Design, synthesis and RNase A inhibition activity of catechin and epicatechin and nucleobase chimeric molecules

Article abstract of DOI:10.1016/j.bmcl.2008.09.051

Several novel catechin/epicatechin and nucleobase chimeric molecules 1-6 have been synthesized via azide-alkyne click chemistry. The structures of these hybrids have been confirmed by NMR and mass spectroscopic data. The synthesized molecules were tested

Full text of DOI:10.1016/j.bmcl.2008.09.051

Bioorganic & Medicinal Chemistry Letters 18 (2008) 5411–5414
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design, synthesis and RNase A inhibition activity of catechin and epicatechin
and nucleobase chimeric molecules
*
Basab Roy, Sansa Dutta, Anupma Chowdhary, Amit Basak , Swagata Dasgupta
Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 June 2008
Revised 17 August 2008
Accepted 10 September 2008
Available online 17 September 2008
Several novel catechin/epicatechin and nucleobase chimeric molecules 1–6 have been synthesized via
azide-alkyne click chemistry. The structures of these hybrids have been confirmed by NMR and mass
spectroscopic data. The synthesized molecules were tested for their RNase A inhibition activities. Gel-
based assays showed inhibition in micromolar concentrations. The extent of inhibition was found to
be dependent upon the nature of base as well as the configuration at C-3 position of catechin.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Catechin
Epicatechin
Nucleobase
Click chemistry
RNase
Inhibition
Proteins belonging to the RNase superfamily are endonucleases
that catalyze the degradation of RNA via a two step transphospho-
rylation-hydrolytic reaction.1 Although some of these enzymes
have purely digestive role, there are others which exhibit potent
and unusual biological properties.2 For example, angiogenin the
protein found in the plasma induces new blood vessel formation
and thus plays a crucial role in the spreading of primary tumors.3
Some RNases are neurotoxic and may produce symptoms associ-
ated with overproduction of eosinophils.4 Thus it is implied that
inhibition for these enzymes might suppress the biological action
of these proteins and hence prove useful for treatment of human
diseases and for mechanistic studies. Bovine pancreatic RNase A
is a convenient model for development of inhibition because of
its well defined active site geometry.5 The nucleolytic center con-
sisted of multiple subsites that bind the phosphate, the base and
the ribose components of its RNA substrate, with the P site stabiliz-
ing the phosphate the B site the bases (Fig. 1). At site P1 cleavage of
the phosphodiester occurs. B1 is the binding site of the base whose
30 oxygen is involved in the phosphate being hydrolyzed. Besides,
there are other subsites P0, P2 and B0 (not shown). Because of the
reported RNase activity of natural polyphenols like EGCG,6 we be-
came interested in synthesizing novel nucleobase-catechin or epi-
catechin chimeric molecules. The two units are joined by a triazole
linker constructed via a click reaction. It is expected that the 5-
membered triazole (although planar) may mimic the ribose unit
Cyt or
Ura
O
B1
O
P
O
R1
O
Sugar
P1
Ade B2
O
P
O
O
R2
Sugar
O
P O
O
Figure 1. Schematic representation of active site of RNase A with RNA.
while the nucleobase will have binding interactions with the B1-
site.
The synthesis of the target molecules require the 3-O-propargyl
tetrabenzyl catechin or epicatechin which was prepared by propar-
gylation of tetrabenzyl catechin (TBC) or epicatechin (TBEC). The
benzyl ether was chosen as the protecting group because of its
deprotection under neutral condition. Acid or base sensitive pro-
tecting groups needed to be avoided because of possible racemiza-
tion at C-2.7 The other component for the click reaction, namely the
azido ethyl nucleobases were prepared using a standard protocol
* Corresponding author. Tel.: +91 3222283300; fax: +91 3222282252.
E-mail address: absk@chem.iitkgp.erne2t.in ( A. Basak).
