An approach to metal-assisted DNA base pairing: Novel β-C-nucleosides with a 2-aminophenol or a catechol as the nucleobase

Article abstract of DOI:10.1016/S0928-0987(00)00210-4

The metal-chelating β-C-nucleoside having a phenylenediamine moiety as the nucleobase was previously found to form a stable 2:1 complex with a Pd²⁺ ion in aqueous media, where hydrogen bonding is replaced by metal coordination in the base pairing, thereby creating a novel hybridization motif in duplex DNA. In this regard, we have further designed two types of artificial β-C-nucleosides possessing a metal-chelating site (a 2-aminophenol or a catechol) as the nucleobase moiety. These artificial nucleosides are directed toward controlling the net charges of the metal-assisted base pairs. This paper describes convenient syntheses of the artificial nucleosides bearing a 2-aminophenol or a catechol moiety. Each nucleoside was directly synthesized through 2′-deoxy derivative via a Friedel-Crafts coupling reaction as the key step between the aromatic ring and ribose moiety, whereas the nucleoside having a phenylenediamine moiety was prepared in rather longer steps through an RNA type intermediate followed by the removal of 2′-hydroxyl group. Copyright

Full text of DOI:10.1016/S0928-0987(00)00210-4

European Journal of Pharmaceutical Sciences 13 (2001) 77–83
www.elsevier.nl/locate/ejps
An approach to metal-assisted DNA base pairing: novel b-C-nucleosides
with a 2-aminophenol or a catechol as the nucleobase
Kentaro Tanakaa, Motoyuki Tasakab, Honghua Caob, Mitsuhiko Shionoyaa ,
*
aDepartment of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
bThe Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585, Japan
Received 21 July 1999; received in revised form 16 August 1999; accepted 26 August 1999
Abstract
The metal-chelating b-C-nucleoside having a phenylenediamine moiety as the nucleobase was previously found to form a stable 2:1
complex with a Pd21 ion in aqueous media, where hydrogen bonding is replaced by metal coordination in the base pairing, thereby
creating a novel hybridization motif in duplex DNA. In this regard, we have further designed two types of artificial b-C-nucleosides
possessing a metal-chelating site (a 2-aminophenol or a catechol) as the nucleobase moiety. These artificial nucleosides are directed
toward controlling the net charges of the metal-assisted base pairs. This paper describes convenient syntheses of the artificial nucleosides
bearing a 2-aminophenol or a catechol moiety. Each nucleoside was directly synthesized through 29-deoxy derivative via a Friedel–Crafts
coupling reaction as the key step between the aromatic ring and ribose moiety, whereas the nucleoside having a phenylenediamine moiety
was prepared in rather longer steps through an RNA type intermediate followed by the removal of 29-hydroxyl group.
2001 Elsevier
Science B.V. All rights reserved.
Keywords: Artificial DNA; Metal-chelating nucleoside; Nucleoside synthesis
1. Introduction
1997; Meggers et al., 1997; Ihara et al., 1997; Magda et
al., 1997; Hurley and Tor, 1998). Thus far, a variety of
oligonucleotides containing photo- and redox-active transi-
tion metal complexes have been developed for energy and
electron-transfer systems through DNA, probing systems
for DNA hybridization, and site-specific DNA cleavage.
An alternative approach we have used for the incorporation
of metal ions into DNA is the more direct modification of
a DNA base itself into a metal-chelating nucleobase. In
this artificial DNA, hydrogen-bonded base pairing is
replaced by metal-induced base pairing thereby creating a
novel hybridization motif in double-stranded DNA (Fig.
1). Such an approach would provide a wide range of
applications based on its use as the alternative base pairs
along with the canonical base pairs, AT and GC.
We have previously found that the metal-chelating
nucleoside 1 having a phenylenediamine moiety forms a
stable 2:1 square-planar complex with a Pd21 ion in
aqueous media, where hydrogen bonding is replaced by
metal coordination in the base pairing (Scheme 1) (Tanaka
et al., 1998; Tanaka and Shionoya, 1999). In this regard,
we have further designed two types of artificial b-C-
nucleosides, 2 and 3, possessing a metal-chelating site (a
2-aminophenol and a catechol, respectively) as the nu-
Hydrogen-bonding plays central roles in the high selec-
tive recognition in the DNA base pair identification.
