Furanose ring conformations in a 1′-alkynyl C-nucleoside and the dinucleotide

Article abstract of DOI:10.1016/j.tet.2012.08.057

A large-scale synthesis was carried out for alkynyl C-nucleotide oligomers in order to clarify the structural details of the artificial DNA. A liquid-phase synthesis by means of phosphoramidite methodology enabled us to handle ～0.1 g of the DNA oligomers possessing a pseudouracil derivative (T*) as a non-natural nucleobase. ¹H NMR measurements in aqueous media were carried out for the oligomers, succeeded in the accurate assignments of the protons accompanying with determination of the coupling constants (J values). These J values revealed that average conformation of the alkynylribose rings in the DNA was substantially biased toward the S-type conformers ( ²T₁/E₁/^OT₁). X-ray crystal analysis of the non-natural nucleoside also supported the S-type conformation (²E/²T₁) as seen in natural B-DNA.

Full text of DOI:10.1016/j.tet.2012.08.057

Tetrahedron 68 (2012) 9045e9049
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Furanose ring conformations in a 10-alkynyl C-nucleoside and the dinucleotide
,
a
a
b
b
Junya Chiba a , Wataru Shirato , Yusuke Yamade , Byung-Soon Kim , Shinya Matsumoto ,
*
Masahiko Inouye a
,
*
a Graduate School of Pharmaceutical Sciences, University of Toyama, Sugitani 2630, Toyama 930-0194, Japan
b Graduate School of Environment and Informational Sciences, Yokohama National University, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 June 2012
Received in revised form 18 August 2012
Accepted 20 August 2012
Available online 24 August 2012
A large-scale synthesis was carried out for alkynyl C-nucleotide oligomers in order to clarify the struc-
tural details of the artificial DNA. A liquid-phase synthesis by means of phosphoramidite methodology
enabled us to handle w0.1 g of the DNA oligomers possessing a pseudouracil derivative (T*) as a non-
natural nucleobase. 1H NMR measurements in aqueous media were carried out for the oligomers, suc-
ceeded in the accurate assignments of the protons accompanying with determination of the coupling
constants (J values). These J values revealed that average conformation of the alkynylribose rings in the
DNA was substantially biased toward the S-type conformers (2T1/E1/OT1). X-ray crystal analysis of the
non-natural nucleoside also supported the S-type conformation (2E/2T1) as seen in natural B-DNA.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Alkynylribose
C-Nucleotide
Puckering conformation
Pseudorotation
1. Introduction
between the artificial and natural DNA in terms of shape, size, and
pitch of the duplexes in spite of the differences in their first order
Duplex and triplex formation of nucleic acids plays a crucial role
not only in the origin of life but also in the fields of medicine,
biotechnology, and nanotechnology. In these fields, non-natural
analogs of DNAs such as peptide nucleic acid (PNA),1e3 threofur-
anosyl nucleic acid (TNA),4,5 and locked nucleic acid (LNA)6,7 are
recently employed as well as natural nucleic acids.8e12 Represented
by such non-natural DNAs, chemists have developed a variety of
artificial DNAs with various chemical modifications in the struc-
tures of natural DNA and RNA.13e16 Many of the single-stranded
artificial oligomers can interact with complementary natural DNA
and RNA to form hetero duplexes. The strength of the hetero-
duplex formation strongly depends on the first order structure of
the artificial oligomers, especially in electrostatic configuration,
nucleoside composition, and detailed conformation such as sugar
puckering and base orientation.
We recently reported an architecturally new artificial DNA
composed exclusively of alkynyl C-nucleotides bearing four types of
non-natural bases.17 In that report, the artificial DNA hybridized
with the complementary artificial DNA in a sequence specific
manner with antiparallel orientation. The resulting right-handed
duplexes had thermal stabilities very close to those of the corre-
sponding natural duplexes. This finding suggests the similarity
structures, especially in the alkynylribose part on first sight. Thus,
we decided to clarify the detailed puckering structure of the alky-
nylribose ring within the artificial DNAs. Nuclear magnetic reso-
nance (NMR) and X-ray crystal analyzes are suitable for the
structural confirmation. In these measurements, however, there is
some difficulty to handle relatively small amounts of the artificial
DNA samples when we use the automated, solid-phase DNA syn-
thesis as described before.17 Here we report a large-scale, liquid-
phase synthesis of the artificial DNA oligomers and determination
of the conformations in solution and solid states (Scheme 1).
