3′-Modified oligodeoxyribonucleotides for the study of 2-deoxyribose damage in DNA

Article abstract of DOI:10.1016/j.bmcl.2012.11.050

Well-defined substrates for the study of oxidative processes are important for the elucidation of the role of DNA damage in the etiology of diseases such as cancer. We have synthesized 3′-modified oligodeoxyribonucleotides (ODNs) using 5′ → 3′ 'reverse' DNA synthesis for the study of 2-deoxyribose oxidative damage to DNA. The modified monomers designed for these studies all share a common feature, they lack the naturally occurring 3′-hydroxyl group found in 2-deoxyribonucleosides. Modified H-phosphonates containing 3′-phenyl selenides as well as saturated and unsaturated sugars were obtained and incorporated in ODNs. These ODNs were used to investigate the fate of C3′-dideoxyribonucleotide radicals in DNA.

Full text of DOI:10.1016/j.bmcl.2012.11.050

Bioorganic & Medicinal Chemistry Letters 23 (2013) 854–859
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
30-Modified oligodeoxyribonucleotides for the study of 2-deoxyribose damage
in DNA
Buthina Al-Oudat a, Alex Salyer b, Kevin Trabbic b, Amanda Bryant-Friedrich b,
⇑
a Department of Medicinal Chemistry and Pharmacognosy, Jordan University of Science and Technology, Irbid 22110, Jordan
b Department of Medicinal and Biological Chemistry, The University of Toledo, 2801 W. Bancroft St., Toledo, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Well-defined substrates for the study of oxidative processes are important for the elucidation of the role
of DNA damage in the etiology of diseases such as cancer. We have synthesized 30-modified oligodeoxy-
ribonucleotides (ODNs) using 50 ? 30 ‘reverse’ DNA synthesis for the study of 2-deoxyribose oxidative
damage to DNA. The modified monomers designed for these studies all share a common feature, they lack
the naturally occurring 30-hydroxyl group found in 2-deoxyribonucleosides. Modified H-phosphonates
containing 30-phenyl selenides as well as saturated and unsaturated sugars were obtained and incorpo-
rated in ODNs. These ODNs were used to investigate the fate of C30-dideoxyribonucleotide radicals in
DNA.
Received 21 August 2012
Revised 9 November 2012
Accepted 14 November 2012
Available online 28 November 2012
Keywords:
H-phosphonate
DNA damage
Oligonucleotides
Sugar damage
Radicals
Ó 2012 Elsevier Ltd. All rights reserved.
It is a long accepted fact that the primary cellular target of ion-
izing radiation is DNA.1 The transfer of energy from ionizing radi-
ation to cellular molecules such as proteins, water and DNA, results
in the production of large numbers of electrons with energies of
<30 eV.2 To fully elucidate the effects of ionizing radiation on bio-
logical systems, it is important to determine the nature of the neu-
tral products formed when these low energy electrons (LEEs) are
captured by DNA. In recent years, it has been determined, through
studies involving DNA in the condensed phase, that LEEs are easily
transferred to the p⁄ orbitals of the nucleobases initiating DNA
damage processes. This damage takes the form of bond dissocia-
tion either at the N-glycosidic bond of the nucleobase or cleavage
of the C–O bond at the 30- and/or 50-phosphodiester moieties.3,4
When cleavage occurs at the carbon–phosphodiester bond C30-
and C50-radical species 2 and 5 are formed along with phosphory-
lated DNA fragments 3 and 4 (Scheme 1). It was reported that in
addition to stable fragments 3 and 4, other fragments were also
produced which have evaded identification.3 To address this issue,
the work described herein seeks to elucidate the structure of frag-
ments originating from the degradation of 2. Through the use of
site-specifically modified oligodeoxyribonucleotides (ODNs) con-
taining photolabile groups capable of generating the C30-dideoxy-
radical 2, as well as expected degradation products, we will seek
to determine the mechanism of degradation as well as the identity
O
P
O
O
O
P
P
5'
5'
O
O
B
3
B
B
P
5'
O
B
O
O
O
O
O
+
+
3'
5'
P O
3'
O
P
O
2
4
*
O
O
B
O
B
P O
3'
P O
3'
O
P
O
1
O
O
5
B (structures) = DNA bases: G, T, C, A
Scheme 1. The proposed pathway of LEE induced formation of C30 and
C50-deoxyribose radicals in DNA.3
of the end products obtained from this reactive intermediate.
