Synthesis of all four nucleoside-based β-amino acids as protected precursors for the synthesis of polyamide-DNA with alternating α-amino acid and nucleoside-β-amino acids

Article abstract of DOI:10.1016/j.tet.2012.11.028

A simple approach is described for the synthesis of all four orthogonally protected nucleoside-β-amino acids from commercially available starting materials. Synthesis of a model tetrameric DNA sequence in 5′-3'direction employing trityl strategy and glycine as α-amino acid alternating with nucleoside-β amino acids is described.

Full text of DOI:10.1016/j.tet.2012.11.028

Tetrahedron 69 (2013) 1210e1216
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of all four nucleoside-based
b-amino acids as protected
precursors for the synthesis of polyamide-DNA with alternating
a-amino acid and nucleoside-b-amino acids
*
Seema Bagmare, Manojkumar Varada, Anjan Banerjee, Vaijayanti A. Kumar
Organic Chemistry Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple approach is described for the synthesis of all four orthogonally protected nucleoside-b-amino
Received 8 September 2012
Received in revised form 1 November 2012
Accepted 7 November 2012
Available online 13 November 2012
acids from commercially available starting materials. Synthesis of a model tetrameric DNA sequence in
50e3’direction employing trityl strategy and glycine as
a
-amino acid alternating with nucleoside-
b amino
acids is described.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Neutral DNA analogues
Nucleoside-b amino acids
Antisense therapeutics
1. Introduction
(6/7 atom repeating units) deoxyribo/ribo4e7 ON analogues are
studied till date, but the chemistry development in most instances
has been restricted to the synthesis of thymine (TeT dimers). The
other dephosphono linkers also have not been extensively explored
for the synthesis of mixed purineepyrimidine sequences probably
because of the difficulties encountered during the synthesis of
building blocks corresponding to all four nucleobases required for
the synthesis of any desired nucleobase sequence. The complete
replacement of sugar phosphate backbone as in PNA has been ex-
tremely successful8 and has found advantages in diagnostic appli-
cations11 as well as in splice correction antisense applications.3 The
development of cyclic, chiral analogues of PNA containing 6/7-atom
repeating units was an offshoot aimed at either getting better RNA
selectivity as antisense agents, or directional selectivity for binding
to specific RNA sequences.8 The relevant examples are those of
pyrrolidine-based NA analogues, such as pyrrolidinyl12/POM-PNA13
and bepPNA14 or those comprising prolyl-aminopyrrolidine-2-
carboxylic acid15 and prolyl-2-aminocyclopentanecarboxylic acid
(prolyl-ACPC backbone) (Fig. 1).16,17 The prolyl-ACPC backbone and
Novel oligonucleotide (ON) analogues that can form stable du-
plexes or triplexes with target nucleic acids are highly desired
synthetic objectives because of their use as therapeutic agents.1
Various types of modified oligonucleotides2 have been developed
over the last two decades as potential antisense and antigene
agents. The more recent developments, such as splice correcting
and exon skipping3 strategies require highly robust nucleic acid
analogues that are stable under physiological conditions as single
strands as well as in the form of duplexes with complementary RNA
sequences. The study of unnatural dephosphono nucleic acid olig-
omers would be highly beneficial for siRNA applications as well.4a
This approach would maintain the chirality and 30e50 di-
rectionality of the backbone, along with the enzymatic stability of
the replaced phosphate linkage by unnatural linker group. The re-
placement of the internucleoside sugarephosphate linkages by the
robust amide,4e8 carbamate,9aec carbazoyl9d or guanidino-10 link-
ages is studied in the literature. The exploration of an amide bond
for this purpose may be an interesting choice because the well
established solid phase peptide synthesis (SPPS) methodology can
be directly extended and could be advantageous as in case of syn-
thesis of peptide nucleic acids (PNA)8aec and analogues containing
its homologues17b are the alternating
a/b amino acid backbone
exhibited preferential binding with complementary DNA. Our ini-
tial work was based on the prolyl-ACPC backbone and in that case
the alternating nucleoside-
acids (Fig. 1) gave rise to similar alternating
bone scaffolds using well-established peptide chemistry.18a We
studied thyminyl homooligomer sequences, in which thyminyl-
amino acid was used in conjunction with natural -amino acids
and found that these sequences displayed RNA selective binding.
