Metallocene - DNA: Synthesis, molecular and electronic structure and DNA incorporation of C5-ferrocenylthymidine derivatives

Article abstract of DOI:10.1002/1521-3765(20020703)8:13<2891::AID-CHEM2891>3.0.CO;2-B

Ferrocenylthymidine derivatives have been prepared by Pd-catalysed cross-coupling between ethynylferrocene or vinylferrocene and 5-iodo-2′-deoxyuridine. In the latter case a mixture of trans (2a) and gem (2b) isomers was obtained. The cis-vinylferrocenyl

Full text of DOI:10.1002/1521-3765(20020703)8:13<2891::AID-CHEM2891>3.0.CO;2-B

FULL PAPER
Metallocene ± DNA: Synthesis, Molecular and Electronic Structure and DNA
Incorporation of C5-Ferrocenylthymidine Derivatives
[
a]
[a]
[a]
[b]
Andrew R. Pike, Lyndsey C. Ryder, Benjamin R. Horrocks, William Clegg,
[
b]
[c]
[a]
Mark R. J. Elsegood, Bernard A. Connolly, and Andrew Houlton*
Abstract: Ferrocenylthymidine deriva-
tives have been prepared by Pd-cata-
lysed cross-coupling between ethynyl-
ferrocene or vinylferrocene and 5-iodo-
the nucleobase is essentially co-planar
with the substituted Cp ring of the
metallocene. The selective reduction of
the linkage between the ferrocenyl and
thymidine moieties, from ±CꢀC- to
indicate that the diminished conjugation
reduces the spin transfer onto the bridg-
ing C group, but has less effect on the
2
extent transferred to the nucleobase
from the ferrocenyl group. Compound
1a was incorporated site-specifically
into DNA oligonucleotides by using
automated solid-phase methods. How-
ever, some interconversion of 1a ! 1b
occurs, even under rapid mild conditions
of deprotection.
2
'-deoxyuridine. In the latter case a
mixture of trans (2a) and gem (2b)
isomers was obtained. The cis-vinylfer-
rocenyl (2c), and ethylferrocenyl (3)
derivatives were obtained by catalytic
hydrogenation of ethynylferrocenyl-dT
-CH CH -, causes a shift in the reduction
2
2
potential of À124 mV. DFTcalculations
for the one-electron oxidised species
Keywords: DNA
¥
ferrocene
¥
(
1a), and 2c respectively. Single-crystal
ferrocenyl-nucleosides
¥
redox
X-ray data for 1a, the ferrocenyl-2'fur-
ano-pyrimidone 1b, and 2a show that
chemistry ¥ thymidine
position of an oligonucleotide[5, 1ꢀ, 2±, 29±ꢁ27 or addition of metal
Introduction
ions to ligand-modified oligonucleotides.[
4, 6, 8, 9, ꢁꢁ7
An alter-
Recently there have been increasing efforts to attach metal-
containing groups to sites on DNA strands.[ Of particular
interest are redox-active metal complexes, since these may
native strategy is the synthesis of metal-modified nucleosides
as phosphoramidite monomers for use in automated solid-
phase synthesis.[ This approach has been well developed in
1±±7
ꢁ47
induce oxidative cleavage for use in the design of artificial
organic chemistry but has only rather recently been explored
nucleases,[
1, 8±1ꢀ7
act as electrochemical probes for sensing
for metal-containing groups.
[±, ꢁ5±4ꢀ7
The advantages of this
[11±1ꢁ7
applications,
and are also important in studies on DNA-
approach include the ease of synthesis and the ability to
incorporate the modified base into the oligonucleotide
sequence site-specifically.
mediated electron transfer.[
4, 14±287
The preparation of DNA oligonucleotides bearing metal-
containing groups has typically relied upon either post-
synthetic conjugation of a preformed complex at the terminal
The ferrocenyl group appears to be well suited for the
development of redox-active oligonucleotides given its stabil-
ity, diverse substitution chemistry and amenable stereochem-
istry. There have been some reports of the terminal attach-
ment of ferrocenyl groups to oligonucleotides using simple
[
a7 Dr. A. Houlton, A. R. Pike, L. C. Ryder, Dr. B. R. Horrocks
Department of Chemistry
[41, 427
University of Newcastle upon Tyne
Newcastle upon Tyne, NE1 ±RU (UK)
Fax : (‡44)191-222-6929
derivatives
amidite.
including ferrocenylhexanol as a phosphor-
More recently, Grinstaff and Beilstein have
[127
reported the on-column attachment of ferrocenylpropargyla-
mide to 5-iododeoxyuridine.[ This method involves inter-
rupting normal solid-phase synthesis to perform site-specific
covalent modification on the reactive nucleoside derivative.
An alternative approach is to attach the ferrocenyl group
prior to oligonucleotide synthesis. A range of ferrocenyl-
nucleosides has been described,[ including C5-pyrimidine
and C8-adenosine derivatives. These appear highly suitable
for incorporation into DNA; however, a very recent report
has highlighted possible problems.[ Here we have reinvesti-
E-mail: andrew.houlton@ncl.ac.uk
ꢁ87
[
b7 Prof. W. Clegg, Dr. M. R. J. Elsegood
Crystallography Laboratory
Department of Chemistry
University of Newcastle upon Tyne
Newcastle upon Tyne, NE1 ±RU (UK)
Fax : (‡44)191-222-6929
4ꢁ7
[
c7 Prof. B. A. Connolly
Department of Biochemistry and Human Genetics
University of Newcastle upon Tyne
Newcastle upon Tyne, NE1 ±RU (UK)
ꢁ67
Chem. Eur. J. 2002, 8, No. 1ꢁ
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2891

A. Houlton et al.
FULL PAPER
gated the synthesis of the ferrocenyl C5-modified thymidine
derivatives, and report on the redox chemistry and the
molecular and electronic structure of the compounds. Also
investigated are modified methods for the incorporation of
C5-ethynylferrocenylthymidine site-specifically into DNA
oligonucleotides.
similar results.[4ꢁ7 Mixtures containing both the C5-ethynyl-
ferrocenylthymidine (1a) and ferrocenylfuranopyrimidone
(1b) compounds were obtained, with 8ꢀ% of the latter
formed with 5-iodouracil, and only 5% with either 5-iodour-
idine or 5-iodo-2'-deoxyuridine.
In our studies it was found that replacing Et N as solvent
ꢁ
with a weaker base such as pyridine inhibited the reaction
between ethynylferrocene and 5-iododeoxyuridine complete-
Results and Discussion
ly. Addition of small quantities of Et
N afforded a mixture of
ꢁ
1
a and 1b in yields of 45 and ꢁꢀ% from reactions at elevated
Synthesis, spectroscopy and structure of C5-modified ferro-
cenylthymidine derivatives: Gautheron et al. have previously
described a series of ferrocenyl-nucleosides, including dT,
temperatures. Robins and Barr reported that the addition of
solid disodium EDTA to reactions moderated the formation
of the cyclised furanopyrimid-2-one product in the synthesis
[4ꢁ7
[457
containing C linkages; ethynyl, vinyl and ethyl. Several
of 5-hexenyldeoxyuridine. However, these reaction condi-
2
synthetic strategies were explored; 1) Pd-catalysed cross-
coupling between ethynylferrocene and the appropriate
halonucleoside, 2) the reaction with 5-chloromercuri-
tions only slightly reduced the degree of cyclisation in the case
of ethynylferrocene. Reducing the reaction temperature to
6ꢀ8C decreased the proportion of the 1b to <1ꢀ%. The
formation of 1b can however be completely eliminated by
careful monitoring of the room temperature reaction over a
period of 4 h with typically ±5% yield of 1a obtained. The
formation of the isomer 1b can be identified by the singlet at
(
deoxy)uridine and vinylferrocene and ꢁ) reaction of
[
Cp ZrCl(CHˆCHFc)7 with halonucleosides (Fc ˆ ferrocen-
2
yl). Of the four nucleosides, thymidine is the most convenient
to modify for use in solid-phase DNA synthesis as it does not
require protection of the nucleobase. We have therefore
investigated modified and alternative routes to the series of
1
d ˆ 6.84 ppm in the H NMR spectrum, attributed to the
uncoupled aromatic proton attached to C . There is also a
±
C -bridged ferrocenyl C5-thymidine derivatives, with partic-
significant up-field shift in the H6 resonance (d(CDCl
ꢁ
) ˆ 8.81
2
ular interest in their redox properties and electronic structure.