Geometrical modification of the Watson–Crick pairing
mode will lead not only to a great understanding of the
molecular programming language developed by nature, but
also to new functional molecular architectures in the fields
of biosciences and material sciences. In recent years, a
large number of nonnatural DNA nucleosides have been
synthesized (Lesser et al., 1990; Kornberg and Baker,
1992; SantaLucia et al., 1992; Smith et al., 1994;
Schweizer and Kool, 1995; Ren et al., 1996; Hildbrand et
al., 1997), and it has been generally accepted that the
incorporation of metal complexes into DNA is a useful
tool for the functionalization of DNA (Dreyer and Dervan,
1985; Chen and Sigman, 1988; Telser et al., 1989;
Bannwarth et al., 1991; Murphy et al., 1993; Bashkin et
al., 1994; Matsumura et al., 1994; Magda et al., 1994;
Meade and Kayyem, 1995; Manchanda et al., 1996;
Schliepe et al., 1996; Mucic et al., 1996; Dandliker et al.,
*Corresponding author. Tel./fax: 181-3-5841-8061.
E-mail address: shionoya@chem.s.u-tokyo.ac.jp (M. Shionoya).
0928-0987/01/$ – see front matter
PII: S0928-0987(00)00210-4
2001 Elsevier Science B.V. All rights reserved.

78
K. Tanaka et al. / European Journal of Pharmaceutical Sciences 13 (2001) 77–83
complex with a divalent metal ion, the complexes of 2 and
3 have no charge and negative double charge, respectively,
and therefore could possibly be incorporated into oligo-
nucleotides at the adjacent positions. These nucleosides are
expected to form metal-assisted base pairs in DNA and
thereby to create novel structures and functions of DNA.
Herein we present convenient synthetic routes for 2 and 3.
2. Materials and methods
2.1. General
Unless otherwise noted, all reactions were carried out in
oven dried glassware under an argon atmosphere with
commercial dehydrated solvents (Wako). 1-O-Methyl-2-
deoxy-D-ribofuranose and 1-O-methyl-3,5-O-ditoluoyl-2-
deoxy-D-ribofuranose were prepared according to previous-
ly reported procedures (Hoffer, 1960). 2-Aminophenol and
trifluoroacetic anhydride were obtained from TCI and tert-
butyldimethylsilyl chloride was obtained from Shin-Etsu.
nBu4NF in THF (1 M) was purchased from Aldrich. All
the other reagents were purchased from Wako and were
used without further purification. Column chromatography
was performed using Wakogel C-300 silica gel (Wako). 1H
and 13C NMR spectra were recorded on a JEOL Lambda
1
500 or a JEOL Alpha 500 spectrometer (500 MHz for H;
125.65 MHz for 13C). The spectra are referenced to
tetramethylsilane. Chemical shifts (d ) are reported in ppm;
multiplicities are indicated by: s (singlet), d (doublet), dd
(doublet of doublets), ddd (doublet of doublets of doub-
lets), q (quartet), m (multiplet), and br (broad). Coupling
constants, J, are reported in Hz. Electron ionization mass
spectra were obtained with a Shimadzu QP1000EX spec-
trometer (low resolution, 70 eV), and fast atom bombard-
ment (FAB) mass and elemental analyses were carried out
at the Research Center for Molecular Materials, the
Institute for Molecular Science. Electrospray ionization
(ESI) mass spectra were recorded on a PE SCIEX API-300
spectrometer.
2.2. 2-Trifluoroacetamidephenol (4)
To a solution of 2-aminophenol (17.6 g, 161 mmol) and
dry pyridine (13.7 ml, 169 mmol) in CH2Cl2 (350 ml) was
added a solution of trifluoroacetic anhydride (22.6 ml, 164
mmol) in CH2Cl2 (90 ml) over 10 min at 08C. The
reaction mixture was stirred for 100 min at 08C. After the
solvent was evaporated, the residue was dissolved in a 1%
HCl aqueous solution (250 ml), which was extracted with
three portions of AcOEt. The combined organic layer was
washed with brine. The aqueous layer was further ex-
tracted with two portions of AcOEt. The combined organic
phase was dried over anhydrous MgSO4, followed by
concentration. The resulting solid was recrystallized from
toluene to give 25.1 g (76%) of 4 as colorless thin plates,
Fig. 1.
cleobase moiety. These artificial nucleosides are directed
toward controlling the net charges of the metal-assisted
base pairs (Fig. 1). When these nucleosides form a 2:1
Scheme 1.