b
-amino acids with natural
a-amino
chiral
a
-amino acids.8dei Several four- and five-atom amide-linked
a/b
amino acid back-
b
-
a-L
* Corresponding author. Tel.: þ91 20 25902340; e-mail address: va.kumar@
ncl.res.in (V.A. Kumar).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.11.028

S. Bagmare et al. / Tetrahedron 69 (2013) 1210e1216
1211
ester at 50-end as a linker of the growing oligomer via succinamide
linker to the support. The succinate ester can be cleaved concom-
itantly with the nucleobase deprotection, using aqueous/meth-
anolic ammonia at the end of synthesis leaving a 50-OH group. This
strategy has been previously applied successfully for the synthesis
of oligonucleotide phosphonamidates on Controlled Pore Glass
(CPG) support.20 The MBHA-succinate linker strategy has been
previously used for the synthesis of glycopeptides21 but has not
been employed for the synthesis of oligopeptides.
The TEMPOeBAIB method22 used earlier by us18a,19 and oth-
ers18cef for oxidation of 50-primary hydroxyl group in 30-deoxy-30-
azidothymidine to get the 40-carboxylic acid would be used as
general procedure for protected tritylaminonucleosides. Thus, the
30-deoxy-30-tritylaminothymidine-40-carboxylic acid 4 was ob-
tained as follows (Scheme 1). The 50-TBDMS-protected 30-deoxy-30-
azidothymidine 120a was hydrogenated over PdeC for reduction of
azide to amine to get compound 2. Reaction of 2 with trityl chloride,
followed by deprotection of 50-silyl ether gave compound 3 in good
yield. The 30-tritylamino-30-deoxythymidine 3 was obtained in
comparable yields and purity as reported earlier.20 Compound 3 was
easily converted to the corresponding acid 4 under TEMPOeBAIB
oxidation conditions18,19 (60% overall yield). The 30-tritylamino-
20,30-dideoxy-N4-benzoyl-5-methylcytidine was synthesized from 1
as shown in Scheme 1. Compound 1 was converted to 1,2,4-triazolyl
nucleoside derivative 5 by treatment with POCl3 and 1,2,4-triazole
in the presence of triethylamine.20b The cytidine derivative was
obtained upon treatment of 5 with aq ammonia in dioxane. The free
exocyclic amino group was further protected by treatment with BzCl
in dry pyridine to get N4-Bz protected cytidine derivative 6 in
good yields. Compound 6 was further reduced to 30-amino-5-
methylcytidine derivative, which was subsequently protected as
30-tritylamino group followed by the removal of silyl ether to get
compound 7.20a These altered reaction sequence as compared to the
reported route20b avoided formation of 30-NHTr-benzoyl derivative,
which was formed during N4-benzoylation. We used H2S/pyridine
for the reduction of azide to amine7a as reduction with PdeC was
found to be sluggish and less efficient. Oxidation to 40-carboxylic
acid derivative 8 was accomplished using TEMPOeBAIB oxidation
conditions and the product was recovered by trituration with ether
and acetone (42% overall yield).
Fig. 1. DNA/RNA and backbone alterations.
The thyminyl-
b
-amino acids were also synthesized by others for
synthesizing phoshonamide internucleoside linkages18b and also
for their use in sequences for studies in foldamer chemistry.18cef For
our interests in synthesizing (
a b) amino acid backbone, we fur-
þ
ther studied the effect of chirality of the
thesizing the thymine dimers linked with
a
-amino acids by syn-
L-proline, -proline and
D
prochiral glycine units on base stacking interactions.19 To further
exploit this interesting backbone, synthesis of all four natural
nucleoside-based
b-amino acids as building blocks is required. In
this paper, we describe the first approach towards the synthesis of
all four appropriately protected natural nucleoside-based
acids and show their utility in the synthesis of a model (
b
a
-amino
-amino
acidþnucleoside-
b amino acid) tetrameric sequence of dephos-
phono polyamide-DNA.
2. Results and discussion
In our earlier strategy, we used Fmoc protected 30-deoxy-30-
aminothymidine as nucleoside- -amino acid monomer and rink
b
amide resin as the solid support.18a Exocyclic amino protection is
not required for thymine nucleobase during oligomer synthesis,
and Fmoc strategy worked well. Also cleavage from the rink amide
MBHA resin requires strongly acidic conditions (TFA:TFMSA/20%
TFA in DCM), which may not be compatible for mixed base se-
quences containing purine nucleosides due to likely depurination
in strongly acidic medium. We decided to use the trityl protection
for 30-sugar-amino function to be deprotected by trichloroacetic
acid at each coupling step. This altered strategy would accommo-
date the orthogonal protection of nucleobases (benzoyl for adenine
and cytosine and isobutyryl for guanine). We chose the succinate
For the synthesis of b-amino acid derivatives of adenine and
guanine nucleosides, we used the commercially available 30-amino-
30-deoxy purine nucleosides as starting materials. The synthesis of
Scheme 1. Synthesis of protected thymine/5-methylcytosine-b-amino acids.