2. Results and discussion
In our previous report, artificial DNA oligomers were obtained
by using a solid-phase synthesis that was found not efficient for
large-scale preparative purposes. In contrast, liquid-phase synthe-
sis enables abundant handling of target compounds. For natural
DNA synthesis, liquid-phase methodologies based on phospho-
diester, phosphotriester, and phosphoramidite have been de-
veloped for short oligomers up to 20mer lengths.18,19 We chose the
phosphoramidite method for the present liquid-phase synthesis. To
simplify the synthesis, a pseudouracil referred to as T* was used as
a protection-free nucleobase. Moreover, TBDMS group was selected
for the protection of the terminal 30-OH because this group is stable
under mild-acidic and basic conditions needed to cleave the
dimethoxytrityl (DMTr) and cyanoethyl (CNE) groups, respectively.
* Corresponding authors. E-mail addresses: chiba@pha.u-toyama.ac.jp (J. Chiba),
inouye@pha.u-toyama.ac.jp (M. Inouye).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.08.057

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J. Chiba et al. / Tetrahedron 68 (2012) 9045e9049
Scheme 1. (a) TBDMSeCl, imidazole, DMF; (b) CCl3CO2H, CH2Cl2; (c) 1) 1H tetrazole, CH3CN; 2) I2, H2O, pyridine; (d) 1) 4, 1H tetrazole, CH3CN; 2) I2, H2O, pyridine; (e) 1) 10% HF/
H2O, CH3CN; 2) conc. NH4OH; (f) i-Pr2NP(Cl)O(CH2)2CN, i-Pr2NEt, CH2Cl2.
On the other hand, the TBDMS group is labile to fluoride solution to
regenerate the eOH group when necessary.
NMR spectrum, T*4mer showed strong resemblance in some sets
of chemical shifts to the native thymidine 4mer T4mer for the
protons bonding to C20, C30, C40, and C50 carbon atoms (Fig. S4). In
contrast, T4mer exhibited unique peaks around 6.2 ppm assigned
as H10 protons, while the anomeric protons for T*4mer were ob-
served in relatively high field around 5.0 ppm. This observation
would be ascribed to the increased shielding effects at the anomeric
protons by modifying from the native N-nucleotides to the acety-
lenic C-nucleotides. In the case of T* nucleoside dT* and the di-
nucleotide T*2mer, precise assignments of the 1H NMR spectrum
could be attained, accompanying with verification of the coupling
constants (J values) for many protons (see Experimental Section
and Fig. 1).
Scheme 1 shows the full synthetic route for the artificial DNA
oligomers T*2e5mers. We have already reported the synthesis of
DMTr-protected nucleoside 1 as a starting material for 30-TBDMS-
protected 3 and T* phosphoramidite 4.17 These derivatives were
coupled into a T*-dimer with 1H-tetrazole as a catalyst, then con-
verted into DMTr-protected T*2mer by oxidation from PIII to PV, and
derivatized to50-deprotected T*2mer bysubsequent treatment with
CCl3CO2H. A series of conditions for the liquid-phase oligomeriza-
tion of the artificial DNAs were almost the same as that for the solid-
phase natural-DNA synthesis. Repeating the synthetic cycles gives
desirable lengths of the oligomeric non-natural nucleotides, 50-
protected T*2e5mers and deprotected T*2e5mers. After all of the
protecting groups were cleaved, T*2e5mers were purified by
reverse-phase HPLC Fig. S1A shows the charts of HPLC for the pu-
rified T*2e5mers, performed with a CHEMCOBOND 5-ODS-H col-
umn (10ꢀ150 mmwith an eluant of 5 mM ammonium formate in 3%
CH3CN at a flow rate of 2.0 mL/min). Sharp peaks corresponding to
the individual oligomers were observed with different retention
times. In spite of the increase of hydrophilicity for the longer oli-
gonucleotides with increment of total negative charge, longer re-
tention times were necessary for longeroligomers under these HPLC
conditions. It can be considered that the outstanding size effect of
lengths for the oligomers appeared over the hydrophilic effect in the
case for these relatively short 2e5mer lengths.