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.09.051

5412
B. Roy et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5411–5414
N N3
Inhibition of RNase A was checked qualitatively by an agarose
gel based assay, wherein the degradation of the 28s and 18s rRNA
was studied. The ribonuclease A of concentration 0.011 M was
mixed with the synthesized compounds and incubated for 6 hrs
at 37 °C. To these preincubated enzyme solutions the RNA
NH2
N
NH2
N
NH2
N
Br
dry DMF
Br
N
N
N
N
50oC, 24hrs
N
l
N
Anhy. K2CO3
dry DMF
rt, 10 hrs
N
H
N
85%
N
14
13
N3
R
12
Br
(90%)
(0.22 lg/ll) was added as substrate and the incubation was al-
lowed for a further 15 min. The resulting solution was mixed with
40% sucrose solution (made from RNase free water) and 0.25% bro-
mophenol blue (dissolved in 1ꢀ TAE buffer) and loaded onto a 1.1%
agarose gel. The bands were visualized under a UV–transillumina-
O
N
O
O
N
MsCl, Et3N
dry DCM
R
R
NH
N
Br
CO2Et
CO2Et
N
CO2Et
0oC, 30 min
O
N
O
Anhy. K2CO3
dry DMF,
O
90%
rt, 3 days
tor by virtue of the ethidium bromide (final concentration 0.5 lg/
OH
15-16
OH
17-18
85%
OMs
19-20
mL) present in the gel. Comparison of the gel pattern (Fig. 2) in var-
ious lanes indicated that the pyrimidine based chimeric molecules
are better inhibitors than the adenine based ones. This was ex-
pected as the site B1 has a preference for pyrimidine bases.10 Be-
tween the two adenine based chimeras, the epicatechin based
one did not show any inhibition while the catechin did show some
inhibition under identical conditions thus showing the importance
of the C-3 OH stereochemistry.
O
NaN3
R
N
CO2Et
dry DMF
R = Me
R = H
Thymine
Uracil
50oC, 10 hrs
N
O
80%
N3
21-22
In conclusion, several novel catechin and epicatechin nucleo-
base chimeric molecules11 have been synthesized. All the pyrimi-
dine based molecules were found to be potent inhibitors of
RNase A and are thus good candidates for further evaluation.
Selected experimental procedure and spectral data. All the 1H and
13C NMR spectra were recorded at 400 and 100 MHz, respectively.
General procedure for the click reaction. Compounds 7–11 were
synthesized by this procedure. 3-O-propargyl tetrabenzyl catechin
or epicatechin (0.13 mmol) and azido-nucleobases 14, 21–22
(0.13 mmol) were dissolved in acetonitrile/H2O mixture (1:1). Cop-
per sulfate (1 equiv) and sodium ascorbate (2 equiv) were added
and the mixture was stirred at room temperature until TLC indi-
cated the disappearance of the starting materials (ꢁ24 h). The mix-
Scheme 1. Synthesis of azido-nucleobases.
as shown in Scheme 1. The click reaction8 was performed in pres-
ence of Cu (I) made in situ by reduction of CuSO4 with -ascorbate.
L
The protected chimeric molecules were isolated by pure Si-gel
chromatography. Deprotection with H2 in presence of Pearlman
catalyst9 furnished the final compounds (Schemes 2 and 3). These
were all fully characterized by NMR and mass spectral data. The
regiochemistry of the cycloaddition was proved to be 1, 3 as
revealed by the NOESY spectrum recorded on the catechin adenine
chimera. The cross peak observed between the NCH2 and the
olefinic hydrogen confirmed the proposed structure.
OBn
OBn
OBn
OBn
BnO
O
NaH, propargyl
.5H2O,
CuSO4
O
O
N
BnO
bromide
dry THF,
Na-ascorbate
R
O
N
N
CO2Et
TBC
O
H
OBn
MeCN -H2O (1:1)
rt, 3days
10 hrs
O
OBn
23
24
N
( 75% )
N
( 85% )
7-8
H2 [2 bar]
20% Pd(OH)2-C
OH
OH
OH
H
THF:MeOH:H2O
=20:1:1
OH
O
O
HO
HO
( 90% )
O
O
N
R
O
N
O
N
N
CO2Et
CO2Eta
OH
H
OH
N
O
O
N
N
N
N
N
1-2
3
For1 R =Me
For2 R =H
OBn
CuSO4.5H2O,
Na -ascorbate
OBn
NaH, propargyl
bromide
O
BnO
dry THF,
MeCN-H2O (1:1)
rt, 3days
( 87% )
TBEC
O
10 hrs
75%
OBn
25
26
OBn
OBn
OH
BnO
OH
O
H
2 [2 bar]
O
N
O
HO
20% Pd(OH)2-C
THF:MeOH:H2O
=20:1:1
O
Me
O
N
N
CO2Et
Me
H
O
N
N
OBn
CO2Et
O
H
OH
( 88%)
N
N
O
N
9
N
N
4
Scheme 2. Synthesis of pyrimidine based chimeras via click reaction.