These investigations build on our long standing interest in the
site-specific generation of sugar radicals in DNA oligomers.5–7
Modified ODNs suitable for the formation and study of 2 contain
modifications at the C30-position of the terminal 2-deoxyribose
moiety. Due to the nature of these substrates, nucleotide mono-
mers lacking the 30-hydroxyl normally found at the C30-position
are required. The synthesis of ODNs containing modifications at
their 30-ends is conventionally accomplished through the use of
specially prepared solid supports.8,9 While this approach has made
⇑
Corresponding author.
E-mail addresses: abryant9@utnet.utoledo.edu, amanda.bryant-friedrich@utoledo.
edu (A. Bryant-Friedrich).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.11.050

B. Al-Oudat et al. / Bioorg. Med. Chem. Lett. 23 (2013) 854–859
855
30-modified oligomers such as 30-phosphoglycolate and 30-phos-
phoglycolaldehyde easily obtainable,10 it is limited in scope due
to the need for highly functionalized substrates attached to solid
supports. Dervan and co-workers reported the use of a ‘reverse’
synthesis approach for nucleic acids to facilitate the formation of
ODNs containing 30–30 linkages.11 The same concept has also been
applied to the synthesis of ODNs containing 50–50,12,13 linkages as
well as oligonucleotide substrates containing segments in the
opposite sense of the rest of the strand.14 Additionally, almost a
decade ago, Hecht and co-workers15 reported ‘reverse’ (i.e.,
50 ? 30) automated synthesis of modified ODNs with a tyrosine res-
idue at the 30-end. The authors pointed out the generality of this
approach for the introduction of modifications at the 30-end of
ODNs through the incorporation of modified phosphoramidites
into a protected ODN during the final synthetic step. This strategy
is highly appropriate for our purposes. The commercial availability
of 50-phosphoramidite monomers, appropriately functionalized for
automated nucleic acid synthesis as well as solid supports with a
50-linkage, makes this approach even more suitable. Due to our
long-standing interest in the synthesis of modified ODNs using
the H-phosphonate method,7,16 we chose to develop methods for
the ‘reverse’ synthesis of the modified ODNs of interest using
50-H-phosphonates 6–9.
7, the reduction product formed upon trapping of radical 2 by a
hydrogen atom donor.
Due to the absence of the 30-hydroxyl, these 30-modified nucle-
otides do not contain the dimethoxytrityl protected hydroxyl
substituent at the 30-position of the nucleoside usually found in
50-phosphoramidites. This change makes these H-phosphonates
significantly more polar than their unmodified counterparts. As re-
ported by Kraszewski et al,23 we found the isolation and purifica-
tion of such H-phosphonate monomers to be highly inefficient
when the synthesis was performed using traditional phosphonyla-
tion conditions.24,25 This became evident in our synthesis of
H-phosphonate derivative 6a using standard phosphonylation con-
ditions employing PCl3 and imidazole in the presence of triethyl-
amine (Method A, Scheme 2). This approach delivered 6a as its
triethylammonium salt in only 22% yield (Scheme 2). The solubility
of 10a26 in the reaction solvent CH2Cl2 was limited and reaction
workup was very tedious.