31P NMR measurements were carried out for these artificial
DNAs in D2O with D3PO4 as an external reference (Fig. S1B). All of
the artificial DNAs displayed some distinctive peaks of PV at the
chemical shifts ranging from ꢁ0.5 to 0 ppm that are almost the
same in natural DNAs.20 The dimer T*2mer and the trimer T*3mer
clearly showed one and two peaks, respectively, corresponding to
the number of PV under different chemical environments. Although
all the peaks were hardly separable one another in T*4mer and
T*5mer, characteristic peaks were observed accompanying with
broadening and overlap.
Fig. 1. (A) 1H NMR spectra in D2O for artificial dT* (top) with partially expanded charts
of H10 and H20/H200 and T*2mer (bottom) with TSP-d4 as an internal reference at room
temperature. (B) Chemical structure and annotation of protons for T*2mer. (C) Sche-
matic projections of a furanose ring in two idealized twisted conformations, looking
toward the oxygen atom from the center of the C20eC30 bond.
1H NMR measurements were performed for these artificial DNA
oligomers (Fig. S2). 1H assignment for all the oligomers was eluci-
dated by 1He1H COSY (Fig. S3), being based on the well-known
assignment for the native thymidine oligomers.21,22 In the 1H

J. Chiba et al. / Tetrahedron 68 (2012) 9045e9049
9047
Table 1
A standard method for evaluating the conformation of a fura-
nose ring is pseudorotational analysis.23e26 In the pseudorotational
wheel depiction (Fig. 2A), furanose conformations occupying
‘Northern’ hemisphere are conveniently represented as belonging
to the N-type family (pseudorotational phase angle P¼0ꢂ90ꢃ),
while sugar rings in the ‘Southern’ half of the circle denoted as the
S-type family (P¼180ꢂ90ꢃ). P can be calculated by eq. 1 with the
Evaluation of furanose ring conformation for dT* in solid state
f0/deg
f1/deg
f2/deg
f3/deg
f4/deg
P/deg
Typea
Ab
Bd
ꢁ24.63
ꢁ35.73
34.23
21.63
ꢁ30.93
ꢁ0.73
17.13
ꢁ20.53
5.03
35.33
154c
91
2E/2T1
OE
a
See Fig. 2 for conformer definitions.
See Fig. 3A.
Fm¼28ꢃ.
b
c
five endocyclic torsion angles (f0ef4, Fig. 2B), and the puckering
d
See Fig. 3B.
amplitude Fm related to P and f3 is determined through eq. 2.
Fig. 3. Two types of dT* determined by X-ray crystal analysis (CCDC number: 881808).
(A) 20-endo/20-endo-10-exo and (B) O-endo conformers.
(DH2 /H2 ) as eqs. 3 and 4.23 Tables 1 and 2 show the P values for dT*
in solid and solution states, which indicates the S-type preferability
to the alkynylribose ring. It is well known that the pseudorotational
equilibrium between S- and N-type modes in native 20-deoxy
N-nucleosides (Fig. 1C) is driven toward N by the O40eC10eN1/9
0
00
Table 2
Evaluation of furanose ring conformation for dT* in D2O
Xsa
Typeb
2T1/E1
0
00
0
0
0 00
DH2 /H2 /Hz
J1 2 þJ1 2 /Hz
P/deg
In D2O
12.0
16.0
136
0.8
a
Approximate mol fraction of S-type conformer (Xsꢂ0.05). Xs¼(25.0ꢁS30)/
(25.0ꢁ7.5):23 S30¼J2 3 þJ2 3 þJ3 4 ¼11.0 (Hz).