K. Tanaka et al. / European Journal of Pharmaceutical Sciences 13 (2001) 77–83
79
1
1
which were sublimed at 808C. H NMR (DMSO-d6 ): d
127.5–128.08C. H NMR (CDCl3): d 2.21 (ddd, 1H, J5
13.8, 11.0, 6.3 Hz), 2.40 (s, 3H), 2.44 (s, 3H), 2.53 (ddd,
1H, J513.9, 5.1, 0.6 Hz), 4.51 (ddd, 1H, J54.0, 4.0, 2.0
Hz), 4.63 (dd, 1H, J511.8, 3.8 Hz), 4.66 (dd, 1H, J512.0,
4.0 Hz), 5.15 (s, 2H), 5.22 (dd, 1H, J510.5, 5.0 Hz), 5.59
(dd, 1H, J55.3, 1.3 Hz), 6.96 (d, 1H, J58.5 Hz), 7.21 (d,
2H, J57.5 Hz), 7.24–7.29 (m, overlapped with residual
CHCl3) 7.35–7.43 (m, 5H), 7.94 (d, 2H, J58.5 Hz), 7.98
(d, 2H, J58.0 Hz), 8.36 (d, 1H, J52.0 Hz), 8.61 (br, 1H,
D2O exchangeable). 13C NMR (CDCl3): d 21.7, 21.7,
41.6, 65.0, 71.4, 77.1, 80.3, 83.0, 112.4, 115.6 (q, JC–F5
288 Hz, CF3), 118.4, 123.2, 125.3, 126.9, 127.1, 127.3,
128.6, 128.9, 129.1, 129.2, 129.8, 134.1, 135.7, 143.7,
144.1, 147.1, 154.3 (q, J2C–F537.2 Hz, CO), 166.1, 166.4.
FAB mass (positive): m/z, proposed ion: 648, [M1H]1.
a-anomer 7: colorless solid, mp 127.0–128.58C. 1H NMR
(CDCl3): d 2.29 (ddd, 1H, J513.7, 6.4, 4.2 Hz), 2.39 (s,
3H), 2.41 (s, 3H), 2.95 (ddd, 1H, J514.0, 7.0, 7.0 Hz),
4.54 (dd, 1H, J511.8, 4.4 Hz), 4.58 (dd, 1H, J511.7, 4.9
Hz), 4.70 (br, 1H), 5.16 (s, 2H), 5.31 (dd, 1H, J56.8, 6.8
Hz), 5.59 (ddd, 1H, J56.9, 4.0, 3.1 Hz), 6.68 (d, 1H,
J58.5 Hz), 7.17 (d, 2H, J57.9 Hz), 7.22–7.25 (m, 3H),
7.35–7.43 (m, 5H), 7.79 (d, 2H, J58.1 Hz), 7.96 (d, 2H,
J58.1 Hz), 8.42 (d, 1H, J52.2 Hz), 8.62 (br, 1H, D2O
exchangeable).
6.83 (ddd, 1H, J57.6, 7.6, 1.4 Hz), 6.94 (dd, 1H, J58.1,
1.2 Hz), 7.14 (ddd, 1H, J58.1, 7.5, 1.6 Hz), 7.32 (dd, 1H,
J57.8, 1.5 Hz), 9.97 (br, 1H, D2O exchangeable), 10.42
(br, 1H, D2O exchangeable). 13C NMR (DMSO-d6 ): d
116.0 (q, JC–F5288 Hz, CF3), 116.0, 118.9, 122.1, 126.3,
127.9, 151.2, 154.9 (q, J2C–F536.2 Hz, CO). LRMS (70 eV,
EI) m/z (relative intensity, proposed ion): 205 (71.4, M1),
136 (100.0, M1–CF3), 108 (50.4, M1–CF3CO).