1212
S. Bagmare et al. / Tetrahedron 69 (2013) 1210e1216
the starting nucleoside derivatives is earlier reported from exo-
cyclic amino protected 20-deoxyxylo nucleosides.20 Studies are also
reported using commercial 30-tritylamino-20,30-dideoxy purine
nucleosides, when exocyclic amino protection involved either
transient protection using TMS-chloride or peracylation/hydroly-
sis.23 In either case, some undesired side products were obtained.
The reactivity of nucleosides is quite sensitive to the substituent
groups and we had to consider appropriate changes in the route to
get the desired products in good yields. To get clean reaction
products, we decided to use 50-O-TBDMS protection as for the py-
rimidine derivatives. The 30-amino-20,30-dideoxyadenosine 9a
(Scheme 2) was converted to 30-tritylamino derivative by treatment
with trityl chloride in pyridine followed by silylation of 50-OH with
TBDMSCl to get the desired 30-and 50-protected derivative 10.
Subsequent protection of the exocyclic amino functionality with
N-benzoyl tetrazole at higher temperature24 afforded 11a in good
yields. The protection of exocyclic amino group proved to be chal-
lenging as extensive depurination was encountered even at ꢀ5 to
ꢀ10 ꢁC when benzoyl chloride was used as the acylating reagent.
Benzoylation of 30-NH-trityl group was not observed under these
conditions. Compound 11a was then desilylated to get the free
hydroxyl function at 50 position in 11b, which was further subjected
to oxidation using TEMPOeBAIB conditions to afford corresponding
acid 14 (27% overall yield). The 30-amino-20,30-dideoxyguanosine
9b (Scheme 2) was converted to 30-tritylamino derivative by
treatment with trityl chloride followed by treatment with TBDMSCl
to afford 30-and 50-protected derivative 12. Exocyclic amino group
was protected with isobutyryl chloride in pyridine to get 13a. Re-
moval of the 50-silyl protecting group in 13a gave compound 13b,
which on oxidation with TEMBOeBAIB afforded 40-carboxylic acid
derivative 15 (37% overall yield) (Scheme 2). All the purified com-
pounds in Schemes 1 and 2 were characterized by 1H, 13C and mass
spectrometry analysis and by comparing with the known com-
towards stability of glycosidic bond. With this in mind, the first
building block was modified with succinate ester linker at its 50
position and was loaded on the resin through an amide bond. The
resulting ester bond at 50 position can be hydrolyzed under basic
condition after oligomer synthesis. We used both CPG20 and
MBHA21 resins for solid phase synthesis. The MBHA resin, which
has better swelling properties gave much higher yield compared
(coupling yield w80% at each step) to the synthesis when CPG was
used. The coupling efficiencies at each step could be monitored
using Kaiser test26 or by measurement of trityl cation. Thus, the
succinate modified monomer20 16 was loaded on the MBHA resin
using solid phase peptide coupling protocol (Scheme 3).21 Tetra-
meric sequence was assembled using repetitive cycles, purified
(21% isolated yield) and analyzed by mass spectrometry using ESI-
MS technique.
3. Conclusions
In conclusion, the synthesis of all four protected natural nucle-
oside based b-amino acids was accomplished using simple TEMPO/
BAIB method and we further tested the synthesis of a model tet-
rameric nucleobase sequence 50-MeC(gly)A(gly)MeC(gly)T 18 where
MeC, A and T are nucleoside-
b
-amino acids corresponding to the 5-
methyl-cytosine, adenine and thymine, and glycine is used as the
alternating -amino acid. Synthesis of longer PNA needs small
a
changes in our protocols. Current studies are going on for opti-
mizing condition for solid phase synthesis for improving coupling
reactions and yield of the final oligomers so that the synthesis of
longer oligomers can be achieved in greater purity.
4. Experimental section
4.1. General
pounds. The new compounds b-amino acid monomers 4, 8, 14 and
15 were further characterized by HRMS analysis.