To overcome this problem, an H-phosphonylation reagent
developed by Fang and co-workers was used facilitating a more
efficient aqueous work-up and purification than Method
A
(Method B, Scheme 2).27
Method B is based on the use of the H-phosphonylation reagent,
ammonium (9H-fluoren-9-yl)methyl H-phosphonate (12), to intro-
duce a lipophilic handle into these polar nucleosides to increase
solubility and to simplify purification. The inclusion of pyridine
in the reaction mixture also increases the solubility of the starting
material. Compound 12 is easily obtained through treatment of
commercially available phosphonic acid with pivaloyl chloride fol-
lowed by the addition of (9H-fluoren-9-yl)methanol.27 The use of
this reagent permitted simple aqueous work-up of the reaction
mixture and at the same time easy removal of the reaction by-
products at the end of the synthesis.
Phenylselenide containing nucleosides and ODNs have been
shown to efficiently generate radicals at both the 2-deoxyribose
as well as the nucleobases of DNA under photochemical conditions
facilitating the homolytic cleavage of the C–Se bond.17–21 Using
these substrates, radicals have been generated at the C40-position
of the sugar as well as the C5- and methyl substituent of thymine
bases in ODNs. Our initial investigations indicate that phenylsele-
nide containing nucleosides are efficient precursors of the radical
of interest in their monomeric form.22
To determine their ability to perform the same task in ODNs, we
have synthesized ODNs containing C30-phenylselenide monomers
6 as well as unsaturated substrates 8 and 9, representing radical
disproportionation products that can arise from 2, and compound
Treatment of nucleoside 10a with 12 in the presence of pivaloyl
chloride, which functions as an activator, allowed introduction of
the lipophilic (9H-fluoren-9-yl)methyl handle to deliver 13a as
an oily residue. Without further workup, treatment of intermediate
O
O
NH
NH
O
P
H
N
O
N
O
HO
O
Et3NH
O
Method A
O
O
1. PCl3, Imidazole, Et3N, CH2Cl2, -10 °C
2. TEAB buffer
X
X
Method
A
B
10a (X = α-SePh)
10b (X = β-SePhl)
11 (X = H)
6a (X = α-SePh) 22% 65%
6b (X = β-SePh)
(X = H)
35%
46%
7
Method B
Pivaloyl Chloride,
CH2Cl2,Pyridine
(95:5)
Et3N/CH3CN (1:2)
O
CH O
P O
H
2
NH4
O
N
NH
O
O
CH2
O
P
O
H
O
X
13a (X = α-SePh)
13b (X = -SePhl)
β
13c (X = H)
Scheme 2. Synthesis of H-phosphonate monomers.

856
B. Al-Oudat et al. / Bioorg. Med. Chem. Lett. 23 (2013) 854–859
O
N
O
N
O
N
O
N
NH
NH
NH
NH
O
O
O
O
O
O
O
O
O
X+
P
O
O
X+
P
O
O
X+
P
O
O
X+
P
O
H
H
H
H
O
O
O
8
O
9
SePh
7
6a, 6b
X = HNEt3
Figure 1. 20,30-dideoxyribonucleotide H-phosphonates.
13a with CH3CN/triethylamine (2:1, v/v) at room temperature re-
sulted in quantitative elimination of the (9H-fluoren-9-yl)methyl
group (Scheme 2). Silica gel purification of the crude products
afforded phenyl selenide derivatives 6a and 6b in 65 and 35% yield,
respectively, while the 50-H-phosphonate of 30-deoxythymidine (7)
was obtained in 46% yield. This approach was also extended to the
synthesis of proposed disproportionation products (8 and 9). Using
the same procedure described for the substrates above (6–7,
Scheme 2) unsaturated H-phosphonates 8 and 9 (Fig. 1) were ob-
tained in 50% and 27% yield, respectively.
The propensity of phenylselenides to undergo oxidation in the
presence of reagents similar to those utilized for the conversion
of the H-phosphonate moiety to the conrresponding phosphate
prompted us to look at the extent of this conversion in the synthe-
sis of ODNs 14a–b. RNA polymers containing alkyl selenide moie-
ties obtained through automated RNA synthesis29 were reported to
deliver 2–5% selenoxide upon exposure of the alkyl selenide con-
taining oligomer to 20 mM I2 for 20 s. This conversion was ob-
served when the modification was near the 30-terminus of the
ODN.
