.94, 8.25, 8.16, 7.18 (5 × s, 5 × 1H), 6.67 (s, 2H);
13C NMR (DMSO-d6) δ 160.7, 157.5, 145.4, 141.9, 137.2, 130.5,
117.7, 115.4; HRMS m/z 201.0753 (M+ [C8H7N7] ) 201.0763).
V. 2-Chloro-6-(imidazol-1-yl)purine (8a). V.1. Method 1 (via
7). Imidazole (820 mg, 12.1 mmol) and 2,6-dichloropurine (7) (380
mg, 2 mmol) were dissolved in freshly distilled DMF (36 mL).
The solution was stirred for 20 h at 65 °C, and volatiles were
evaporated. The residue was washed with a large volume of CH2-
Cl2 to give 8a (290 mg, 66%).
II.2. Method 2. A suspension of 924 (4.0 g, 10 mmol), Ph3P
(6.4 g, 24 mmol), I2 (5.3 g, 20.9 mmol), and imidazole (2.5 g, 37
mmol) in EtN(iPr)2 (8.8 mL, 6.53 g, 50.5 mmol) and toluene (100
mL) was stirred overnight at 95 °C.8a Volatiles were evaporated in
vacuo, and the residue was extracted with boiling EtOAc. The
combined EtOAc extracts were evaporated to dryness. The residue
was dissolved in HOAc (400 mL), and AcCl [4.2 mL, 4.64 g, 58
(24) Bredereck, H.; Martini, A. Chem. Ber. 1947, 80, 401-405.
4220 J. Org. Chem., Vol. 71, No. 11, 2006