Scheme 1 summarises the synthetic routes explored.
As a general point it was found that protection of the 5'-OH
of 5-iododeoxyuridine (5-I-dU) with dimethoxytrityl (DMT)
improved the yields due to an enhanced solubility in the
solvent systems used. Such protection is of course also
necessary for the reagent to be used in solid-phase oligonu-
cleotide synthesis.
(1a), ±.95 ppm (1b)). The two isomers are also distinguished
by electronic absorption spectroscopy. The more extended
ring system of 1b gives rise to a red-shifted band in the visible
region (lmax ˆ ꢁ55 nm) as has been reported elsewhere.
[ꢁ67
De-tritylation of 1a and 1b aided in the preparation of
crystals suitable for analysis by X-ray diffraction and these
data confirmed unequivocally the structural assignments
based on spectroscopic data. Both structures contain two
independent molecules in the asymmetric unit; molecular
structures are shown in Figure 1 and Figure 2, respectively. In
each case, the two molecules are related by pseudo-inversion
inversion symmetry if the sugar groups are ignored, but these
sugars have the same chirality for both molecules.
It is known that alkynylthymidine derivatives are suscep-
tible to cyclisation to the corresponding furanopyrimi-
done.[
44±467
This type of rearrangement generally occurs under
rather forcing conditions, as noted in couplings of o-iodoani-
[4±7
lines with copper(i) acetylides and by treatment of (Z)-5-
hexynyl-2'-deoxyuridine with triethylamine under reflux.[ In
reactions with ethynylferrocene as the alkyne, Gautheron
457
In 1a the FeÀC bond lengths range from 2.ꢀꢀ2(8) to
2.ꢀ64(9) ä with an average of 2.ꢀꢁ4 ä. The FeÀC
H perpen-
5 5
et al. used similar conditions (CuI, Et N, reflux) and observed
dicular distances are 1.64ꢁ and 1.656 ä in the two molecules,
ꢁ
Scheme 1. Synthetic route to C-5 ferrocenyl thymidine derivatives.
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Chem. Eur. J. 2002, 8, No. 1ꢁ

Metallocene ± DNA
2891±2899
product which contained both the trans-vinyl (2a) and gem-
vinyl (2b) isomers (see Scheme 1) in an approximate 1:1 ratio
1
(
based on integration of the H NMR spectra). The isomers
are distinguished by the coupling constants of the vinylic
protons (gem ˆ 1.2 Hz and trans ˆ 16.2 Hz). Separation of the
isomers was not possible by TLC and HPLC; however, after
removal of the DMTgroup, the trans isomer 2a was isolated
by fractional crystallisation from a methanol/chloroform
(
1ꢀ:9ꢀ) solvent system. This method also afforded crystals of
suitable quality for X-ray diffraction.
The molecular structure of 2a is shown in Figure ꢁ and
1
confirms the stereochemical assignment based upon H NMR
spectroscopy. The structure contains two independent mole-
cules in the asymmetric unit, and is approximately isomor-
phous with 1a. The CÀFe bond lengths vary between
Figure 1. Structure of one independent molecule of 1a. Selected bond
lengths [ä7 and angles [87: FeÀC (av) 2.ꢀꢁ4, FeÀC
5
H
1.64ꢁ, FeÀC
5 4
H 1.6ꢁ6,
5
C±ÀC8 1.1±5(6), C8ÀC9 1.4ꢁ2(6); CpÀCp 1.48; CpsubÀpyrimidine 6.98.
insignificantly different from the values of 1.6ꢁ6 and 1.64± ä
for the FeÀC H distances. The intramolecular interplanar
5
4
angles between the cyclopentadienyl rings are 1.4 and 2.ꢁ8,
and those between the nucleobase and the substituted Cp ring
are 6.98 and 5.ꢁ8. The two cyclopentadienyl rings within each
molecule are virtually eclipsed (mean torsional angle C-X1-
X2-C is 5.18, where X1 and X2 are the centres of the rings).
The ethynyl unit linking the nucleobase and the ferrocenyl
group deviates only slightly from linearity (C5ÀC±ꢀC8
1
±6.±(5), 1±ꢁ.8(6) and C±ꢀC8ÀC9 1±8.5(6), 1±6.9(6) ä for
the two independent molecules), with a triple bond length of
.1±5(6) and 1.18ꢁ(6) ä and adjacent single bonds: C5ÀC±
1
Figure ꢁ. Structure of one independent molecule of 2a. Selected bond
1
.4ꢁ2(6) and 1.456(6) ä, C8ÀC9 1.4ꢁ2(6) and 1.4ꢀ8(±) ä.
lengths [ä7 and angles [87: FeÀC (av) 2.ꢀ4ꢀ, FeÀ-C
5
H
1.652, FeÀC
5 4
H 1.649,
5
In 1b the FeÀC bond lengths range from 2.ꢀ26(2) to
.ꢀ46(2) ä (av 2.ꢀꢁ6 ä). The FeÀC H perpendicular distan-
C±ÀC8 1.ꢁꢀ(2), C8ÀC9 1.4±(2), CpÀCp ꢀ.6; CpsubÀpyrimidine ꢁ.ꢁ.
2
5
5
ces are 1.64ꢀ and 1.645 ä, compared to 1.6ꢁ± and 1.6ꢁ9 ä for
FeÀC H . The interplanar angles between the cyclopenta-
1.986(19) and 2.1ꢀꢀ(1±) ä with an average of 2.ꢀ4ꢀ ä. The
iron atom lies 1.649 or 1.641 ä above the substituted cyclo-
pentadienyl ring and 1.652 or 1.6ꢁ4 ä below the unsubstituted
ring in the two molecules. The ferrocenyl rings are virtually
co-planar, orientated at an interplanar angle of ꢀ.6 or 2.48, and
are arranged in an almost eclipsed configuration (mean
torsional angle C-X1-X2-C is ꢁ.58, where X1 and X2 are the
centres of the cyclopentadienyl rings). The trans-vinyl linkage
C±ˆC8 has a bond length (1.ꢁꢀ(2) or 1.25(2) ä) consistent
5
4
dienyl rings are ꢀ.6 and 2.ꢁ8, and those between the
nucleobase and the substituted Cp rings are 11.4 and 14.98.
The two cylcopentadienyl rings are virtually eclipsed (mean
torsional angle C-X1-X2-C is 1.58). The furanopyrimidone is
linked directly to the ferrocene through C8ÀC9 (1.446(ꢁ) and
1
.4ꢁ8(ꢁ) ä), and C±ÀC8 is lengthened to a typical CˆC double
bond (1.ꢁ4ꢀ(ꢁ) and 1.ꢁ4ꢀ(ꢁ) ä). As a furanopyrimidone, 1b
has lost the ability to act as a hydrogen-bond donor through
Nꢁ and can no longer base-pair effectively with adenine.