While not impressive these yields are a substantial improve-
ment over those obtained using traditional methods (data not
shown). A possible explanation for the low yields obtained lies in
the fact that the conversion of the nucleosides to intermediates
13 was found to be incomplete in all cases (based on TLC). Extend-
ing the reaction time as well as using more equivalents of phos-
phonylating agent 12 failed to lead to improvement of the yields.
The utilization of 1-adamantanecarbonyl chloride as activator also
did not result in improvement in yield (data not shown). Moreover,
the synthesis of 9 was even more challenging due to the instability
of the starting material (30,40-didehydro-20,30-dideoxythymidine).28
Efforts are underway to improve the synthetic yields of H-phos-
phonates 6–9.
In the synthesis of ODNs 14a–f, the oxidation of the H-phospho-
nate linkage is performed using 4% I2 in pyridine/H2O/THF (1:1:8)
with THF/H2O/triethylamine (8:1:1). Under these conditions
oxidation of the selenide moiety in ODNs 14a and b could not be de-
tected either by HPLC or by MALDI-ToF analysis (data not shown).
These oligomers were also found to be very stable with no oxidation
or degradation in the presence of air over several months.
Oligonucleotides 14c and d were also shown to be highly stable
over long periods of time. However, analysis of oligomers 14e and f
after storage at -20 °C in aqueous solution for several months indi-
cated decomposition (Fig. 3). MALDI-ToF MS analysis detected the
presence of a fragment corresponding to the product of base
hydrolysis (15) as well as an oligomer containing a 30-phosphate
(17) resulting from the complete loss of the modified nucleoside
(Scheme 3). This observation prompted us to further investigate
the stability of the 30,40-unsaturated nucleotide containing ODN.
After incubation of 14f for 45 min in 100 mM phosphate buffer,
pH = 2.0, HPLC analysis showed the presence of remaining 14f,
along with a considerable amount of another fragment with a
retention time of 12.47 min (see Supplementary data). ESI-MS
analysis of the isolated peaks confirmed the presence of the start-
ing material and fragment 15 (see Supplementary data). ODN 17
was not detected by HPLC analysis. The facile conversion of enol
ethers similar to 14e/f to the corresponding ketoaldehyde has been
reported.28
With the required precursors in hand as well as a set of plausi-
ble degradation products, we turned to the identification of the
lesions resulting from the photochemical generation of C30-
dideoxyradical 2 from C30-phenylselenides. ODNs 14a and b were
photochemically activated under anaerobic conditions in 100 mM
phosphate buffer. After photolysis, the crude photolysate was in-
jected directly onto a reverse-phase column with detection at
260 nm. Elution was achieved by applying a linear gradient of
50 mM TEAA buffer (pH = 7.0) in acetonitrile. The fractions were
collected and analyzed by MALDI-ToF MS, and the products were
compared to independently synthesized standards. Standard
curves of all oligonucleotides were constructed by injection of
independently synthesized substrates with UV dection in order
to quantify the products from the photolysis experiments (see Sup-
plementary data).
Oligonucleotides containing 30-modified nucleotides at their 30-
termini (14) were synthesized using the above-described H-phos-
phonates (6–9) through combined semi-automated and manual
synthesis techniques. Beginning with commercially available sup-
ports and 50-phosphoramidites, the unmodified portion of the se-
quence was formed on an Applied Biosystems 391 PCR-MATE
DNA synthesizer. The final dimethoxytrityl protecting group of
the unmodified oligomer was removed in the last automated step.
The incorporation of the modified nucleotide of interest was then
achieved utilizing the corresponding H-phosphonate derivatives
via manual coupling as previously described.16
The resulting protected ODNs were cleaved from the solid sup-
port and fully deprotected via treatment with ammonium hydrox-
ide for 15–18 h at 55 °C. The resulting crude oligonucleotides were
purified using reverse phase HPLC (Fig. 2), 14a. Oligomers 14a–f
were obtained in high yield, and purity as indicated by HPLC and
MALDI-ToF MS analysis (See Table 1).