All the reagents were purchased from SigmaeAldrich and used
without purification. 30-Amino purine nucleosides were purchased
from Metkinen Chemistry, Finland. All solvents used were dried and
distilled according to standard protocols. Analytical TLCs were per-
formed on Merck 5554 silica 60 aluminium sheets. Column chro-
matography was performed for purification of compounds on silica
gel (60e120 mesh, Merck). For acid-sensitive (trityl-containing)
compounds, the column packed and equilibrated with 0.5% trie-
thylamine. TLCs were performed using dichloromethaneemethanol
or petroleum ethereethyl acetate solvent systems. Visualization
These trityl protected nucleoside amino acids were stored in
refrigerator with trace amounts of pyridine. Synthesis of a model
tetrameric nucleoside sequence 50-MeC(gly)A(gly)MeC(gly)T (18)
was then planned using synthesized monomeric units and trityl
glycine (17).25 Choice of the resin for solid phase synthesis was
influenced by the swelling properties of the resin and avoiding
harsh acidic deprotection conditions considering sensitivity of the
glycosidic bond towards acid. Amino functionalized resin has to be
selected such that the final cleavage conditions are compatible
Scheme 2. Synthesis of protected adenine/guanine-b-amino acids.

S. Bagmare et al. / Tetrahedron 69 (2013) 1210e1216
1213
Scheme 3. Schematic representation of solid phase synthesis of (
aþb amino acid) tetramer.
was accomplished with UV light and/or by spraying with perchloric
acid reagent and heating. The analytical data, such as melting point
and specific rotation was obtained for the new compounds 4, 8, 14
and 15 and these were further characterized using IR, 1H and 13C
NMR spectra and HRMS 1H and 13C NMR spectra were obtained
using Bruker ACF 200 (200 MHz) or 400 (400 MHz) spectrometers
5.41 mmol) was added. Reaction was stirred at RT for 1 h. THF was
removed in vacuo and residue dissolved in dichloromethane,
washed with water. Organic layer was dried over Na2SO4 and
concentrated. Compound was purified by column chromatography
(30% EtOAc/petroleum ether) to afford 30-tritylamino-30-deoxy-
thymidine 3 (1.49 g, 90%) as a white foam.1H NMR (200 MHz,
CDCl3): 8.72 (br s, 1H), 7.54 (m, 6H), 7.14e7.33 (m, 10H), 6.02 (dd,
J¼6.57, 6.19 Hz, 1H), 3.60e3.87 (m, 3H), 3.32 (m, 1H), 2.26 (br s, 1H),
1.81 (s, 3H), 1.26e1.52 (m, 2H); 13C NMR (50 MHz, CDCl3): 163.8,
150.3, 146.0, 135.9, 128.5, 128.0, 126.7, 110.8, 86.6, 84.5, 71.1, 61.9,
53.2, 39.7, 12.4. MS (ESI) m/z 506 (MþþNa, 100).
and chemical shifts are reported on d scale in parts per million (ppm)
with the solvent indicated as the internal reference. The IR spectra
were recorded on a Perkin Elmer Spectrum one-FT-IR spectropho-
tometer in chloroform.1H NMR data are reported in the order of
chemical shift, multiplicity (s, singlet; d, doublet; t, triplet; br, broad;
br s, broad singlet; m, multiplet and/or multiple resonance), number
of protons. Mass spectra were recorded on Thermo Finnigan Sur-
veyor MS-Q spectrometer (Mass spectra for tetramer was recorded
on Aquity Ultra performance LC-MS, Waters), while HRMS was
recorded by using a Micromass Q-Tof micro (YA-105) spectrometer
and Thermo Scientific Q-exactive mass spectrometer. Specific ro-
tation was recorded on BellinghamþStanley Ltd, ADP 220 polarim-
eter and the concentration (c) is expressed in gram per 100 mL.
4.4. (10R, 30S, 40R)-30-Azido-50-O-(tert-butyldimethylsilyl)-5-
methyl-30-deoxy-4-(1,2,4-triazol-1-yl)thymidine (5)
Triethylamine (10.95 mL, 78.94 mmol) was added drop wise
over a portion of 10 min to a stirred mixture 1,2,4-triazole (5.97 g,
86.61 mmol) and phosphorus oxychloride (1.83 mL, 19.68 mmol) in
60 mL CH3CN at 0 ꢁC. The solution of compound 1 (3 g, 7.87 mmol)
in 15 mL dry CH3CN was then added drop wise and reaction mix-
ture was allowed to stir at room temperature for 2.5 h. The solvent
was evaporated in vacuo. The resulting brown solid was dissolved
in ethyl acetate (100 mL). Organic layer washed with saturated
NaHCO3 solution followed by water, dried over Na2SO4 and con-
centrated to afford crude product 5 (3.26 g, 96%) as a white solid,
which was used immediately for next reaction.