The reaction of vinylferrocene with 5'-DMT-5-iodo-2'-
deoxyuridine, using [PdCl (PPh ) 7 as catalyst, gave a crude
with a carbon-carbon double bond and is connected to the
ferrocene unit through C8ÀC9 (1.4±(2) or 1.44(2) ä). Figure ꢁ
clearly shows the virtually planar trans-vinyl link between the
thymine ring and the ferrocene
2
ꢁ 2
cyclopentadienyl ring. The inter-
planar angle between the nucleo-
base and the substituted Cp ring is
ꢁ
.ꢁ8 or 5.88. The dihedral angle
between the vinylic bond and the
pyrimidine ring is 6.ꢁ or 6.48, and
between the adjoining cyclopen-
tadienyl ring and the vinylic bond
it is ꢁ.2 or 11.ꢁ8. This geometric
alignment looks promising as a
rigid and planar delocalised elec-
tronic system between the nucleo-
base and the redox group.
Figure 2. Structure of one independent molecule of 1b. Selected bond lengths [ä7 and angles [87: Fe
ÀC (av)
2
.ꢀꢁ6, FeÀC 1.64ꢀ, FeÀC 1.6ꢁ±, C±ÀC8 1.ꢁ4ꢀ(ꢁ), C8ÀC9 1.446(ꢁ), CpÀCp ꢀ.48, CpsubÀpyrimidine 11.68.
5
H
5
5 4
H
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A. Houlton et al.
FULL PAPER
o
The trans isomer 2a has been synthesised previously by two
different routes. The reaction of vinylferrocene with C5-
HgCl-dU in methanol using Li [PdCl 7 as a catalyst followed
Table 1. Electrode potentials, E , of ferrocene ± ferrocenium couple of
ferrocenylthymidine and related derivatives (from CV measurements in
, Ag quasi-reference electrode, tungsten counter
electrode, gold disc working electrode).
‡
CH
ꢁ
CN/ꢀ.1m TBAPF
6
2
4
by treatment with either H S or NaBH led to a mixture of
2
4
o
Compound
E [mV7
three products, including the trans-vinylferrocenyl deriva-
[
4ꢁ7
ferrocene
ꢀ
‡ 226
‡ 14ꢀ
‡ 1ꢁ1
‡ 56
tive. This compound was also isolated in about 15 ± 2ꢀ%
FcCꢀCH
yield by the reaction of [Cp ZrCl(CHˆCHFc)7 with 5-iodo-
2
1
1
a
b
deoxyuridine. To our knowledge, the gem and cis isomers of
C5-vinylferrocenylthymidine have not previously been re-
ported. The latter compound was prepared as a single isomer
FcCˆCH
2
2a
‡ 2±
2
2
b
c
‡ 21
in ꢁꢀ% yield by catalytic reduction of 1a using NiCl /
2
‡ 29
[487
NaBH . Distinguishing features of the cis-isomer, 2c, are
4
FcCH
2
CH
ꢁ
À ꢁ2
1
the H NMR vinylic coupling constants (J ˆ 11Hz). The
3
‡ 16
electronic absorption spectrum for each C5-vinylferrocenyl-
thymidine isomer is characteristic (Figure 4). The more
extended conjugation of the trans isomer gives rise to a red-
shifted band compared to the cis form (298 nm c.f. 2±5 nm).
All three isomers exhibit the weak band at about 45ꢀ nm
attributed to (d ± d) transitions.
The interest in DNA-mediated electron transfer[4, 14±21, 497
has focused attention on methods for incorporating site-
specific redox-active centres.[
±, 2±, ꢁ6, ꢁ8±4ꢀ7
While nucleobase
modification has been used in such studies, very few contain
a conjugated pathway with the redox centre.[ It is therefore
worthwhile to consider the extent of interaction between the
ferrocenyl group and the nucleobase for the oxidised para-
magnetic ferrocenium species. Density functional calculations
(DFT) for 1-methyl analogues of 1 ± 3 and their oxidised
counterparts provide some insights.
Calculations for the parent ferrocenyl derivatives show a
qualitative correlation between the measured redox poten-
tials and DE for the neutral and oxidised counterparts
ꢁ67
The C5-ethylferrocenylthymidine 3 was prepared from 2c
by using an excess of NiCl /NaBH mixture. The reactions
2
4
were performed at low temperature (Tˆ À±88C) and moni-
tored by TLC. Yields were typically rather low (~25%) and
there were some problems with reproducibility. The previous
report on the synthesis of 3 from the trans-vinyl compound 2a
employed hydrogenation on Pd/C but did not record a
yield.[
4ꢁ7
Redox behavior and electronic structure calculations: Cyclic
voltammetry was used to probe the redox properties of the
compounds and in particular to assess the effect of the
(DE(a.u.) ˆ EFc À EFc‡
:
EtFc ˆ Àꢀ.245; VinylFc ˆ Àꢀ.246;
EthynylFc ˆ Àꢀ.25ꢁ a.u.). This suggests that solvent interac-
tions are consistent for all these compounds upon oxidation.
For the 1-methyl analogues of 1 ±3 the ethynyl derivative
has the largest DE value (Àꢀ.242 a.u.) however, the vinyl
derivatives have a smaller value than that calculated for the
ethyl compound 3. This indicates that for these compounds
the correlation between the gas-phase calculations and the
solution-phase redox potentials is not good. This may well
reflect solvation effects on the nucleobase.
The calculated spin density
different C linkages, -CꢀC-, -CˆC-, -CÀC-, on the ferrocenyl
2
oxidation. Table 1 summarises the formal electrode potentials
of the derivatives measured in MeCN; values for the parent
ferrocenyl derivatives are also included. All compounds
exhibit reversible one-electron waves with formal potentials,
o
E , lying in the Àꢁ2 to ‡226 mV range versus ferrocenium ±
ferrocene.
serves extremely well to illus-
trate the extent of interaction
between the ferrocene moiety
and the thymine. The spin den-
sity distributions of 1-methyl
analogues of the prepared com-
pounds are shown in Figure 5.
Table 2 contains the major
atomic contributions based on
an NAPC (natural atomic pop-
ulations and charges) analysis
as a quantitative guide. As can
be seen from the spin density
figures there is considerable
delocalisation to the nucleo-
base for all types of C linkage.
2
A simplistic analysis based on a
representation of the molecules
as comprising of three compo-
nents; ferrocene-bridge-nucleo-
Figure 4. UV/Vis spectra of vinyl isomers of ferrocenyl-dTin acetonitrile.
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Chem. Eur. J. 2002, 8, No. 1ꢁ

Metallocene ± DNA
2891±2899
4
± 9% for the ferrocene, C bridge
2
and thymine, respectively. These
non-planar configurations result in
a reduction in conjugation and
hence loss of spin density trans-
ferred to the thymidine unit.
The 1-methyl analogue for 1b is
quite different as might be expect-
ed. The extended aromatic system
to which the ferrocene is attached
serves to transfer spin density ef-
fectively from the metallo group.
The NAPC analysis shows about
ꢁ
1% transferred across the furano-
pyrimidone ring system.
The electrostatic potential atom-
ic charges derived from these cal-
culations also explain the suscepti-
bility of 1a to cylcisation. Com-
pared to the results for the model
C5-phenylacetylene-dT, C is sig-
8
nificantly more positive in 1a
(
Àꢀ.1ꢁ4 c.f. Àꢀ.211 eV) accounting
for the observed susceptibility to
nucleophilic attack which effects
ring closure.
Site-specific incorporation of 1a in
DNA: Nucleoside derivatives that
are highly conjugated to a redox
centre have not been widely con-
Figure 5. DFT-calculated spin density distribution on 1a, 1b, 2a ± c, and 3.
sidered for studies on DNA-medi-
ated charge transport. Compound
1
a is therefore of particular interest as the alkynyl linkage
Table 2. NAPC analysis percentages of spin density on the ferrocenyl-C
thymine components.