2.3. O-Benzyl-2-trifluoroacetamidephenol (5)
To a solution of 2-trifluoroacetamidephenol 4 (47.7 g,
233 mmol) and iPr2EtN (60 ml, 350 mmol) in dry 1,2-
dichloroethane (470 ml) was added benzyl chloride (40
ml, 348 mmol) at room temperature. The reaction mixture
was stirred for 30 min at 558C, and then heated at reflux
for 15.5 h. After cooling down to room temperature,
iPr2EtN (60 ml) and benzyl chloride (40 ml) were added
to the solution. After refluxing for 4 h, the solution was
poured into a 2% HCl aqueous solution (1.23 l). The
organic layer was separated, and then the aqueous layer
was extracted with CH2Cl2. The organic layer was washed
with brine, and then the aqueous layer separated was
further extracted with two portions of CH2Cl2. The
combined organic extracts were dried over anhydrous
MgSO4 and the solvent was concentrated. The residue was
purified by silica gel column chromatography (9f324 cm)
with AcOEt-n-hexane (3:97), followed by recrystallization
from n-hexane to afford 33.9 g (49%) of 5 as colorless
2.5. O-Benzyl-2-trifluoroacetamide-4-(1,2-dideoxy-b-D-
ribofuranos-1-yl)-phenol (8)
To a solution of 6 (2.00 g, 3.09 mmol) in dry MeOH (47
ml) was added in one portion a solution of 28% MeONa in
MeOH (1.65 ml, 8.55 mmol) at room temperature. The
reaction mixture was stirred for 2.2 h at room temperature.
After adding Dowex 50W38 (pyridinium form) (31.7 g),
the mixture was further stirred for 2 min. After removing
the resin, the filtrates were combined and then concen-
trated. The resulting oil was purified by silica gel column
chromatography (3f310.5 cm) with MeOH–CH2Cl2
(5:95) to afford 1.21 g (95%) of 8 as colorless solid, mp
1
needles, mp 76.0–76.58C. H NMR (CDCl3): d 5.17 (s,
2H), 7.01 (dd, 1H, J58.3, 1.5 Hz), 7.03 (ddd, 1H, J57.8,
7.8, 1.5 Hz), 7.15 (ddd, 1H, J57.9, 7.9, 1.7 Hz), 7.36–7.43
(m, 5H), 8.32 (dd, 1H, J58.3, 1.8 Hz), 8.64 (br, 1H, D2O
exchangeable). 13C NMR (CDCl3): d 71.3, 112.2, 115.7 (q,
J
C–F5288 Hz, CF3), 120.3, 121.7, 125.5, 126.0, 127.3,
128.5, 128.9, 135.8, 147.5, 154.3 (q, J2C–F537.2 Hz, CO).
LRMS (70 eV, EI) m/z (relative intensity, proposed ion):
295 (100.0, M1), 204 (9.7, M1–CH2C6H5), 135 (48.4,
M1–CH2C6H5 –CF3).
1
126.58C. H NMR (CDCl3): d 1.85 (d, 1H, J54.2 Hz,
D2O exchangeable), 1.98 (dd, 1H, J56.2, 6.2 Hz, D2O
exchangeable), 2.06 (ddd, 1H, J513.3, 10.0, 6.2 Hz), 2.26
(ddd, 1H, J513.3, 5.7, 2.1 Hz), 3.77 (ddd, 1H, J511.6,
6.6, 4.9 Hz), 3.86 (ddd, 1H, J511.8, 5.7, 4.2 Hz), 4.02
(ddd, 1H, J54.3, 4.3, 3.1 Hz), 4.47 (ddd, 1H, J58.9, 3.4,
2.7 Hz), 5.16 (dd, 1H, J510.3, 5.9 Hz), 5.17 (s, 2H), 6.97
(d, 1H, J58.3 Hz), 7.14 (dd, 1H, J58.5, 2.2 Hz), 7.37–
7.42 (m, 5H), 8.38 (d, 1H, J52.0 Hz), 8.64 (s, 1H, D2O
exchangeable).