4.2. (10R, 30S, 40R)-30-Amino-50-O-(tert-butyldimethylsilyl)-30-
deoxythymidine (2)
A mixture of 30-azido-50-O-(tert-butyldimethylsilyl)-30-deoxy-
thymidine 1 (3 g, 7.87 mmol) and 10% palladium on charcoal (0.3 g)
in 30 mL methanol/ethyl acetate mixture (1:1, v/v) was stirred
under hydrogen atmosphere (60 psi) at room temperature for 4 h.
The reaction mixture was filtered through Celite and the solvent
was evaporated in vacuo to give 30-amino-50-O-(tert-butyldime-
thylsilyl)-30-deoxythymidine 2 (2.37 g, 85%) as white foam.1H NMR
(200 MHz, CDCl3): 7.51 (s, 1H), 6.26 (t, J¼6.07 Hz, 1H), 3.90 (m, 2H),
3.72 (m, 1H), 3.64 (m, 1H), 2.19 (m, 2H), 1.91 (s, 3H), 0.93 (s, 9H), 0.11
(s, 6H); 13C NMR (50 MHz, CDCl3): 164.1, 150.5, 135.4, 110.5, 87.4,
84.3, 62.9, 51.4, 41.7, 25.8, 18.3, 12.5, ꢀ5.42, ꢀ5.47; MS (ESI) m/z 356
(Mþþ1, 7), 378 (MþþNa, 100).
4.5. (10R, 30S, 40R)-N4-Benzoyl-30-azido-50-O-(tert-butyldime-
thylsilyl)-5-methyl-20,30-dideoxycytidine (6)
Compound 5 (3.26 g, 7.54 mmol) was dissolved in 1,4-dioxane
(30 mL) and 15 mL concentrated aqueous ammonia was added.
Reaction mixture was allowed to stir at rt for 2 h. After completion
of the reaction, solvents were removed in vacuo. The residue was
dissolved in dichloromethane and washed with water. Organic
layer was dried over Na2SO4 and concentrated to afford a quanti-
tative yield of 30-azido-50-O-(tert-butyldimethylsilyl)-5-methyl-
20,30-dideoxycytidine (2.56 g, 6.73 mmol) as a colourless foam. This
crude material was then azeotroped with pyridine and redissolved
in pyridine (15 mL), and cooled externally at 0 ꢁC. To this solution,
benzoyl chloride (0.938 mL, 8.08 mmol) was added drop wise and
the mixture was then allowed to attain rt and stir additionally for
14 h at rt. The reaction was quenched with 2 N NH3 (5 mL) and was
further stirred for 30 min. The solvent was evaporated in vacuo and
residue was dissolved in ethyl acetate, washed with saturated
4.3. (10R, 30S, 40R)-30-Tritylamino-30-deoxythymidine (3)
Compound 2 (2 g, 5.63 mmol) was dissolved in dry pyridine
(10 mL) and to it trityl chloride (2.04 g, 7.32 mmol) and catalytic
amount of DMAP was added. Reaction mixture was allowed to stir
at room temperature for 8 h. Solvent was removed under reduced
pressure to afford crude 50-O-(tert-butyldimethylsilyl)-30-trityla-
mino-30-deoxythymidine (3.03 g, 90%), which was then dissolved
in 15 mL anhydrous THF and to it 1 N TBAF in THF (5.41 mL,

1214
S. Bagmare et al. / Tetrahedron 69 (2013) 1210e1216
NaHCO3 solution, dried over Na2SO4 and concentrated in vacuo.