0
0
00
0
0 0
b
See Fig. 2 for conformer definitions.
anomeric effect, and toward S by steric effect of the nucleobase and
the gauche effect of [O30eC30eC40eO40] fragment.27 In dT*, owing
to the acetylenic bond with two sp1-carbons between the
furanoseeC10 and the T*-C5, stereoelectronic effets of dT* are
thought to be significantly different from those of native N-nucle-
osides as well as conventional C-nucleosides having an
O40eC10eC(sp2) anomeric effect with a direct bond between the
furanoseeC10 and an aromatic sp2-carbon of a nucleobase. The
S-type preferability of dT* might result from the minimal contri-
bution of the O40eC10eC(sp1) anomeric effect of the alkynylribose
compared with that of N-nucleosides (O40eC10eN1/9) and con-
ventional C-nucleosides (O40eC10eC(sp2)).
Fig. 2. (A) A pseudorotational wheel for a D-aldofuranose ring. (B) Definition of en-
docyclic torsion angles f0ef4. (CeG) Representative schematic projections of a fura-
nose ring in this study, looking toward the oxygen atom from the center of the C20eC30
bond.
tanP ¼ fðf0
þ
f2Þ ꢁ ðf1
þ
f4Þg=0:377f3
(1)
Fm
¼
f
3=cosP
(2)
X-ray structural analysis that directly provides
f0ef4 was per-
formed for dT* (Table 1). Fig. 3 displays the two different structures
for dT* in the unit cell. One has the C20-endo/C20-endo-C10-exo
(2E/2T1) conformer (P¼154ꢃ, Fm¼28ꢃ, Fig. 3A) and the other shows
the O-endo (OE) conformation (P¼91ꢃ, Fig. 3B). On the other hand,
isochronous H20/H200 signals ‘deceptively simple’ splitting patterns
were observed from 1H NMR measurement for dT*: a triplet for the
H10 resonance and a quartet for H20/H200 (Fig.1A, partially expanded
charts). In this case, P is calculated through the first-order distance
between the outer peaks of the isochronous H20/H200 quartet
P ¼ 162 ꢁ 13:58ðJ2030 þ J20030 ꢁ 6:1Þ
(3)
(4)
2DH20=H200 ¼ J1020 þ J10200 þ J2030 þ J20030
For T*2mer, sugar-ring conformational analysis was performed
based on PSEUROT calculation.23e26 In solution, furanose rings
within native DNAs are under rapid dynamic SN equilibrium
(Fig. 1C). The PSEUROT analysis provides the furanose

9048
J. Chiba et al. / Tetrahedron 68 (2012) 9045e9049
Table 3
associated with the physical properties of the artificial duplexes
that show almost the same thermal stabilities with natural DNA
duplexes.
Evaluation of furanose ring conformation for T*2mer in D2O
S10/Hz
S20/Hz
S200/Hz
S30/Hz
Xsa
P/deg
Typeb
dT*p-c
-pdT*e
15.5
15.5
15.5
15.5
30.8
ndf
30.8
30.8
22.5
ndf
21.6
21.1
12.3
ndf
12.8
11.5
0.91
0.91
d
120d
E1/OT1
ndf
ndf
4. Experimental section
g
117ꢃ
126ꢃ
117
126
E1/OT1
E1
g
d
4.1. ESI-TOF mass measurements of T*-oligomers
a
Approximate mol fraction of S-type conformer (Xsꢂ0.05). Xs¼(S10ꢁ9.4)/
(15.7e9.4)23
.
ESI-TOF mass spectra were recorded on a JEOL JMS-T100LC mass
spectrometer operating in the positive ion mode with MeOH or H2O
as a solvent (Fig. S5). DMTr, TBDMS-protected T*2mer: calcd for
MNaþ, C54H62N5O14PSiNa: 1086.37; found 1086.33, DMTr, TBDMS-
protected T*3mer: calcd for MNaþ, C69H78N8O21P2SiNa: 1467.44;
found 1467.29, DMTr, TBDMS-protected T*4mer: calcd for MNaþ,
b
c
See Fig. 2 for conformer definitions.
The 50-side furanose ring of T*2mer.
Estimation from the calculated values in Ref. 23. Fm¼28ꢃ and root mean square
d
(rms)¼0.5 Hz.
e
The 30-side furanose ring of T*2mer.
f
Not determined.
g
The values (Fm¼28ꢃ) from PSEUROT (version 5.4) calculation in Ref. 23.