2.4. O-Benzyl-2-trifluoroacetamide-4-(1,2-dideoxy-3,5-O-
ditoluoyl-b-D-ribofuranos-1-yl)-phenol (6)
A solution of 1-O-methyl-3,5-O-ditoluoyl-2-deoxy-D-
ribofuranose (520 mg, 1.35 mmol) and O-benzyl-2-tri-
fluoroacetamidephenol 5 (403 mg, 1.36 mmol) in CH2Cl2
(2 ml) in the presence of SnCl4 (310 ml, 2.65 mmol) was
stirred for 30 min at 08C. The reaction mixture was
quenched by saturated NaHCO3 aqueous solution (10 ml)
and the resulting precipitates were removed through a thin
pad of Celite. The organic filtrate was washed with brine,
dried over anhydrous MgSO4, and then concentrated. The
residue was chromatographed (2.4f314 cm) on silica gel
with AcOEt-n-hexane (1:9) to give 134 mg (15%) of 6 and
13 mg (1.5%) of 7. b-anomer 6: colorless solid, mp
2.6. 2-Amino-4-(1,2-dideoxy-b-D-ribofuranos-1-yl)-
phenol (2)
A solution of 8 (132 mg, 0.321 mmol) in 40% (w/v)
MeNH2 –MeOH (0.64 ml, 8.24 mmol) was stirred for 11 h.

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K. Tanaka et al. / European Journal of Pharmaceutical Sciences 13 (2001) 77–83
The solvent was evaporated in vacuo to give 105 mg of
colorless solid. Without further purification, a solution of
room temperature. The reaction mixture was diluted with
CH2Cl2 (200 ml) and washed with three portions of 2%
H2SO4 aqueous solution and brine, dried over anhydrous
MgSO4, and concentrated. The residue was purified by
silica gel column chromatography with n-hexane–diethyl
ether (5:2) to afford 16.3 g (78%) of 11 as a colorless oil.
1-a-,b-O-Methyl-2-deoxy-3,5-di-O-(ethoxycarbonyl)-D-
ribofuranose (11) (20.0 g, 68.4 mmol) and O,O9-bis(tert-
butyldimethylsilyl)catechol (10) (20.6 g, 60.8 mmol) were
dissolved in dry CH2Cl2 (80 ml), and SnCl4 (13.7 ml,
127.0 mmol) in CH2Cl2 (130 ml) was added dropwise to
the solution within 10 min at 08C. After stirring for 15 min
at 08C, the reaction was quenched by saturated NaHCO3
aqueous solution (1 l), and the solution was extracted with
four portions of CH2Cl2. The organic layer was washed
with brine, dried over MgSO4, and concentrated. The
residue was purified by silica gel column chromatography
with n-hexane–diethyl ether (8:1) to afford 13.2 g (36%)
of 12 (b-anomer) as a pale yellow oil and 3.0 g (8.2%) of
13 (a-anomer) as a pale yellow oil. 1H NMR (CDCl3): d
b-anomer 12: 0.18 (br, 12H), 0.98 (s, 18H), 1.31 (t, 3H,
J57.2 Hz), 1.34 (t, 3H, J57.2 Hz), 2.05 (ddd, 1H, J5
14.0, 11.2, 6.1 Hz), 2.32–2.35 (m, 1H), 4.18–4.28 (m, 5H),
4.36 (dd, 1H, J511.4, 3.8 Hz), 4.40 (dd, 1H, J511.5, 4.2
Hz), 4.98 (dd, 1H, J511.2, 4.6 Hz), 5.16–5.18 (m, 1H),
6.78 (d, 1H, J58.1 Hz), 6.80 (dd, 1H, J58.2, 1.8 Hz),
6.84 (d, 1H, J51.7 Hz); a-anomer: d 0.18 (s, 3H), 0.18 (s,
3H), 0.19 (s, 3H), 0.19 (s, 3 H), 0.97 (s, 9 H), 0.98 (s, 9
H), 1.30 (t, 3H, J57.5 Hz), 1.32 (t, 3H, J57.3 Hz), 2.10
(ddd, 1H, J513.5, 8.0, 5.3 Hz), 2.79 (ddd, 1H, J513.8,
7.0, 7.0 Hz), 4.19 (q, 2H, J57.2 Hz), 4.22 (q, 2H, J57.2
Hz), 4.32 (dd, 1H, J511.5, 5.2 Hz), 4.35 (dd, 1H, J511.3,
3.7 Hz), 4.40–4.43 (m, 1H), 5.04 (dd, 1H, J57.5, 7.5 Hz),
5.18 (ddd, 1H, J57.4, 5.3, 3.8 Hz), 6.78 (d, 1H, J58.3
Hz), 6.81 (dd, 1H, J58.3, 2.0 Hz), 6.85 (d, 1H, J52.0 Hz).