Table 1
MALDI-ToF MS analysis of ODNs 14a–f
Sequence#
Sequences
Theoretical [M+H]+
Experimental [M+H]+
14a
14b
14c
14d
14e
14f
TTATTp (10a)
TTATTp (10b)
TTATTp (11)
TTATT (X)a
1912.3
1912.3
1756.3
1754.3
1754.3
1739.3
1912.2
1912.3
1756.3
1754.4
1754.2
1739.3
TTATT (Y)b
b
TTATC (Y)
In the presence of 6 mM glutathione (GSH), photochemical
cleavage of the C30-selenium bond upon exposure to UV light,
P320 nm, resulted in the formation of the same products from
a
X = 20,30-didehydro-20,30-dideoxythymidine.
Y = 30,40-didehydro-20,30-dideoxythymidine.
b

B. Al-Oudat et al. / Bioorg. Med. Chem. Lett. 23 (2013) 854–859
857
220
mAU
1 - 17.467
150
100
min
20
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0
Time (min)
Figure 2. HPLC chromatogram of oligonucleotide 14a.
x104
2.0
1754.291
14f
17
1548.238
1.5
1.0
0.5
0.0
1814.221
15
1646.269
1830.189
1164.241
1468.281
1244.219
1200
1400
1600
1800
2000
2200
m/z
Figure 3. MALDI-ToF MS analysis of an aqueous solution of 14e stored at -20 °C for 2 months (compound 14e: expected 1754.3 m/z found 1754.3; compound 15: expected
1646.3 found 1646.3; compound 17: expected 1548.3 [M+H] found 1548.2 [M+H]).
O
Aqueous solution
NH
O
pH = ~7
-20 °C, 2 months
or
N
O
O
O
NH
P
O
P
H
O
100 mM phosphate buffer
O
P
O
N
H
O
pH = 2
37 °C, 45 min
14f
15
17
16
Scheme 3. Decompostion products of 14f.
both precursors. Utilizing the independently synthesized oligo-
mers reported herein, we were able to easily identify the unsatu-
rated sugar containing oligomers 14d and 14e as the major
degradation products observed in this system with 14d
formed in an ꢀ3:1 ratio relative to 14e in the case of alpha phenyl-
selenide 14a and 6:1 in the case of beta isomer 14b. In addition,

858
B. Al-Oudat et al. / Bioorg. Med. Chem. Lett. 23 (2013) 854–859
O
O
N
O
N
NH
NH
P
P
O
O
NH
N
O
O
O
O
P
O
O
O
14e
14d
SePh
14a
h
> 320 nm
100 mM phosphate
buffer
O
6 mM GSH
O
N
O
NH
NH
NH
P
O
N
O
P
O
P
O
O
O
N
O
O
O
PhSe
H
OH
14b
18
19
Scheme 4. Products identified in the photolysate derived from radical precursors 14a and 14b in the presence of glutathione.
Table 2
Product yields from the photolysis of phenylselenides 14a and 14b in the presence of GSH
Phenyl selenide
14d
14e
18
19
Inversion products
Mass balance (%)
14a
14b
14a
14b
42.8
43.6
15.7
7.3
4.8
2.0
3.2
5.2
—
21.6
29.2
—
95.7
79.7
C30-phenylselenide containing oligomers (14a–14b) with inverted
configuration at the C-Se bond, relative to the starting material
(14a ? 14b; 14b ? 14a), as well as C30-phenylthymidine contain-
ing oligomers (19) were observed. Finally, trace amounts of thymi-
dine terminated oligomers (18) were also observed (Scheme 4). It
should be noted that these precursors do not deliver either the
expected reduction product 14c or a 30-phosphorylated ODN
resulting from the loss of the modified nucleotide (see Table 2).