B. Roy et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5411–5414
5413
OH
OBn
H2 [2 bar]
20% Pd(OH)2-C
THF:MeOH:H2O
=20:1:1
OH
OBn
N
O
HO
O
BnO
H2N
H2N
O
O
( 88% )
N
N
OH
N
H
OBn
H
N
N
N
N
N
N
N
N
N
N
10
5
OH
OBn
H2 [2 bar]
20% Pd(OH)2-C
THF:MeOH:H2O
=20:1:1
OH
OBn
N
O
HO
O
BnO
H2N
H2N
O
( 85% )
O
N
N
OH
H
N
OBn
11
Scheme 3. Synthesis of adenine based chimeras via click reaction.
H
N
N
N
N
N
N
N
N
N
N
6
127.1, 123.2, 119.8, 114.4, 113.8, 101.3, 94.7, 93.9, 77.8, 72.2,
71.3, 71.2, 70.1, 69.9, 62.4, 48.7, 43.1, 24.7. Mass (ES+): m/z 893
[MH+].
For 6. m
max (KBr, cmꢂ1): 3431, 1637, 1385, 1220, 1112, 1078; dH
(MeOH-d4): 8.18 (1H, s), 7.69 (1H, s), 7.23 (1H, s), 7.10 (1H, s),
6.75–6.72 (2H, m), 5.93–5.88 (2H, m), 4.83 (1H, s), 4.76–4.75
(1H, br s), 4.68–4.67 (2H, m), 4.49, 4.38 (2ꢀ 1H, ABq, J = 13.2 Hz),
3.89 (1H, s), 3.35 (1H, s), 2.88 (1H, d, J = 16.8 Hz), 2.72 (1H, dd,
J = 13.2, 3.8 Hz); dC (MeOH-d4): 158.1, 157.9, 157.5, 157.4, 154.1,
150.8, 146.8, 146.0, 142.6, 132.5, 125.3, 119.6, 116.0, 115.6,
115.5, 99.9, 96.5, 95.9, 79.2, 75.0, 63.4, 44.9, 30.8, 25.8. Mass
(ES+): m/z 555 [MNa+], 533 [MH+].
For 10. mmax (CHCl3, cmꢂ1): 2942, 1618, 1445, 1260, 1028; dH
(CDCl3): 8.26 (1H, s), 7.52–7.28 (22H, m), 6.94 (1H, s), 6.90–6.85
(2H, m), 6.79–6.71 (2H, m), 6.25 (1H, d, J = 2.0 Hz), 6.18 (1H, d,
J = 2.0 Hz), 5.14 (2H, s), 5.09 (2H, d, J = 8.0 Hz), 5.05 (2H, s), 5.01
(2H, s), 4.72 (2H, d, J = 7.6 Hz), 4.61 (4H, s), 4.45, 4.23 (2ꢀ 1H,
ABq, J = 12.8 Hz), 3.77 (1H, dd, J = 13.2, 8.0 Hz), 3.04 (1H, dd,
J = 16.4, 5.6 Hz), 2.62 (1H, dd, J = 16.4, 8.4 Hz); dC (CDCl3): 158.7,
157.6, 155.3, 155.1, 152.6, 148.7, 148.6, 145.5, 140.5, 137.1,
137.0, 136.9, 136.8, 132.3, 128.6, 128.5, 128.4, 128.0, 127.9,
127.8, 127.5, 127.3, 127.2, 123.04, 120.6, 114.6, 113.6, 102.0,
94.3, 93.7, 79.4, 62.7, 48.7, 43.3, 25.5. Mass (ES+): m/z 893 [MH+].