2
-
provides an efficient means for delocalisation between the
redox group and the nucleobase.
Derivative
Ferrocene[a7
C2 bridge[b7
Thymine[c7
Yu et al. have very recently reported on attempts to
incorporate 1a into synthetic oligonucleotides using solid-
phase phosphoramidite methods.[ They found that during
the course of synthesis, using standard reagents and protocols,
1a ! 1b interconversion occurred. Interestingly, these work-
ers noted that in hybridization studies 1b forms a more stable
interaction with dG compared to dA. Figure 6 shows sche-
matically the two corresponding base pairs. The observed
1
1
2
2
2
3
a
b
a
b
c
Fe: ±ꢀ.4 (±8.ꢀ)
Fe: 61.ꢁ (68.5)
Fe: 69.4 (±±.ꢁ)
Fe: 85.5 (92.9)
Fe: 6ꢁ.4 (±2.2)
Fe: ±±.5 (8ꢁ.2)
C±: ꢁ.8, C8: 5.ꢁ
C±: 5.4, C8: 5.2
C±: 4.ꢀ, C8: 6.2
C±: ꢀ.6, C8: 2.8
C±: 5.4, C8: ±.ꢀ
C±: ꢀ.6, C8: ꢀ.8
12.9
2ꢀ.9
12.5
ꢁ.±
15.4
15.1
ꢁ67
[
[
a7 Percentage on iron atom and total ferrocene unit in brackets.
b7 Percentages on C± and C8. [c7 Total percentage on thymine ring.
increase in T values for the duplex containing the 1b:dG pair
m
can be easily rationalised, though clearly the pairing is not
optimised.[ In light of these findings we have explored
alternative procedures in an effort to retain the structure of 1a
in synthetic oligonucleotides. In particular, given the known
susceptibility to base-catalysed cyclisation, prime concern was
given to the deprotection step. This is typically treatment with
ꢁ67
base, reveals that between ±2 ± 9ꢁ% of the spin density is
located at the ferrocenyl group and between about 4 ± 15% is
transferred to the thymine. This is predominantly located at
C5, C6, and the heteroatoms N1, O4 and O2. The conjugated
bridging groups, CꢀC and CˆC, are affected similarly, with 9 ±
12%, while very little spin density resides in the non-
aqueous NH for 12 h at room temperature, or 5 h at 558C. As
ꢁ
conjugated gem-bridge (ꢁ.4%) and the saturated CÀC bridge
an alternative, we have used ULTRAMILD base phosphor-
amidites (Glen Research, Va, USA) for all syntheses. These
amidites are de-protected in a few hours in ꢀ.ꢀ5m methanolic
(
<2%).
The extent of ferrocenyl ± nucleobase interaction is con-
[5ꢀ7
formationally dependent as demonstrated for models of 1a.
Calculations of two extreme conformers of 1a, where the
thymine ring was rotated Æ9ꢀ8 to the substituted cyclo-
pentadienyl ring gave spin density values of 84 ± 89%, ±% and
K CO or several minutes with anhydrous methylamine.
2 ꢁ
Compound 1a was converted to the corresponding cyano-
ethylphosphoramidite for incorporation into oligonucleotides
by reaction with 2-cyanoethoxy-N,N-diisopropylaminochloro-
Chem. Eur. J. 2002, 8, No. 1ꢁ
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2ꢀꢀ2
ꢀ94±-65ꢁ9/ꢀ2/ꢀ81ꢁ-2895 $ 2ꢀ.ꢀꢀ+.5ꢀ/ꢀ
2895

A. Houlton et al.
FULL PAPER
Analysis of the 2ꢀ-mer oligo-
nucleotide after digestion with
snake venom phosphodiester-
ase and alkaline phosphatase
indicated in addition to incor-
poration of 1a, the formation of
1
b. The presence of the two
isomeric ferrocenyl derivatives
was not observed in the HPLC
of the 2ꢀ-mer due to the small
difference in the retention
times. Figure 8 shows the HPLC
Figure 6. Base-pairing for 1b with dG (left) and dA (right). The former is preferred after Yu et al.[ꢁ67
phosphoramidite as phosphitylating
agent. This was preferred to
bis-(N,N-diisopropylamino)-2-cyano-
ethoxyphosphine which gave poorer
yields.
The
2ꢀ-mer
oligonucleotide,
TATCGTATCGT ATCGTATCG,
Fc
(
ꢀ
where T ˆ 1a) was prepared on
Fc
.2 mmol CPG columns using stand-
ard protocols with the exception of a
prolonged coupling reaction time
(
ꢁ min) for the 1a phosphoramidite.
Deprotection with ꢀ.ꢀ5m methanolic
K CO was monitored by HPLC and
2
ꢁ
it was found that after five hours the
oligonucleotide is fully deprotected.
Similar data was obtained after de-
protection with anhydrous MeNH2
(
2
2ꢀ min). The purified ferrocenyl-
ꢀmer was characterised by
MALDI-TOF mass spectrometry,
calcd: [M(ÀCp)7 622±.ꢀ4; found:
Figure ±. Differential pulse voltammogram of TATCGTATCGTFcATCGTATCG, E8 ˆ 286 mV (Pulse
À1
‡
height 25 mV, scan rate 2ꢀ mVs , Ag quasi-reference electrode, tungsten counter electrode, gold disc
working electrode, ꢀ.1m TEAA buffer pH 6.5).
(
6
222.4(Æ5)) and illustrates success-
ful incorporation of ferrocenylthymidine into an oligonucleo-
tide.
of the digested 2ꢀ-mer. Approximately 5ꢀ% of the modified
nucleoside remains in the ethynylferrocenyl form. This
compares with complete conversion to the furanothymidone
form in the work of Yu et al.[ However, despite the ability to
retain the ethynylferrocenyl dTmoiety intact, the mixture
could not be separated.
Re-analysis of the purified 2ꢀ-mer by HPLC revealed a new
peak with a retention time of 16.5 min in addition to the
original peak at 21 min. The two species are attributed to the
ferrocenyl group in different oxidation states. The latter
species can be converted to the former species by addition of
the reducing agent dithiothreitol (DTT) to a solution of the
ꢁ67
2ꢀ-mer. The action of DTT on a sample of the oligonucleotide
was monitored by HPLC. Treatment showed a decrease in the
latter peak concomitant with an increase in the earlier peak.
The difference in retention times between the two forms is
attributed to the change in overall charge upon oxidation/
reduction of the iron centre. As expected the less negatively
charged form bearing the ferrocenium cation has a longer
retention time on the reverse-phase column. Differential
pulse voltammetry confirmed that collected fractions of both
peaks were electrochemically active, with a redox potential of
Conclusion
The conjugated ferrocenyl-nucleoside 1a can be incorporated
into synthetic oligonucleotides to give redox-active DNA
strands. However, even with the use of rapid mild methods for
de-protection of the oligomers, a considerable fraction of 1a
(ca. 5ꢀ%) still undergoes cyclisation to 1b. The difficulties in
separating the resulting mixture are non-trivial. DFTcalcu-
lations show however that delocalisation between the ferro-
cenyl group and the thymidine occurs for the series of C2-
bridged nucleosides. This indicates that a range of derivatives
may be used to prepare site specific redox-active ferrocenyl
oligonucleotides in which the electronic structure of the
nucleobase is significantly perturbed by redox reactions at the
ferrocenyl group.
286 mV (Figure ±).
In view of these observations and the known susceptibility
of the oxidised ferrocenyl species, ferrocenium, to nucleo-
philic attack, a precautionary post-synthetic reduction step,
using a solution of DTT in acetonitrile, was employed before
de-protection and removal from the solid support.