1
the solid (25 mg) in MeOH (8 ml) in the presence of 10%
Pd–C (34 mg) was stirred for 2 h at room temperature
under a hydrogen atmosphere. The Pd–C was filtered off,
and washed with MeOH repeatedly. The filtrate was
evaporated in vacuo to give colorless syrup, which was
then chromatographed on silica gel (1.6f310 cm) with
MeOH–CH2Cl2 (16:84) to give 2.8 mg (16% from 8) of 2
as a colorless oil. 1H NMR (DMSO-d6 ): d 1.72 (ddd, 1H,
J512.7, 10.6, 5.7 Hz), 1.92 (ddd, 1H, J512.8, 5.3, 1.3
Hz), 3.45 (m, 1H), 3.69 (ddd, 1H, J56.1, 5.4, 2.2 Hz),
4.12 (m, 1H), 4.44 (br, 2H, D2O exchangeable), 4.70 (dd,
1H, J55.6, 5.6 Hz, D2O exchangeable), 4.76 (dd, 1H,
J510.5, 5.1 Hz), 4.97 (d, 1H, J53.9 Hz, D2O exchange-
able), 6.37 (dd, 1H, J58.1, 2.2 Hz), 6.55 (d, 1H, J58.1
Hz), 6.57 (d, 1H, J52.2 Hz), 8.90 (br, 1H, D2O exchange-
able). The signal of 59-H overlapped with that of H2O in
DMSO-d6. ESI mass (MeOH, positive): m/z, proposed ion:
226.2, [M1H]1.
2.7. O,O9-Bis(tert-butyldimethylsilyl)catechol (10)
To a solution of tert-butyldimethylsilyl chloride (9.7 g,
64 mmol) in pyridine (10 ml) was added catechol (3.54 g,
32.2 mmol). After stirring for 2 days at 808C, the reaction
mixture was poured into ice-cold water (500 ml). The
solution was extracted with three portions of AcOEt, and
the combined organic layer was washed with brine, dried
over anhydrous MgSO4, and concentrated. The residue
was purified by silica gel column chromatography with
n-hexane-triethylamine (100:1) to give 9.11 g (84%) of 10
as a colorless oil. 1H NMR (CDCl3): d 0.19 (s, 12H), 0.99
(s, 18H), 6.81 (m, 4H).
2.8. O,O9-Bis(tert-butyldimethylsilyl)-4-[1,2-dideoxy-3,5-
O,O9-bis(ethoxycarbonyl)-a- and -b-D-ribofuranos-1-
yl]catechol (12: b, 13: a)
2.9. O,O9-Bis(tert-butyldimethylsilyl)-4-(1,2-dideoxy-b-D-
ribofuranos-1-yl)catechol (14)
O,O9-Bis(tert-butyldimethylsilyl)-4-[1,2-dideoxy-3,5-
O,O9-bis(ethoxycarbonyl)-b-D-ribofuranos-1-yl]catechol
(12) (2.57 g 4.30 mmol) was treated with 1% (w/v)
K2CO3 in MeOH (25 ml) for 50 min at room temperature.
The mixture was neutralized with aqueous acetic acid
solution and then concentrated. The residue was dissolved
in CH2Cl2 (200 ml) and washed with water, dried over
anhydrous MgSO4, and concentrated. The residue was
purified by silica gel column chromatography with
CHCl3 –MeOH (20:1) to afford 1.64 g (84%) of 14 as a
pale yellow oil. 1H NMR (CDCl3): d 0.18–0.19 (m, 12H),
0.98 (s, 9H), 0.98 (s, 9H), 1.88 (d, 1H, J53.7 Hz), 1.91 (d,
1H, J56.2 Hz), 2.01 (ddd, 1H, J513.3, 10.3, 6.2 Hz), 2.20
(ddd, 1H, J513.4, 5.6, 2.0 Hz), 3.72 (ddd, 1H, J511.6,
5.7, 5.7 Hz), 3.81 (ddd, 1H, J511.4, 4.8, 4.8 Hz), 3.99
(ddd, 1H, J54.5, 4.5, 3.0 Hz), 4.42–4.43 (br, 1H), 5.06
(dd, 1H, J510.3, 5.6 Hz), 6.82–6.79 (m, 3H).