C
84H94N11O28P3SiNa: 1848.52; found 1849.29, DMTr, TBDMS-
protected T*5mer: calcd for MNaþ, C99H110N14O35P4SiNa:
2229.59; found 2230.21, TBDMS-protected T*2mer: calcd for
MNaþ, C33H44N5O12PSiNa: 784.24; found 784.06, TBDMS-protected
T*3mer: calcd for MNaþ, C48H60N8O19P2SiNa: 1165.31; found
conformations with their approximate equilibrium composition by
fitting the experimental data of the three-bond 1He1H coupling
constants (3JHH). Table 3 summarizes the results of the pseudor-
otational analysis in terms of T*2mer. The 50-side alkynylribose in
T*2mer (50-dT*pdT*-30) is referred to as ‘dT*p-’, while the 30-side as
‘-pdT*’ in Table 3. The calculated mol fraction of S-type conformer
(Xs) for the both furanose rings is 0.91ꢂ0.05, indicating the S-
1165.13,
TBDMS-protected
T*4mer:
calcd
for
MNaþ,
C
63H76N11O26P3SiNa: 1546.38; found 1546.34, T*2mer: calcd for
MHþ, C24H28N4O12P: 595.14; found 595.13, T*3mer: calcd for
MNaþ, C36H40N6O19P2: 945.17; found 945.02, T*4mer: calcd for
MNaþ, C48H53N8O26P3Na: 1273.22; found 1273.16, T*5mer: calcd
for MNaþ, C60H66N10O33P4Na: 1601.26; found 1601.12.
conformer preference of alkynylribose in T*2mer. Fortunately, we
could get important JHH vicinal coupling constants including J1 2
3
0
0
,
0
00
0
00
0
0
00
0
0 0
J1 2 , J2 2 , J2 3 , J2 3 , and J3 4 for ‘dT*p-’. These values are organized
4.2. Detailed assignments of 1H NMR spectra for dT* nucleo-
side and T*2mer in D2O
into eqs. 5e8 in order to compare with the calculated values in
Ref. 23 (Table 3).
X
10 ¼ J1020 þ J10200
(5)
(6)
dT* (See Fig. S6, the same annotation with 3). 1H NMR (500 MHz,
D2O, TSP-d4)
d
7.95 (s, 1H, H6), 5.03 (t, J¼8.0 Hz, 1H, H10), 4.41 (ddd,
J¼3.0(H4 ), 3.25, 4.75 Hz, 1H, H30), 3.96 (ddd, J¼3.0(H3 ), 4.5(H5 )
0
0
0
,
X
X
X
6.0(H5 ) Hz, 1H, H40), 3.71 (dd, J¼4.5(H4 ), 12.0(geminal) Hz, 1H, H50),
20 ¼ J1020 þ J2030 þ J20200
00
0
3.66 (dd, J¼6.0(H4 ), 12.0(geminal) Hz, 1H, H500), 3.40 (s, 3H, N-Me),
0
2.32e2.30 (dd, J¼4.3, 7.8 Hz, 2H, H20 and H200) ppm.
T*2mer (see Fig. 3B). 1H NMR (500 MHz, D2O, TSP-d4)
d 8.00 (s,
200 ¼ J10200 þ J20030 þ J20200
(7)
1H, H6), 7.94 (s, 1H, H60), 5.05 (dd, J¼6.5(Hi), 9.0(Hh) Hz, 1H, Hg), 4.99
(dd, J¼6.0(Hc), 9.5(Hb) Hz, 1H, Ha), 4.73 (septet [ddd], J¼2.5(Hc and He)
,
7.3(Hb) Hz, 1H, Hd), 4.53 (fivlet [ddd], J¼2.5(Hk), 5.0 Hz, 1H, Hj), 4.13
(ddd, J¼2.5(Hd), 4.0(Hf or Hf ), 5.5(Hf or Hf ) Hz, 1H, He), 4.10 (dt [ddd],
0
00
0
00
30 ¼ J2030 þ J20030 þ J3040
(8)
0
00
J¼2.5(Hj), 6.5(Hl) Hz, 1H, Hk), 3.99e3.97 (m, 2H, Hl and Hl ), 3.75 (dd,
0
00
J¼4.0(He), 12.3(geminal) Hz, 001H, Hf or Hf ), 3.70 (dd, J¼5.5(He)
,
In the case of ‘dT*p-’, experimental S10, S20, S200, and S30 (eqs.