Figure 2. Lane 1: RNA + water + methanol, lane 2: RNA + RNase A + methanol, lane
3: RNA + RNase A + catechin adenine chimera 5 (0.476 mM), lane 4: RNA + RNase
A + epicatechin adenine chimera 6 (0.476 mM), lane5: RNA + RNase A + catechin
thymine chimera
chimera (0.412 mM), lane 7: RNA + RNase A + catechin uracil chimera 2+3
(0.423 mM).
1 (0.412 mM), lane 6: RNA + RNase A + epicatechin thymine
4
For 5. m
max (KBr, cmꢂ1): 3428, 1635, 1384, 1222, 1145, 1078; dH
ture was poured into satd aq NH4Cl solution (1:1, 20 mL) and the
product was extracted four times with EtOAc. The organic layer
was dried with Na2SO4 and filtered, and the solvent was removed
under reduced pressure. The residue was purified by flash
chromatography.
General procedure of debenzylation reaction. Compounds 1–6
were synthesized by this procedure. Solutions of compounds 7–
11 (0.041 mmol) in a mixture of THF/MeOH/H2O (20:1:1) (22 mL)
were hydrogenated at 2 bar pressure over 20% Pd(OH)2/C (5 mg)
for 2 h at room temperature. Filtration and concentration afforded
a pale brown solid, which was purified by Si-gel (230–400 mesh,
N2 flash) short column chromatography (MeOH–CHCl3) to give
pure 1–6 (95%) as pale brown amorphous solids.
(MeOH-d4): 8.34 (1H, s), 8.15 (1H, s),7.76 (1H, s), 7.41 (1H, s), 6.77–
6.74 (2H, m), 6.68–6.66 (1H, m), 5.93 (1H, s), 5.85 (1H, s), 4.7 (2H,
br s), 4.68 (1H, d, J = 7.22 Hz), 4.45 , 4.34 (2ꢀ 1H, ABq, J = 12.8 Hz),
3.77–3.72 (1H, m), 2.84 (1H, dd, J = 16.0 , 5.2 Hz), 2.52 (1H, dd,
J = 16.0 , 8.0 Hz); dC (MeOH-d4): 158.1, 157.7, 157.3, 157.0, 154.0,
150.8, 146.8, 146.3, 146.6, 146.2, 132.5, 125.4, 120.1, 116.3,
115.3, 100.5, 96.5, 95.7, 80.9, 76.7, 63.37, 44.9, 30.9, 26.2. Mass
(ES+): m/z 555 [MNa+], 533 [MH+].
For 9. mmax (CHCl3, cmꢂ1): 2927, 1747, 1705, 1652, 1461, 1377,
1264, 1218, 1146; dH (CDCl3):7.48–7.29 (20H, m), 6.97 (1H, s),
6.95–6.91 (2H, m), 6.71 (1H, s), 6.50 (1H, s), 6.27 (2H, s), 5.14
(2H, s), 5.10 (2H, s), 5.03 (2H, s), 5.01 (2H, s), 4.90 (1H, s), 4.67
(2H, s), 4.63–4.47 (2H, m), 4.36 (2H, t, J = 5.6 Hz), 4.27–4.21 (2H,
m), 4.03–4.01 (3H, m), 3.04 (1H, d, J = 17.2 Hz), 2.82 (1H, dd,
J = 17.2, 4.0 Hz), 1.67 (3H, s), 1.26 (3H, t, J = 7.2 Hz); dC (CDCl3):
167.7, 162.8, 158.6, 158.0, 155.4, 150.9, 148.2, 148.1, 145.5,
138.7, 137.1, 137.0, 136.9, 136.8, 132.3, 128.6, 128.5, 128.4,
128.3, 128.0, 127.9, 127.8, 127.6, 127.5, 127.4, 127.2, 123.6,
119.8, 114.2, 113.5, 110.0, 101.3, 94.7, 93.9, 77.8, 72.3, 71.2, 71.1,
70.1, 69.9, 62.3, 61.6, 49.1, 47.6, 42.0, 37.7, 24.8, 14.1, 12.6. Mass
(ES+): m/z 970 [MH+].