2896
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ꢀ94±-65ꢁ9/ꢀ2/ꢀ81ꢁ-2896 $ 2ꢀ.ꢀꢀ+.5ꢀ/ꢀ
Chem. Eur. J. 2002, 8, No. 1ꢁ

Metallocene ± DNA
2891±2899
2
H: H-5', H-5∫), ꢁ.±1 (s, 6H: OMe), 4.ꢀ±
(
2
t, 1H: H-ꢁ'), 4.1ꢀ (m, ±H: Fc), 4.19 (m,
H: Fc), 4.49 (m, 1H: H-4'), 6.ꢁꢀ (t, 1H:
H-1'), 6.8ꢀ (m, 4H: DMT), ±.2± ± ±.44 (m,
‡
9
±
H: DMT), ±.95 (s. 1H: H-6); MS: FAB :
ꢁ±.9 for C42
ꢁ8 2 ±
H N O Fe.
The DMT group was removed by treat-
ment with a 2% CCl COOH solution in
CH Cl to give 1a, which was crystallised
ꢁ
2
2
from chloroform/methanol (9ꢀ:1ꢀ).
Ferrocenyl-2'-furanopyrimidone (1b): A
sample of 1b was obtained from the
following reaction, which also yielded
1
1
a.
.4 mmol) was dissolved in deoxygenated
acetonitrile (5ꢀ mL). Dry triethylamine
5 mL), ethynylferrocene (55ꢀ mg,
.6 mmol), copper(i) iodide (5ꢀ mg) and
PdCl (PPh 7 (5ꢀ mg) were added and
5-Iodo-2'-deoxyuridine
(5ꢀꢀ mg,
(
2
[
2
ꢁ 2
)
the reaction mixture stirred under nitro-
gen at 658C for 5 h. The reaction mixture
was dried by rotary evaporation to a dark
foam and then redissolved in chloroform
Figure 8. HPLC of digested TATCGTATCGTFcATCGTATCG.
and washed twice with 5% (v/w) disodi-
Experimental Section
um ETDA and then once with water
before drying over sodium sulfate and concentration to a dark oil. The
crude mixture contained both isomers in a ratio of 2:1 (1a/1b). These were
Materials: Reagents were purchased from Aldrich and Lancaster and used
as received unless otherwise stated. Solvents were dried and distilled under
ꢁ
separated by column chromatography on silica using a CHCl :MeOH
gradient ꢀ ± 1ꢀ% methanol. The cyclised 1b eluted from the column first
nitrogen prior to use. All reactions were performed under N
2
using
1
1
and was obtained in ꢁ5% yield. H NMR (2ꢀꢀ MHz, CDCl
ꢁ
, 258C): d ˆ
standard Schlenk techniques. H NMR spectra were performed on a
ꢀꢀ MHz Bruker Spectrospin AC 2ꢀꢀE spectrometer and ꢁ1P NMR spectra
2.16 ± 2.52 (m, 2H; H-2', H-2™), ꢁ.±6 (m, 2H; H-5', H-5∫), 4.ꢁ± (s, 5H; Fc),
2
4
(
.5± (d, 2H; Fc), 4.94 (d, 2H; Fc), 5.2± (t, 1H; H-ꢁ'), 5.4ꢀ (d, 1H; H-4'), 6.29
on a ꢁꢀꢀ MHz Bruker Spectrospin WM ꢁꢀꢀ WB spectrometer. UV/Vis
electronic spectra were recorded on a Shimadzu UV-21ꢀ1PC Scanning
Spectrophotometer. Mass spectra of the ferrocenyl derivatives were
obtained at the MS Service Centre, University of Wales, Swansea, whilst
the MALDI-TOF spectrum of the modified 2ꢀ-mer was measured by
Nathan Harris at Moredun Research Institute.
‡
t, 1H; H-1'), 6.84 (s, 1H; H-±), 8.81 (s, 1H; H-6); MS: FAB : 4ꢁ5.9 for
21 2ꢀ 2 5
C H N O Fe.
5
5
'-DMT-5-trans-vinylferrocene-2'-deoxythymidine, 2a-DMT and 5'-DMT-
-gem-vinylferrocene-2'-deoxythymidine, 2b-DMT: 5-Iodo-2'-deoxyuri-
dine (ꢀ.ꢁꢁ g, ꢀ.5 mmol) was dissolved in dry acetonitrile (2ꢀ mL). Triethyl-
amine (2 mL), vinylferrocene (1ꢀꢀ mg, ꢀ.4± mmol) and [PdCl (PPh
5ꢀ mg, ꢀ.ꢀ± mmol) were added sequentially. The mixture was stirred and
2
ꢁ 2
) 7
Preparation of 5'-Dimethoxytrityl-5-iodo-2'-deoxyuridine: 5-Iodo-2'-deoxy-
uridine (2.ꢀ g, 5.6 mmol) was dissolved in dry pyridine and then dimethy-
laminopyridine (DMAP) (±ꢀ mg) and dimethoxytrityl chloride (2.ꢁ g,
(
heated at ꢁ58C for 9ꢀ h followed by a further 9ꢀ h at reflux. After cooling
and evaporation to a sticky oil the crude product was dissolved in
chloroform and washed twice with disodium EDTA 5% (v/w) and once
with water before being dried with sodium sulfate. After filtration and
concentration by rotary evaporation the reaction mixture was purified by
column chromatography using chloroform ± methanol (9ꢀ:1ꢀ) as eluent.
The appropriate fractions were combined to yield a mixture of the title
products as an orange powder (ꢀ.21 g, 6ꢀ%). This was found to be a
mixture of the gem and trans isomers. Attempts to separate the mixture by
TLC or HPLC were unsuccessful. Treatment of the mixture with a 2%
6
.±± mmol) were added. The mixture was stirred at ambient temperature
under nitrogen for 16 h. Then 5% (v/w) sodium bicarbonate (5 mL) was
added before the reaction mixture was evaporated to dryness. Final traces
of pyridine were removed by repeated co-evaporation with toluene. The
reaction mixture was then dissolved in chloroform, washed twice with 5%
v/w) sodium bicarbonate and then dried over magnesium sulfate. The
crude product was dissolved in chloroform ± methanol (98:2) and loaded
onto a silica column packed in chloroform–methanol ± triethylamine
9±:2:1). The product was eluted by using a gradient of 2 ± 1ꢀ% methanol in
chloroform and the appropriate fractions collected and solvent removed by
rotary evaporation to give a white crystalline powder (2.96 g, 8ꢀ%).
(
(
ꢁ 2 2
CCl COOH solution in CH Cl afforded the de-protected nucleosides.
Repeated recrystallisation of the mixture gave sample 2a in 45% yield.
Crystals of 2a suitable for single-crystal X-ray analysis were obtained by
slow evaporation of a saturated solution of 2a in chloroform ± methanol
1
H NMR (2ꢀꢀ MHz, CDCl
.ꢁ6 (m, 2H: H-5', H-5∫), ꢁ.±6 (s, 6H: OMe), 4.ꢀ9 (m, 1H: H-4'), 6.ꢁ1 (t,
ꢁ
, 258C): d ˆ 2.2ꢀ ± 2.54 (m, 2H: H-2', H-2™),
ꢁ
1
(
95:5).
H: H-1'), 6.82 (d, 4H: DMT), ±.22 ± ±.41(m, 9H: DMT), 8.1ꢁ (s, 1H: H-6);
1
‡
2a: H NMR (2ꢀꢀ MHz, CD OD, 258C): d ˆ 2.49 (t, 2H; H-2'), 4.ꢀ2 (m,
MS: FAB : 655.8 for Cꢁꢀ
H
29
N
2
O
±
I.