To a solution of 1-O-methyl-2-deoxy-D-ribofuranose
(Hoffer, 1960) (9.6 g, 65 mmol) and ethyl chloroformate
(12.4 ml, 130 mmol) in dry CH2Cl2 (70 ml) was added
pyridine (10.5 ml, 130 mmol) at room temperature. After
stirring for 1.5 h, ethyl chloroformate (12.4 ml, 130 mmol)
and pyridine (10.5 ml 129.8 mmol) were added to the
reaction mixture, which was further stirred for 40 min at
1After purification by silica gel column chromatography (1.8f38.5 cm)
with MeOH–CH2Cl2 (1:9), 9 was obtained as colorless solid in 90%
yield, mp 121.5–123.58C. 1H NMR (CDCl3): d 1.82 (d, 1H, J54.0 Hz,
D2O exchangeable), 1.92 (dd, 1H, J56.0, 6.0 Hz, D2O exchangeable),
2.05 (ddd, 1H, J513.4, 10.1, 6.4 Hz), 2.19 (ddd, 1H, J513.3, 5.5, 2.3
Hz), 3.73 (ddd, 1H, J511.5, 6.5, 5.3 Hz), 3.83 (ddd, 1H, J511.8, 4.5, 4.5
Hz), 3.87 (br, 2H, D2O exchangeable), 3.98 (ddd, 1H, J54.8, 4.0, 3.3
Hz), 4.43 (br, 1H), 5.06 (dd, 1H, J510.0, 5.5 Hz), 5.08 (s, 2H), 6.68 (dd,
1H, J58.5, 2.0 Hz), 6.74 (d, 1H, J52.0 Hz), 6.82 (d, 1H, J58.0 Hz),
7.33 (m, 1H), 7.38 (m, 2H), 7.43 (m, 2H).

K. Tanaka et al. / European Journal of Pharmaceutical Sciences 13 (2001) 77–83
81
2.10. 4-(1,2-Dideoxy-b-D-ribofuranos-1-yl)catechol (3)
nBu4NF in THF (1 M, 1.14 ml, 1.14 mmol) was added
dropwise to a solution of O,O9-bis(tert-butyldimethylsilyl)-
4-(1,2-dideoxy-b-D-ribofuranos-1-yl)catechol (14) (0.26 g,
0.57 mmol) in THF (6 ml). After stirring for 10 min, a 5%
NaHCO3 aqueous solution (2.6 ml) was added to the
reaction mixture, which was then concentrated. The res-
idue was purified by silica gel column chromatography
with CHCl3 –MeOH (4:1) to give 0.13 g (99%) of 3 as
colorless form. After recrystallization from acetonitrile,
colorless needles were obtained, mp 182.0–184.08C (dec.).
1H NMR (CD3OD): d 1.93 (ddd, 1H, J513.2, 10.6, 6.0
Hz), 2.08 (ddd, 1H, J513.2, 5.3, 1.5 Hz), 3.61 (dd, 1H,
J511.7, 5.4 Hz), 3.65 (dd, 1H, J511.7, 5.1 Hz), 3.88 (ddd,
1H, J55.3, 5.3, 2.3 Hz), 4.29 (ddd, 1H, J55.7, 1.8, 1.8
Hz), 4.96 (dd, 1H, J510.5, 5.4 Hz), 6.69 (dd, 1H, J58.1,
1.7 Hz), 6.71 (d, 1H, J58.1 Hz), 6.81 (d, 1H, J51.7 Hz).
Scheme 3. Reagents and conditions: (a) tert-butyldimethylsilyl chloride,
pyridine, 808C, 84%; (b) ethyl chloroformate, pyridine, CH2Cl2, rt, 78%;
(c) SnCl4, CH2Cl2, 08C, 36% (b-anomer), 8% (a-anomer); (d) K2CO3,
MeOH, rt, 78%; (e) nBu4NF, THF, rt, 99%.
3. Results and discussion
Synthetic routes for the 2-aminophenol b-C-nucleoside
2 and the catechol b-C-nucleoside 3 are shown in Schemes
2 and 3, respectively. Each nucleoside was efficiently
synthesized via a Friedel–Crafts coupling reaction as the
key step between the aromatic ring and ribose moiety,
whereas the phenylenediamine b-C-nucleoside 1 was
prepared in rather longer steps through an RNA type
intermediate followed by the removal of 29-hydroxyl group
(Tanaka and Shionoya, 1999).