For 11. mmax (CHCl3, cmꢂ1): 2930, 1609, 1445, 1262, 1219,
1143,1027; dH (CDCl3): 8.32 (1H, s), 7.46–7.28 (21H, m), 7.24
(1H, m), 7.20 (1H, s), 6.83 (2H, m), 6.40 (1H, s), 6.27 (2H, s), 5.66
(2H, br s), 5.13–5.10 (4H, m), 5.03–4.98 (4H, m), 4.87 (1H, s),
4.54–4.42(5H, m), 3.92 (1H, s), 3.03 (1H, d, J = 16 Hz), 2.80 (1H,
dd, J = 17.2, 4.0 Hz), dC (CDCl3): 158.6, 158.0, 155.4, 155.1, 152.5,
149.6, 148.3, 148.2, 145.5, 140.5, 137.1, 137.0, 136.9, 136.8,
132.4, 128.5, 128.4, 128.3, 127.9, 127.8, 127.5, 127.4, 127.3,

5414
B. Roy et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5411–5414
For 4. mmax (KBr, cmꢂ1): 3420, 1742, 1640, 1466, 1380, 1232,
For 3. mmax (KBr, cmꢂ1): 3406, 1745, 1658, 1503, 1420, 1360,
1342, 1225, 1122; dH (MeOH-d4): 7.57 (1H, s), 6.78–6.74 (3H, m),
5.92 (1H, d, J = 1.6 Hz), 5.84 (1H, d, J = 1.6 Hz), 4.60–4.56 (3H, m),
4.50 (1H, d, J = 12.8 Hz), 4.43 (1H, dd, J = 12.4, 4.4 Hz), 4.22–4.11
(3H, m), 3.85–3.77 (3H, m), 3.21 (2H, t, J = 6.4 Hz), 2.84 (1H, dd,
J = 16.0, 5.6 Hz), 2.62 (2H, t, J = 6.4 Hz), 2.54 (1H, dd, J = 16.0,
7.6 Hz), 1.23 (3H, t, J = 7.2 Hz); dC (MeOH-d4): 168.1, 163.0, 156.5,
156.2, 155.4, 145.3, 144.8, 144.0, 130.9, 124.1, 118.5, 114.6,
113.7, 100.2, 94.8, 94.0, 79.3, 75.0, 61.8, 61.3, 61.1, 49.4, 48.1,
47.7, 41.3, 30.5, 24.7, 13.0. Mass (ES+): m/z 598 [MH+].
1146; dH (MeOH-d4): 7.22 (1H, s), 6.94 (1H, s), 6.89 (1H, s), 6.74
(2H, s), 5.93 (1H, d, J = 2.0 Hz), 5.87 (1H, d, J = 2.0 Hz), 4.83 (1H,
s), 4.62 (2H, s), 4.52 (1H, d, J = 12.8 Hz), 4.39 (1H, d, J = 12.8 Hz),
4.18 (2H, q, J = 7.2 Hz), 4.12 (2H, t, J = 5.6 Hz), 3.94 (1H, s), 3.59
(2H, dd, J = 14.0, 7.2 Hz), 2.90 (1H, d, J = 16.0 Hz), 2.73 (1H, dd,
J = 16.8 , 4.0 Hz), 1.74 (3H, s), 1.25 (3H, t, J = 7.2 Hz); dC (MeOH-
d4): 168.2, 163.5, 156.5, 156.3, 155.9, 151.0,145.2, 144.4, 144.3,
139.9, 130.9, 124.2, 118.1, 114.4, 114.0, 109.0, 98.4, 94.9, 94.3,
77.6, 73.5, 62.0, 61.3, 48.9, 47.8, 41.7, 24.4, 13.0,11.3. Mass (ES+):
m/z 632 [MNa+], 610 [MH+].