ꢁ
2
2
1
H; H-5'), 4.14 (m, 1H; H-ꢁ'), 4.ꢁꢀ (s, 5H; Fc), 4.44 (m, 2H; Fc), 4.62 (m,
5
'-Dimethoxytrityl-5-ethynylferrocene-2'-deoxyuridine, 1a-DMT: 5'-Di-
methoxytrityl-5-iodo-2'-deoxyuridine (1.5 g, 2.29 mmol) was dissolved in
dry acetonitrile. Pyridine (5 mL), triethylamine (2 mL), ethynylferrocene
5ꢁꢀ mg, 2.5 mmol), copper(i) iodide (15ꢀ mg, ꢀ.±9 mmol) and bis(triphen-
ylphosphine)palladium(ii) chloride (15ꢀ mg, ꢀ.21 mmol) were added se-
quentially under nitrogen. The reaction mixture was stirred at ꢁ58C for 5 h.
Disodium EDTA 5% (v/w) (5 mL) was added to the resulting suspension
before evaporation to dryness. The crude product was redissolved in
chloroform (1ꢀꢀ mL) and washed twice with disodium EDTA 5% (v/w)
and once with water before being dried over sodium sulfate. After filtration
and concentration by rotary evaporation the reaction mixture was loaded
onto a silica gel column packed in chloroform ± methanol ± triethylamine
ꢁ
H; Fc), 4.64 (m, 1H; H-4'), 6.51 (t, 1H; H-1'), 6.64 (d, J(H,H) ˆ 16.2 Hz,
ꢁ
H; trans-H), ±.ꢁ6 (d, J(H,H) ˆ 16.2 Hz, 1H; trans-H), 8.ꢁ± (s, 1H; H-6);
UV/Vis (CH
FAB : ±ꢁ9.9 for C42
ꢁ
CN):lmax (e) ˆ 298 (±66ꢀ), ꢁ25 (5ꢀꢁ9), 449 nm (4ꢀ8); MS:
(
‡
4ꢀ 2 ±
H N O Fe.
1
2
2
2
1
b: H NMR (2ꢀꢀ MHz, CD
ꢁ
OD, 258C): d ˆ 2.49 (t, 2H; H-2'), 4.ꢀ2 (m,
H; H-5'), 4.14 (m, 1H; H-ꢁ'), 4.ꢁꢀ (s, 5H; Fc), 4.44 (m, 2H; Fc), 4.62 (m,
ꢁ
H; Fc), 4.64 (m, 1H; H-4'), 5.ꢁꢁ (d, J(H,H) ˆ 1.2 Hz, 1H; gem-H), 5.±9 (d,
H; gem-H, (J ˆ 1.24Hz) ), 6.51 (t, 1H; H-1'), 8.ꢁ± (s, 1H; H-6); UV/Vis
(
(CH
ꢁ
CN):lmax (e) ˆ 264 (1ꢀ5ꢁ2), ꢁ25 (459ꢀ), 446 nm (448).
5'-DMT-5-cis-vinylferrocene-2'-deoxythymidine, 2c-DMT: Compound 1a-
DMT (1 equiv) was dissolved in distilled, anhydrous MeOH under nitrogen
with heating. The orange solution was then cooled to ˱88C and a
(
95:4:1) and eluted by using chloroform ± methanol (95:5). Fractions
containing the product were combined and solvent removed to yield the
title compound as a dark orange powder (1.ꢀ9 g, 65%). H NMR
2 4
suspension of NiCl (1 equiv) in MeOH was added followed by NaBH
1
(1.ꢁ equiv). The reaction mixture was stirred under nitrogen for 1.5 h and
then allowed to warm to room temperature. At the point which the mixture
(
2ꢀꢀ MHz, CDCl
ꢁ
, 258C): d ˆ 2.19 ± 2.4 (m, 2H: H-2', H-2™), ꢁ.ꢁ± (m,
Chem. Eur. J. 2002, 8, No. 1ꢁ
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2ꢀꢀ2
ꢀ94±-65ꢁ9/ꢀ2/ꢀ81ꢁ-289± $ 2ꢀ.ꢀꢀ+.5ꢀ/ꢀ
2897

A. Houlton et al.
FULL PAPER
turned black, flash silica was added (equal weight to nucleoside) and
measured at a pulse height of 25 mV, a pulse width of 5ꢀ ms and at a scan
À1
solvent was removed. Crude mixture was purified by chromatography (2%
rate of 2ꢀ mVs
.
1
MeOH in DCM). Yield ꢁꢀ%. H NMR (2ꢀꢀ MHz, CDCl
(
ꢁ
, 258C): d ˆ 1.9
Electronic structure calculations: Density functional calculations (DFT)
were performed using the Titan program package (Wavefunction Inc.,
USA). Geometries were optimised at the Becke-Perdew level of theory
using the LA-CVP* basis set.
m, 1H; H-2'), 2.ꢁ (m, 1H; H-2'), ꢁ.ꢀ (q, 1H; H-5'), ꢁ.2 (q, 1H; H-5'), ꢁ.8
(
6
1
m, 8H; OMe, H-4'), 4.ꢀ (m, ꢁH; Fc, H-ꢁ'), 4.ꢀ5 (s, 5H; Fc), 4.1 (t, 2H; Fc),
.ꢀ (d, ꢁJ(H,H) ˆ 11 Hz, 1H; cis-H), 6.2 (t, 1H; H-1'), 6.ꢁ (d, J(H,H) ˆ
ꢁ
1 Hz, 1H; cis-H), 6.8 (d, 4H; DMT), ±.2 (m, 9H; DMT), ±.4 (s, 1H; H-6);
Oligonucleotide synthesis: An Applied Biosystems ꢁ81A DNA synthesizer
was used for the preparation of oligonucleotides. The base-phosphorami-
dites were ULTRAMILD (Glen Research, VA, USA) and the standard
Cap A was replaced with phenoxyacetic anhydride. Standard coupling
protocols (45 s) were used with the exception of the ferrocenyl-phosphor-
amidite for which the coupling time was increased to ꢁ min. On completion
of the synthesis the column was washed thoroughly with acetonitrile and
then dried with argon. Deprotection of the oligonucleotides involved
UV/Vis (CH
EI: ±4ꢀ.22 for C42
'-DMT-5-ethylferrocene-2'-deoxythymidine (3): A solution of 2c-DMT
1 equiv) in distilled anhydrous methanol under nitrogen was cooled to
˱88C. NiCl (1 equiv) was added as a solution in methanol followed by
NaBH
ꢁ
CN): lmax (e) ˆ 2±5 (1468±), ꢁ1ꢀ (1±ꢀ25), 451 nm(6±ꢀ); MS:
4ꢀ 2 ±
H N O
Fe.
5
(
2
4
(1.ꢁ equiv). Reaction mixture was stirred at À±88C for one hour
then allowed to warm to ambient temperature. At the point at which the
solution went black, silica was added and solvent was removed. Crude
treatment with either ꢀ.ꢀ5M methanolic K
2 ꢁ
CO for about 5 h or exposure
mixture was separated by column chromatography (1% MeOH in DCM)
to MeNH at a pressure of about 2 bar for 2ꢀ min.
2
1
to give 3 in 25% yield). H NMR (2ꢀꢀ MHz, CDCl
ꢁ
, 258C): d ˆ 1.4± (t, 2H;
), ꢁ.45 (m, 2H; H-5', H-5∫),ꢁ.8ꢁ (s,
H; OMe), ꢁ.8± (t, 1H; H-ꢁ'), ꢁ.9ꢁ(m, 4H; Fc), 4.ꢀ9 (s, 5H; Fc), 4.54 (m, 1H;
HPLC were run using Gilson system ±12 controller software. Analytical
runs were performed on a Jones APEX ODS 5m column with an injection
loop of 25 mL. The column was incubated at ꢁꢀ8C. The solvent gradient was
made up of solvent A (1ꢀ% acetonitrile in water containing TEAA buffer
pH6.5) and solvent B (65% acetonitrile in water containing TEAA buffer
pH 6.5), increasing from ꢀ% B to 4ꢀ% B over 25 min and returning to
1ꢀꢀ% A at ꢁꢀ min.