5e8) were in good agreement with the calculated values having P
between 117ꢃ and 126ꢃ. The best-fitted P is 120ꢃ (Xs¼0.91, Fm¼28ꢃ,
and root mean square (rms)¼0.5 Hz), indicating that the sugar
conformation is biased toward S (ca. 91%) with C10-exo/O-endo-C10-
exo (E1/OT1) conformer. All of the P values obtained from dT* and
T*2mer show that 10-alkynyldeoxyribose prefers around C10-exo
conformation within the S-type category. C10-exo conformer
(108ꢃ<P<144ꢃ) can easily transform to C20-endo, B-DNA-favoring
structure (144ꢃ<P<180ꢃ) with relatively low energy barriers
compared to furanoses in the N-type family. This S-type similarity
of the sugar-puckering mode may result in the closeness of higher-
order structures and physical properties between the artificial DNA
and native B-DNA as described in our previous report.17
0
12.3(geminal) Hz, 1H, Hf or Hf ), 3.40 (s, 3H, N-Me), 3.36 (s, 3H, N-Me),
2.60 (ddd, J¼2.5(Hd), 6.0(Ha), 14.0(geminal) Hz, 1H, Hc), 2.36e2.29 (m,
3H, Hb and Hh and Hi) ppm.
4.3. X-ray analysis of dT*
A single crystal of nucleoside dT* suitable for X-ray analysis was
obtained from H2O with vapor diffusion of EtOH. Measurements
were made on a Rigaku RAXIS RAPID imaging plate diffractometer
with graphite monochromated Cu-Ka radiation at 40 kV and 30 mA.
A colorless chip crystal of C12H16N2O6 (]C12H14N2O5$H2O) is
ꢀ
monoclinic, space group P21(#4), with a¼10.3322(4) A,
3
ꢀ
ꢀ
ꢀ
b¼5.3618(2) A, and c¼23.9597(9) A, V¼1311.3(1) A , Z¼4 with
calculated density 1.440 g/cm3. The data were collected at room
temperature to a maximum 2q
value of 136.4ꢃ. Of the 11,791 re-
3. Conclusion
flections that were collected, 4497 were unique (Rint¼0.086);
equivalent reflections were merged. The linear absorption co-
In summary, we demonstrated a large-scale synthesis of the
alkynyl C-nucleotide oligomers and the detailed analysis about the
puckering conformations of the alkynylribose rings. Not only from
1H NMR studies in aqueous media but also from X-ray crystal
analysis, the sugar rings of the T* monomer and the dimer are
substantially biased toward the S-type conformer that exists in
native B-DNAs. This structural resemblance would be strongly
efficient, m, for Cu-Ka
radiation is 9.972 cmꢁ1. An empirical ab-
sorption correction was applied, which resulted in transmission
factors ranging from 0.726 to 0.980. The data were corrected for
Lorentz and polarization effects. The structure was solved by direct
method (SHELX 9728) and refined by full-matrix least-squares cal-
culations. The non-hydrogen atoms were refined anisotropically.
Four hydrogen atoms of the included water molecules were