For 2. mmax (KBr, cmꢂ1):3406, 1740, 1652, 1458, 1284, 1226,
1143, 1060, 1025; dH (MeOH-d4):7.54 (1H, s), 7.12 (1H, d,
J = 8.0 Hz), 6.71–6.69 (3H, m), 5.92 (1H, d, J = 1.6 Hz), 5.84 (1H, d,
J = 1.6 Hz), 5.53(1H, d, J = 8.0 Hz), 4.67 (2H, d, J = 6.8 Hz), 4.60–
4.56 (3H, m), 4.50 (1H, d, J = 12.4 Hz), 4.43 (1H, dd, J = 12.4,
4.0 Hz), 4.21 (2H, q, J = 7.6 Hz), 3.85–3.77 (3H, m), 2.84 (1H, dd,
J = 16.4, 5.6 Hz), 2.54 (1H, dd, J = 16.4, 8.0 Hz), 1.23 (3H, t,
J = 7.2 Hz); Mass (ES+): m/z 596 [MH+].
For 7. mmax (CHCl3, cmꢂ1): 2922, 1746, 1652, 1460, 1264, 1228,
1145; dH (CDCl3):7.44–7.28 (20H, m), 6.98–6.91 (4H, m), 6.49
(1H, s), 6.25 (1H, d, J = 2.0 Hz), 6.19 (1H, d, J = 2.0 Hz), 5.15 (2H,
s), 5.11 (2H, d, J = 6.0 Hz), 5.00 (2H, s), 4.96 (2H, s), 4.68–4.67
(1H, m), 4.50–4.42 (2H, m), 4.19 (2H, q, J = 7.2 Hz), 4.11 (2H, t,
J = 5.6 Hz), 4.07 (2H, t, J = 6.4 Hz), 3.84–3.82 (1H, m), 3.03 (1H,
dd, J = 16.4, 5.6 Hz), 2.67–2.63 (1H, m), 1.69 (3H, s), 1.26 (3H, t,
J = 7.2 Hz); dC (CDCl3): 167.8, 162.8, 158.7, 157.6, 155.1, 151.0,
148.7, 148.6, 145.5, 138.6, 132.3, 137.0, 136.9, 136.8, 136.7,
128.5, 128.4, 127.9, 127.8, 127. 7, 127.6, 127.5, 127.4, 127.3,
127.2, 123.9, 120.7, 14.5, 113.4, 110.1, 102.0, 94.3, 93.7, 79.5,
74.9, 71.2, 71.1, 70.0, 69.9, 62.6, 61.6, 49.3, 47.8, 42.0, 37.7, 25.7,
14.1, 12.6. Mass (ES+): m/z 970 [MH+].
Acknowledgments
The author B.R. is grateful to CSIR, Government of India for a fel-
lowship. D.S.T. is thanked for providing funds for 400 MHz NMR
facility under the IRPHA program.
For 1. mmax (KBr, cmꢂ1): 3426, 1740, 1650, 1462, 1380, 1232,
1145; dH (MeOH-d4): 7.53 (1H, s), 6.98 (1H, s), 6.76–6.68 (3H, m),
5.92 (1H, d, J = 2.4 Hz), 5.84 (1H, d, J = 2.4 Hz), 4.67–4.66 (3H, m),
4.61 (2H, s), 4.49, 4.41 (2ꢀ 1H, ABq, J = 12.8 Hz), 4.20–4.15 (5H,
m), 3.60 (2H, dd, J = 14.0 , 7.2 Hz), 2.85 (1H, dd, J = 16.4 , 5.2 Hz),
References and notes
1. Richards, F. M.; Wyckoff, H. W.. In The Enzymes; Boyer, P. D., Ed.; Academic
Press: New York, 1971; Vol. 4, p 647; Beintema, J. J.; Fitch, W. M.; Carsena, A.
Mol. Biol. Evol. 1986, 3, 262.
2.54 (1H, dd, J = 16.4
,
8.0 Hz), 1.74 (3H, s), 1.26 (3H, t,
2. D’Alessio, G. Trends Cell. Biol. 1993, 3, 106.
3. Fett, J. W.; Strydom, D . J.; Lobb, R. R.; Alderman, E. M.; Bethune, J. L.; Riordan, J.
F.; Vallee, B. L. Biochemistry 1985, 24, 5480; Olson, K. A.; Fett, J. W.; French, T. C.;
Key, M. E.; Vallee, B. L. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 3146.