Hence, the majority of products formed upon photolysis of phe-
nylselenides 14aand14b arederivedfromthegenerationofthephe-
nylselenyl radical in a solvent cage.30 This phenomenon, which is the
result of the slow diffusion of the selenyl radical from the site of for-
mationdue to itshydrophobicity, facilitates theintramolecular reac-
tion of the resulting radicals to give unsaturated sugars 14d and 14e.
These modified oligomers are formed by hydrogen atom
abstraction at the C20- and C40-positons of the 20,30-dideoxyradical,
respectively. Additional evidence of this phenomenon is found in
the formation of the phenylated oligomer 19. Related 40-phenylat-
ed oligomers were found to be a major reaction product in the gen-
eration of the C40-radical from phenylselenide precursors. The
formation of these oligomers was believed to result from the addi-
tion of the sugar radical to the ipso position of the phenylselenyl
radical.21 This delivered a biradical species which loss selenium
to deliver the modified oligomer. Due to the hydrophobic character
of the phenylselenide radical, access to the sugar radical is com-
pletely hindered and reduction by GSH prohibited. The rate of
the hydrogen atom abstraction at the alpha sugar carbons success-
fully competes with all other radical reactions to deliver the unsat-
urated sugars as the major products. The formation of thymidine
terminated oligomers can be explained by the oxidation of the ini-
tially formed radical followed by solvolysis.
The required H-phosphonates are easily obtained using an alterna-
tive synthesis strategy involving the introduction of a lipophilic
moiety at the 50-H-phosphonate. Even though this method facili-
tated the delivery of the desired H-phosphonates, efforts are
underway to increase the conversion of the modified nucleosides
to intermediate 13 and subsequently to increase the yields. Sub-
strates containing phenyl selenide substituents at the 30-position
are obtained in high purifity with no indication of oxidation
through exposure to the dilute iodide reagent required for oxida-
tion of the H-phosphonate linkage. 20,30-Dideoxynucleosides can
also be converted to their corresponding H-phosphonates and
incorporated in ODNs using these methods.
These substrates have been successfully used to determine the
products obtained from the generation of the 20,30-thymidinyl rad-
ical in DNA oligomers from phenylselenide precursors under
anaerobic conditions. Due to the nature of these precursors several
products were observed which are specifically related to reactions
of the radical pair. Even though these selenide precursors do not
appear to function well for our purposes under anaerobic condi-
tions, we will utilize these substrates for future investigations to
determine the products of degradation of the 30-deoxy-C30-thymid-
inyl radical under aerobic conditions. The high rate of trapping of
the radical of interest by oxygen may suppress the side reactions
we have seen in the presence of the weak hydrogen atom donor
GSH. We will also utilize stronger hydrogen atom donors to poten-
tially suppress the unwanted side reactions observed under anaer-
obic conditions.
Acknowledgment
We are grateful for support of this research (CHE-512973)
by the National Science Foundation who also provided
funding for instrumentation used for MALDI-ToF MS analysis
(DBI-923184).
These data indicate that the introduction of 30-deoxynucleo-
sides of varying structure can be easily accomplished through ‘re-
verse’ (50 ? 30) oligonucleotide synthesis using 50-H-phosphonates.

B. Al-Oudat et al. / Bioorg. Med. Chem. Lett. 23 (2013) 854–859
859
14. van de Sande, J. H.; Ramsing, N. B.; Germann, M. W.; Elhorst, W.; Kalisch, B. W.;
von Kitzing, E.; Pon, R. T.; Clegg, R. C.; Jovin, T. M. Science 1988, 241,
551.
15. Claeboe, C. D.; Gao, R.; Hecht, S. M. Nucleic Acids Res. 2003, 31, 5685.
16. Lahoud, G.; Fancher, J.; Grosu, S.; Cavanaugh, B.; Bryant-Friedrich, A. Bioorg.
Med. Chem. 2006, 14, 2581.