J. Chiba et al. / Tetrahedron 68 (2012) 9045e9049
3. Nielsen, P. E. Chem. Biodivers. 2010, 7, 786e804.
9049
determined from the difference Fourier maps and were refined
isotropically. The remaining hydrogen atoms were located on the
calculated positions and refined using the riding model. The ab-
solute structure was deduced based on Flack parameter. The final
cycle of the refinement was based on the all observed reflections
and 406 variable parameters and converged. The final R1 value
(I>2s(I)), the final Rw value, and GOF were 0.0595, 0.0876, and
0.914, respectively. All calculations were performed using the
CrystalStructure crystallographic software package.29
4. Eschenmoser, A. Orig. Life Evol. Biosph. 2004, 34, 277e306.
5. Schoning, K.-U.; Scholz, P.; Guntha, S.; Wu, X.; Krishnamurthy, R.; Eschenmoser,
A. Science 2004, 290, 1347e1351.
6. Doessing, H.; Vester, B. Molecules 2011, 16, 4511e4526.
7. Veedu, R. N.; Wengel, J. Chem. Biodivers. 2010, 7, 536e542.
8. Singh, J.; Kaur, H.; Kaushik, A.; Peer, S. Int. J. Pharm. 2011, 7, 294e315.
9. Engelhart, A. E.; Hud, N. V. Cold Spring Harb. Perspect. Biol. 2010, 2, a002196.
10. Amin, R.; Kim, S.; Park, S. H. NANO 2009, 4, 119e139.
11. Simmel, F. C. Nanomedicine 2007, 2, 817e830.
€
12. Jortner, J. Phil. Trans. R. Soc. B 2006, 361, 1877e1891.
13. Prakash, T. P. Chem. Biodivers. 2011, 8, 1616e1641.
14. Chiba, J.; Inouye, M. Chem. Biodivers. 2010, 7, 259e282.
ꢁ
ꢂ
ꢁ
ꢂ
Acknowledgements
15. Stambasky, J.; Hocek, M.; Kocovsky, P. Chem. Rev. 2009, 109, 6729e6764.
16. Egli, M.; Pallan, P. S. Annu. Rev. Biophys. Biomol. Struct. 2007, 36, 281e305.
17. Doi, Y.; Chiba, J.; Morikawa, T.; Inouye, M. J. Am. Chem. Soc. 2008, 130,
8762e8768.
18. Crockett, G. C. Aldrichimica Acta 1983, 16, 47e55.
19. Agarwal, K. L.; Yamazaki, A.; Cashion, P. J.; Khorana, H. G. Angew. Chem., Int. Ed.
Engl. 1972, 6, 451e550.
This study was performed through Special Coordination Funds
for Promoting Science and Technology of the Ministry of Education,
Culture, Sports Science and Technology, the Japanese Government.
20. Gorenstein, D. G. Chem. Rev. 1994, 94, 1315e1338.
Supplementary data
21. Wood, D. J.; Hruska, F. E.; Ogilvie, K. K. Can. J. Chem. 1974, 52, 3353e3366.
22. Wood, D. J.; Ogilvie, K. K.; Hruska, F. E. Can. J. Chem. 1975, 53, 2781e2790.
23. van Wijk, J.; Huckriede, B. D.; Ippel, J. H.; Altona, C. Methods Enzymol. 1992, 211,
286e306.
24. de Leeuw, F. A. A. M.; Altona, C. J. Chem. Soc. Perkin Trans. II 1982, 375e384.
25. Altona, C.; Sundaralingam, M. J. Am. Chem. Soc. 1973, 95, 2333e2344.
26. Altona, C.; Sundaralingam, M. J. Am. Chem. Soc. 1972, 94, 8205e8212.
27. Thibaudeau, C.; Acharya, P.; Chattopadhyaya, J. Stereoelectronic Effects in
Nucleosides and Nucleotides and Their Structural Implications; Uppsala
University: Sweden, 2005.
Synthetic procedures; ESI-TOF mass spectra; HPLC Charts; 1H,
13C, 31P NMR spectra; 1He1H COSY spectra; crystallographic data in
CIF. Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.tet.2012.08.057.
References and notes
28. Sheldrick, G. M. Program of the Solution of Crystal Structures; University of
€
Gottingen: Germany, 1997.
1. Sforza, S.; Corradini, R.; Tedeschi, T.; Marchelli, R. Chem. Soc. Rev. 2011, 40,
221e232.
2. Singh, R. P.; Oh, B.-K.; Choi, J.-W. Bioelectrochemistry 2010, 79, 153e161.
29. CrystalStructure 3.8: Crystal Structure Analysis Package; Rigaku and Rigaku/
MSC: The Woodlands, TX, USA, 1998e2007.