2 2
CH ), 2.24 ± 2.ꢁ8 (m, 4H; H-2', H-2™, CH
6
H-4'), 6.4ꢁ (t, 1H; H-1'), 6.88 (m, 4H; DMT), ±.ꢁꢁ ± ±.48 (m, 1ꢀH; DMT,
‡
H-6); MS: ES : ±42.ꢀ for C42
H
52
N
2
O
±
Fe.
Fe.CHCl
a ˆ 9.9869(±), b ˆ 2ꢀ.418ꢁ(15), c ˆ 12.±998(9) ä, b ˆ
Crystal Data: 1a: C21
group P2
H
2ꢀ
N
2
O
5
ꢁ
, M
r
ˆ 555.6, monoclinic, space
1
,
ꢁ
Àꢁ
1
ꢀ9.±51(2)8, Vˆ 2456.5(ꢁ) ä , Z ˆ 4, 1calcd ˆ 1.5ꢀ2 gcm ; MoKa radiation,
The 2ꢀ-mer oligonucleotide was digested into its monodeoxynucleoside
constituents using snake venom phosphodiesterase and alkaline phospha-
tase following literature procedure.[517
À1
l ˆ ꢀ.±1ꢀ±ꢁ ä, m ˆ ꢀ.9±6 mm , Tˆ 16ꢀ K. Of 1951± reflections, corrected
for absorption, 1ꢀ481 were unique (Rint ˆ ꢀ.ꢀꢁ6, q ˆ 28.± 8); R ˆ ꢀ.ꢀ62 (F
2
2
values, F > 2s), R
w
ˆ ꢀ.1±2 (F values, all data), GOF ˆ ꢀ.929 for 6ꢁ2
Àꢁ
parameters, final difference map extremes ˆ ‡ꢀ.9ꢁ and Àꢀ.69 e ä . T he
structure was solved by direct methods. All non-H atoms were refined
anisotropically. H atoms were refined with a riding model. The asymmetric
unit contains two independent molecules of the complex together with one
ordered and one disordered chloroform molecule.
Acknowledgements
1
b: C21
H
2ꢀ
N
2
O
5
Fe, M
r
ˆ 4ꢁ6.2, monoclinic, space group P2
1
, a ˆ 6.5896(ꢁ),
This work was financially supported in part by the EPSRC.
ꢁ
b ˆ 22.1ꢀꢀ4(1ꢀ), c ˆ 12.±4ꢀ5(6) ä, b ˆ 96.±8ꢀ(2)8, Vˆ 1842.46(15) ä , Z ˆ
Àꢁ
4
ꢀ
,
1
calcd ˆ 1.5±ꢁ g cm
;
synchrotron radiation (SRS station 9.8), l ˆ
À1
.6884 ä, m ˆ ꢀ.85± mm , Tˆ 16ꢀ K. Of 1ꢁꢁ18 reflections, corrected for
[
17 D. Magda, S. Crofts, A. Lin, D. Miles, M. Wright, J. L. Sessler, J. Am.
Chem. Soc. 1997, 119, 229ꢁ.
absorption and incident beam decay, ±29ꢀ were unique (Rint ˆ ꢀ.ꢀꢁ4, q ˆ
2
2
2
9.48); R ˆ ꢀ.ꢀꢁ2 (F values, F > 2s), R
w
ˆ ꢀ.ꢀ±8 (F values, all data),
[27 K. Tanaka, M. Shionoya, J. Org. Chem. 1999, 64, 5ꢀꢀ2.
[ꢁ7 J. J. Rack, E. S. Krider, T. J. Meade, J. Am. Chem. Soc. 2000, 122, 628±.
[47 F. D. Lewis, S. A. Helvoigt, R. L. Letsinger, Chem. Commun. 1999,
GOF ˆ ꢀ.99± for 528 parameters, final difference map extremes ˆ ‡ꢀ.ꢁ6
Àꢁ
and Àꢀ.28 e ä . Structure solution and refinement were as for 1a. T he
asymmetric unit contains two independent molecules, but there is no
solvent present.
ꢁ
2±.
[
[
[
57 E. Meggers, D. Kusch, B. Giese, Helv. Chim. Acta. 1997, 80, 64ꢀ.
67 H. Weizman, Y. Tor, J. Am. Chem. Soc. 2001, 123, ꢁꢁ±5.
±7 C. J. Yu, H. Wang, Y. J. Wan, H. Yowanto, J. C. Kim, L. H. Donilon,
C. L. Tao, M. Strong, Y. C. Chong, J. Org. Chem. 2001, 66, 29ꢁ±.
87 G. B. Dreyer, P. B. Dervan, Proc. Natl. Acad. Sci. USA 1985, 82, 968.
97 D. S. Sigman, Acc. Chem. Res. 1986, 19, 18ꢀ.
2
9
a: C21
H
22
N
2
O
5
Fe ¥ CHCl
ꢁ
, M
r
ˆ 55±.6, monoclinic, space group P2
1
, a ˆ
ꢁ
.885(2), b ˆ 2ꢀ.4±4(4), c ˆ 12.949(ꢁ) ä, b ˆ 112.156(4)8, Vˆ 242±.2(9) ä ,
Àꢁ
À1
Z ˆ 4, 1calcd ˆ 1.526 g cm ; MoKa radiation, l ˆ ꢀ.±1ꢀ±ꢁ ä, m ˆ ꢀ.988 mm
,
Tˆ 16ꢀ K. Of 92ꢁ8 reflections, corrected for absorption, 58±8 were unique
[
[
2
2
(
R
int ˆ ꢀ.ꢀ4ꢁ, q ˆ 22.58); R ˆ ꢀ.116 (F values, F > 2s), R
w
ˆ ꢀ.ꢁ54 (F
values, all data), GOF ˆ 1.151 for 6ꢀꢀ parameters, final difference map
[
[
1ꢀ7 P. J. Dandliker, M. E. Nunez, J. K. Barton, Biochemistry 1998, 37, 6491.
117 S. Takenaka, K. Yamashita, M. Takagi, Y. Uto, H. Kondo,Anal. Chem.
Àꢁ
extremes ˆ ‡1.56 and Àꢀ.56 e ä . Structure solution and refinement were
as for 1a, except that H atoms were not located on the sugar groups. The
precision of the structure is limited by the rather weak diffraction data, by
pseudo symmetry, and by twinning. Chloroform sites (two in the
asymmetric unit, together with two independent molecules of the complex)
appear to be only partially occupied. Geometry and displacement
parameter restraints were applied to aid refinement.
2
000, 72, 1ꢁꢁ4.
[
[
[
127 R. C. Mucic, M. K. Herrlein, C. A. Mirkin, R. L. Letsinger, Chem.
Commun. 1996, 555.
1ꢁ7 Y. Mishima, J. Motonaka, K. Maruyama, K. Minagawa, S. Ikeda, Sens.
Act. B. 2000, 65, ꢁ4ꢀ.
147 A. Harriman, Angew. Chem. 1999, 111, 996; Angew. Chem. Int. Ed.
Programs used: SHELXTL (G. M. Sheldrick, SHEXLTL manual, Bruker
AXS Inc., Madison, WI., USA, 1998, version 5.1), together with standard
Bruker SMARTand SAINTdiffractometer control and data integration
programs and local software.