C-Glycoside synthesis, see Levy and Tang, 1995; Postema,
1995; Jaramillo and Knapp, 1994). The Friedel–Crafts
approach proceeding via electrophilic aromatic substitution
was used to build up the carbon skeletons of the nu-
cleosides 2 and 3, where SnCl4 was used as the Lewis acid
promotor. In the synthesis of nucleoside 2, O-benzyl-2-
trifluoroacetamidephenol 5 was used as the aromatic
nucleophile, which was prepared from 2-aminophenol in
two steps. The reaction of 5 with 3,5-protected 2-deoxy-D-
ribofuranose bearing a methoxy group at the anomeric
position was examined at 08C in CH2Cl2 in the presence of
SnCl4, and the b-C-nucleoside 6 was found to be produced
in good selectivity (a-7:b-651:10), albeit in low yield.
Stereochemistry of these isomers was determined by the
comparison of their 1H NMR spectra. The anomeric
The most common method for carbon–carbon bond
formation at the anomeric carbon involves nucleophilic
attack on this naturally electrophilic center (for reviews of
1
configuration for 6 was determined by H NOE differentia-
tion experiments (Table 1) and by examination of coupling
constants for 19- and 29-protons as done by Kool et al.
(1996). In b-anomers, the 29Ha is only near the 19-proton.
When the 19Ha and 29Ha were separately irradiated, we
observed 4.2 and 10% enhancements at the 29a- and
19-protons, respectively. The epimer 6 had a 19-resonance
which appeared as nearly evenly spaced doublet of doub-
lets (J510.5, 5.0 Hz). This 19–29 coupling constant trend
is consistent with similar coupling constants reported for
related b-C-nucleosides (Ren et al., 1996). These results
clearly demonstrated that the aryl nucleoside 6 is a b-
isomer. Removal of toluoyl groups of 6 with MeONa in
MeOH provided 8 quantitatively, which was then con-
verted into the free C-nucleoside 2 by treatment with
MeNH2 in MeOH and subsequently Pd–C under a hydro-
gen atmosphere.
Scheme 2. Reagents and conditions: (a) trifluoroacetic anhydride,
pyridine, CH2Cl2, 08C, 76%; (b) benzyl chloride, iPr2EtN, 1,2-dichloro-
ethane, reflux, 49%; (c) 1-O-methyl-3,5-O-ditoluoyl-2-deoxy-D-ribofuran-
ose, SnCl4, CH2Cl2, 08C, 15% (b-anomer), 1.5% (a-anomer); (d)
MeONa, MeOH, rt, 95%; (e) MeNH2, MeOH, rt; (f) H2, Pd–C, MeOH,
rt, 16% (two steps).
As for the synthesis of the catechol nucleoside 3, silyl

82
K. Tanaka et al. / European Journal of Pharmaceutical Sciences 13 (2001) 77–83
Table 1
Proton NOE differentiation data for a b-isomer of aryl nucleoside 6 in CDCl3
Irradiation at
H19
H29a
H29b
H39
H49
H5
NOE observed:
H19
H29a
H29b
H39
H49
H5
–
4.2
0
0
2.5
5.1
10
–
–
0
0
0
–
–
8.0
0
0
0
3.0
–
3.8
0
2.0
0
0
3.2
–
0
4.1
0
2.0
0
0
–
0
2.9
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protected catechol 10 was employed for the aromatic
moiety in the Friedel–Crafts coupling reaction with 1-O-
methyl-3,5-protected 2-deoxy-D-ribofuranose 11 to afford
fully protected nucleosides 12 (36%) and 13 (8%). The
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Epimer 12 showed two distinct H-19 to H-29 coupling
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The present work demonstrates convenient syntheses of
b-C-nucleosides containing a 2-aminophenol or a catechol
as the ‘chelator-base’ moiety, which would provide alter-
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This work was supported by Grant-in-Aids for Scientific
Research on Priority Areas (No. 11136255 ‘Metal-assisted
Complexes’, No. 11116230 ‘Biometalics’, and No. 706
‘Dynamic Control of Stereochemistry’ for MS) from
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Synthesis and kinetic properties of ribozyme analogues prepared using
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