17. Hong, I. S.; Ding, H.; Greenberg, M. M. J. Am. Chem. Soc. 2006, 128, 485.
18. Hong, I. S.; Greenberg, M. M. J. Am. Chem. Soc. 2005, 127, 10510.
19. Hong, I. S.; Greenberg, M. M. Org. Lett. 2004, 6, 5011.
20. Tallman, K. A.; Greenberg, M. M. J. Am. Chem. Soc. 2001, 123, 5181.
21. Giese, B.; Beyrich-Graf, X.; Erdmann, P.; Petretta, M.; Schwitter, U. Chem. Biol.
1995, 2, 367.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2012.11.
050.
References and notes
1. Sonntag, C. v. Free-Radical-Induced DNA Damage and its Repair: A Chemical
Perspective; Springer: Berlin, London, 2011.
2. Pimblott, S. M.; LaVerne, J. A. Radiat. Phys. Chem. 2007, 76, 1244.
3. Zheng, Y.; Cloutier, P.; Hunting, D. J.; Sanche, L. O.; Wagner, J. R. J. Am. Chem. Soc.
2005, 127, 16592.
4. Zheng, Y.; Cloutier, P.; Hunting, D. J.; Wagner, J. R.; Sanche, L. J. Am. Chem. Soc.
2004, 126, 1002.
5. Becker, D.; Bryant-Friedrich, A.; Trzasko, C.; Sevilla, M. D. Radiat. Res. 2003, 160,
174.
6. Bryant-Friedrich, A. C. Org. Lett. 2004, 6, 2329.
7. Lahoud, G. A.; Hitt, A. L.; Bryant-Friedrich, A. C. Chem. Res. Toxicol. 2006, 19,
1630.
8. Lyttle, M. H. Curr. Pro. Nucl. Acid Chem. 2001, 4.6.1.
9. Greenberg, M. M. Curr. Pro. Nucl. Acid Chem. 2001, 4.5.1.
10. Urata, H.; Akagi, M. Tetrahedron Lett. 1993, 34, 4015.
11. Horne, D. A.; Dervan, P. B. J. Am. Chem. Soc. 1990, 112, 2435.
12. Koga, M.; Moore, M. F.; Beaucage, S. L. J. Org. Chem. 1991, 56, 3757.
13. Masakazu Koga, O.; Wilk, A.; Moore, M. F.; Scremin, C. L.; Zhou, L.; Beaucage, S. L.
J. Org. Chem. 1995, 60, 1520.
22. Audat, S. A. S.; Trzasko Love, C.; Al-Oudat, B. A. S.; Bryant-Friedrich, A. C. J. Org.
Chem. 2012, 77, 3829.
23. Romanowska, J.; Szymanska-Michalak, A.; Pietkiewicz, M.; Sobkowski, M.;
Boryski, J.; Stawinski, J.; Kraszewski, A. Lett. Org. Chem. 2009, 6, 496.
24. Garegg, P. J.; Regberg, T.; Stawinski, J.; Stroemberg, R. Chem. Scr. 1986, 26, 59.
25. Gosselin, G.; Perigaud, C.; Lefebvre, I.; Pompon, A.; Aubertin, A.-M.; Kirn, A.;
Szabo, T.; Stawinski, J.; Imbach, J.-L. Antivir. Res. 1993, 22, 143.
26. Cosford, N. D. P.; Schinazi, R. F. J. Org. Chem. 1991, 56, 2161.
27. Yang, Z.-W.; Xu, Z.-S.; Shen, N.-Z.; Fang, Z.-Q. Nucleosides Nucleotides 1995, 14,
167.
28. Imwinkelried, P., The University of Basel, 1995.
29. Carrasco, N.; Buzin, Y.; Tyson, E.; Halpert, E.; Huang, Z. Nucleic Acids Res. 2004,
32, 1638.
30. Ouchi, A.; Liu, S.; Li, Z.; Kumar, S. A.; Suzuki, T.; Hyugano, T.; Kitahara, H. J. Org.
Chem. 2007, 72, 8700.