CCDC-1±5814 (1a), 1±5815 (1b), and 1±5816 (2a) contain the supple-
mentary crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1
999, 38, 945.
[
[
157 B. Giese, Acc. Chem. Res. 2000, 33, 6ꢁ1.
167 B. Giese, S. Wessely, M. Spormann, U. Lindemann, E. Meggers, M. E.
Michel-Beyerle, Angew. Chem. 1999, 111, 1ꢀ5ꢀ; Angew. Chem. Int. Ed.
1
999, 38, 996.
[
[
1±7 F. D. Lewis, T. F. Wu, Y. F. Zhang, R. L. Letsinger, S. R. Greenfield,
M. R. Wasielewski, Science 1997, 277, 6±ꢁ.
187 F. D. Lewis, R. L. Letsinger, M. R. Wasielewski, Acc. Chem. Res. 2001,
1
EZ, UK; fax: (‡44)122ꢁ-ꢁꢁ6-ꢀꢁꢁ; or e-mail: deposit@ccdc.cam.ac.uk).
3
4, 159.
Electrochemistry: Electrochemical data were recorded on an EG&G
Princeton Applied Research Potentiostat, Model 26ꢁA (using Echem
software 4.11) and using a silver wire reference electrode, a gold disc
working electrode and a tungsten wire counter electrode. Cyclic voltammo-
[197 C. J. Murphy, M. R. Arkin, Y. Jenkins, N. D. Ghatlia, S. H. Bossmann,
N. J. Turro, J. K. Barton, Science 1993, 262, 1ꢀ25.
[2ꢀ7 R. E. Holmlin, R. T. Tong, J. K. Barton,J. Am. Chem. Soc. 1998, 120,
9±24.
[217 M. R. Arkin, E. D. A. Stemp, R. E. Holmlin, J. K. Barton, A.
Hormann, E. J. C. Olson, P. F. Barbara, Science 1996, 273, 4±5.
[227 M. E. Nunez, J. K. Barton, Curr. Opin. Chem. Biol. 2000, 4, 199.
À1
grams were recorded at a scan rate of 1ꢀꢀ mVs in ꢀ.1m tetrabutylammo-
‡
nium hexafluorophosphate in acetonitrile and E8 referenced against Fc/Fc
as an external measurement. Differential pulse voltammograms were
2898
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Chem. Eur. J. 2002, 8, No. 1ꢁ

Metallocene ± DNA
2891±2899
[
2ꢁ7 S. O. Kelley, R. E. Holmlin, E. D. A. Stemp, J. K. Barton, J. Am.
Chem. Soc. 1997, 119, 9861.
[ꢁ97 S. I. Khan, M. W. Grinstaff, J. Am. Chem. Soc. 1999, 121, 4±ꢀ4.
[4ꢀ7 S. I. Khan, A. E. Beilstein, M. W. Grinstaff, Inorg. Chem. 1999, 38,
418.
[417 T. Ihara, M. Nakayama, M. Murata, K. Nakano, M. Maeda, Chem.
Commun. 1997, 16ꢀ9.
[247 P. Lincoln, E. Tuite, B. Norden, J. Am. Chem. Soc. 1997, 119, 1454.
257 M. W. Grinstaff, Angew. Chem. 1999, 111, ꢁ845; Angew. Chem. Int. Ed.
[
1
999, 38, ꢁ629.
[
267 K. Fukui, K. Tanaka, Angew. Chem. 1998, 110, 16±; Angew. Chem. Int.
Ed. 1998, 37, 158.
[427 T. Ihara, Y. Maruo, S. Takenaka, M. Takagi, Nucleic Acids Res.1996,
24, 42±ꢁ.
[
2±7 T. J. Meade, J. F. Kayyem, Angew. Chem. 1995, 107, ꢁ58; Angew.
Chem. Int. Ed. Engl. 1995, 34, ꢁ52.
[4ꢁ7 P. Meunier, I. Ouattara, B. Gautheron, J. Tirouflet, D. Camboli, J.
Besancon, Eur. J. Med. Chem. 1991, 26, ꢁ51.
[
[
[
287 A. M. Brun, A. Harriman, J. Am. Chem. Soc. 1994, 116, 1ꢀꢁ8ꢁ.
297 W. Bannwarth, D. Schmidt, Tetrahedron. Lett. 1989, 30, 151ꢁ.
ꢁꢀ7 X. Hu, G. D. Smith, M. Sykora, S. J. Lee, M. W. Grinstaff, Inorg.
Chem. 2000, 39, 25ꢀꢀ.
[447 C. McGuigan, C. J. Yarnold, G. Jones, S. Velazquez, H. Barucki, A.
Brancale, G. Andrei, R. Snoeck, E. De Clercq, J. Balzarini, J. Med.
Chem. 1999, 42, 44±9.
[457 M. J. Robins, P. J. Barr, J. Org. Chem. 1983, 48, 1854.
[467 C. McGuigan, H. Barucki, S. Blewett, A. Carangio, J. T. Erichsen, G.
Andrei, R. Snoeck, E. De Clercq, J. Balzarini, J. Med. Chem. 2000, 43,
499ꢁ.
[4±7 C. E. Castro, E. J. Gaughan, D. C. Owsley, J. Org. Chem. 1966, 31,
4ꢀ±1.
[487 S. E. Lee, J. S. Vyle, D. M. Williams, J. A. Grasby, Tetrahedron. Lett.
2000, 41, 26±.
[497 F. D. Lewis, X. Y. Liu, S. E. Miller, M. R. Wasielewski, J. Am. Chem.
Soc. 1999, 121, 9±46.
[5ꢀ7 M. P. Reddy, N. B. Hanna, F. Farooqui, Nucleosides Nucleotides 1997,
16, 1589.
[517 B. A.Connolly in Oligonucleotides and Analogues, (Ed.: F. Eckstein),
Oxford University Press, New York, 1991, pp. 155 ± 18ꢁ.
[
[
[
[
[
[
ꢁ17 R. E. Holmlin, P. J. Dandliker, J. K. Barton, Angew. Chem. 1997, 109,
2
8ꢁꢀ; Angew. Chem. Int. Ed. Engl. 1997, 36, 2±14.
ꢁ27 R. E. Holmlin, P. J. Dandliker, J. K. Barton, Bioconjugate Chem. 1999,
0, 1122.
ꢁꢁ7 K. Tanaka, M. Tasaka, H. Cao, M. Shionoya,Eur. J. Pharm. Sci. 2001,
3, ±±.
ꢁ47 T. Brown, D. J. S. Brown in Oligonucleotides and Analogues, (Ed.: F.
Eckstein), Oxford University Press, New York, 1991, pp. 1 ± 2ꢁ.
ꢁ57 C. J. Yu, Y. Wan, H. Yowanto, J. Li, C. Tao, M. D. James, C. L. Tan,
G. F. Blackburn, T. J. Meade, J. Am. Chem. Soc. 2001, 123, 11155.
ꢁ67 C. J. Yu, H. Yowanto, Y. J. Wan, T. J. Meade, Y. Chong, M. Strong,
L. H. Donilon, J. F. Kayyem, M. Gozin, G. F. Blackburn, J. Am. Chem.
Soc. 2000, 122, 6±6±.
1
1
[
ꢁ±7 E. Bucci, L. De Napoli, G. Di Fabio, A. Messere, D. Montesarchio, A.
Romanelli, G. Piccialli, M. Varra, Tetrahedron 1999, 55, 144ꢁ5.
ꢁ87 A. E. Beilstein, M. W. Grinstaff, Chem. Commun. 2000, 5ꢀ9.
[
Received: December 19, 2ꢀꢀ1 [Fꢁ±487
Chem. Eur. J. 2002, 8, No. 1ꢁ
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