4. Gleich, G. J.; Loegering, D. A.; Bell, M. P.; Checkel, J. L.; Ackerman, S. J.; Mckean,
D. J. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 3146.
5. Raines, R. T. In Artificial Nucleobases; Zenkova, M. A., Ed.; Springer: Heidelberg,
2004; pp 19–32; Russo, N.; Acharya, K. R.; Vallee, B. L. Proc. Natl. Acad. Sci. U.S.A.
1996, 93, 804.
6. Ghosh, K. S.; Maiti, T. K.; Dasgupta, S. Biochem. Biophys. Res. Commun. 2004, 325,
807; Ghosh, K. S.; Maiti, T. K.; Debnath, J.; Dasgupta, S. Proteins 2007, 69, 566.
7. Kawamoto, H.; Nakatsubo, F.; Murakami, K. Synth. Commun. 1996, 26, 531; Basak,
A.; Mandal, S.; Bandhyopadhyay, S. Bioorg. Med. Chem. Lett. 2003, 13, 1083.
8. Kolbe, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004;
Rodinov, V. O.; Fokin, V. V.; Finn, M. G. Angew. Chem., Int. Ed. 2005, 44, 2210.
9. Kozikowski, A. P.; Tuckmantel, W.; Bottcher, G.; Romanczyk, L. J., Jr. J. Org.
Chem. 2003, 68, 1641; Saito, A.; Doi, Y.; Tanaka, A.; Matsuura, N.; Ubukata, M.;
Nakajima, N. Bioorg. Med. Chem. 2004, 12, 4783.
10. Fischer, B. M.; Grilley, J. E.; Raines, R. T. J. Biol. Chem. 1998, 273, 34134.
11. Some examples of chimeric molecules from our laboratory: Kar, M.; Basak, A.
Chem. Rev. 2007, 107, 2861; Pal, R.; Basak, A. Chem. Commun. 2006, 2992;
Basak, A.; Mandal, S. Tetrahedron Lett. 2002, 43, 4241; Basak, A.; Khamrai, U. K.
Tetrahedron Lett. 1996, 37, 2475.
J = 7.2 Hz); dC (MeOH-d4): 168.2, 163.5, 156.5, 156.1, 155.3, 151.0,
145.3, 144.7, 139.8, 130.9, 127.2, 124.2, 118.4, 114.6, 113.6,
109.0, 98.9, 94.8, 94.0, 79.3, 75.1, 61.8, 61.3, 49.0, 48.0, 41.7, 24.6,
13.0, 11.2. Mass (ES+): m/z 632 [MNa+], 610 [MH+].
For 8. mmax (CHCl3, cmꢂ1): 2926, 1741, 1670, 1451, 1262, 1219,
1027; dH (CDCl3): 7.45–7.28 (20H, m), 6.97 (1H, d, J = 4.8 Hz),
6.93 (1H, m), 6.60 (1H, d, J = 8.0 Hz), 6.25 (1H, s), 6.19 (1H, s),
5.44 (1H, d, J = 7.6 Hz), 5.15 (2H, s), 5.11 (2H, d, J = 5.2 Hz), 5.02
(2H, s), 5.00 (2H, s), 4.73 (1H, d, J = 8.0 Hz), 4.63 (2H, s), 4.52–
4.46 (2H, m), 4.18 (2H, q, J = 7.2 Hz), 4.12–4.04 (3H, m), 3.86–
3.81 (2H, m); 3.05 (1H, dd, J = 16.4 , 5.2 Hz), 2.63 (1H, dd, J = 16.4
, 8.4 Hz), 1.26 (3H, t, J = 7.2 Hz); dC (CDCl3): 167.6, 161.8, 158.7,
157.6, 155.1, 151.0, 148.7, 148.6, 145.6, 142.6, 137.0, 136.9,
136.8, 136.7, 132.3, 128.6, 128.5, 128.4, 127.9, 127.8, 127.5,
127.4, 127.3, 123.9, 120.7, 114.5, 113.5, 101.9, 101.6, 94.3, 93.7,
79.5, 74.9, 71.3, 71.1, 70.0, 69.9, 62.6, 61.7, 49.6, 47.7, 41.7, 25.8,
14.1. Mass (ES+): m/z 956 [MH+].