This crude was purified on column chromatography (25% EtOAc/
petroleum ether) to afford 6 (2.79 g, 86%) as a colourless foam.1H
NMR (CDCl3, 200 MHz): 8.33 (m, 2H), 7.66 (s, 1H), 7.44e7.58 (m,
3H), 6.23 (t, J¼6.31 Hz, 1H), 4.27 (m, 1H), 3.90e4.02 (m, 2H),
3.80e3.86 (m, 1H), 2.49e2.58 (m, 1H), 2.30 (m, 1H), 2.12 (s, 3H),
0.95 (s, 9H), 0.15 (s, 6H).
in vacuo. Product was purified by column chromatography
(3% MeOH/CH2Cl2) to afford 50-O-(tert-butyldimethylsilyl)-30-tri-
tylamino-20,30-dideoxyadenosine 10 (3.19 g, 78%). 1H NMR
(200 MHz, CDCl3): 8.30 (s, 1H), 7.91 (s, 1H), 7.51e7.56 (m, 6H),
7.17e7.32 (m, 9H), 6.29 (dd, J¼6.19, 5.93 Hz, 1H), 5.71 (br s, 2H), 3.87
(m, 1H), 3.60e3.82 (m, 2H), 3.44 (m, 1H), 2.02 (br s, 1H), 1.71e1.79
(m, 2H), 0.82 (s, 9H), ꢀ0.03 (s, 3H), ꢀ0.04 (s, 3H); 13C NMR (50 MHz,
CDCl3): 155.3, 152.8, 149.4, 146.1, 138.4, 128.6, 128.0, 126.6, 119.4,
87.1, 83.6, 71.2, 63.6, 54.7, 41.1, 25.8, 18.3, ꢀ5.4; MS (ESI) m/z 607
(Mþþ1,10), 629 (MþþNa,100); HRMS (ESI): calcd for C35H42N6O2Si:
607.3211, found: 607.3205.
4.6. (10R, 30S, 40R)-N4-Benzoyl-5-methyl-30-tritylamino-20, 30-
dideoxycytidine (7)
Compound 6 (2.7 g, 5.57 mmol) was dissolved in 15% Et3N in dry
pyridine (10 mL) and H2S gas was bubbled into the solution at 0 ꢁC
for 10 min. Reaction mixture was then stirred at room temperature
for 30 min. Solvent was removed in vacuo and the residue was
purified by column chromatography (5% MeOH/CH2Cl2) to afford in
90% yield of N4-benzoyl-5-methyl-30-amino-50-O-(tert-butyldime-
thylsilyl)-20,30-dideoxycytidine (2.29 g, 4.99 mmol). The following
amounts were used in subsequent tritylation reaction; solvent,
anhydrous pyridine, trityl chloride (1.58 g, 5.67 mmol) and catalytic
amount of DMAP. Reaction mixture was allowed to stir at room
temperature for 8 h. Solvent was removed in vacuo and the residue
was purified by column chromatography (30% EtOAc/petroleum
ether) to afford N4-benzoyl-5-methyl-30-tritylamino-50-O-(tert-
butyldimethylsilyl)-20,30-dideoxycytidine (2.90 g, 78%), which was
then subjected for desilylation. Following amounts were used in
desilylation reaction; N4-benzoyl-5-methyl-30-tritylamino-50-O-
(tert-butyldimethylsilyl)-20,30-dideoxycytidine (2.90 g, 4.14 mmol),
15 mL anhydrous THF as a solvent and TBAF 1 N in THF (6.21 mL,
6.21 mmol). Reaction was stirred at rt for 1 h. THF was removed in
vacuo and residue dissolved in dichloromethane, washed with
water. Organic layer was dried over Na2SO4 and concentrated.
Compound was purified by column chromatography (35% ethyl
acetate/petroleum ether) to give product 7 (2.18 g, yield 90%) as
a white solid. 1H NMR (200 MHz, CDCl3): 8.31 (s, 1H), 8.28 (m, 1H),
7.39e7.55 (m, 10H), 7.18e7.39 (m, 10H), 6.03 (dd, J¼6.70, 5.30 Hz,
1H), 3.66e3.93 (m, 3H), 3.32 (q, J¼13.26, 6.94 Hz, 1H), 2.03 (s, 3H),
1.73 (br s, 1H), 1.25e1.42 (m, 2H); 13C NMR (50 MHz, CDCl3): 179.4,
159.6, 147.8, 146.0, 137.2, 137.0, 132.4,129.8,129.5, 128.1, 126.8,111.6,
86.7, 85.4, 71.1, 61.8, 52.9, 40.3, 13.5; MS (ESI) m/z 587 (Mþþ1, 15),
609 (MþþNa, 100).
4.9. (10R, 30S, 40R)-N6-Benzoyl-50-O-(tert-butyldimethylsilyl)-
30-tritylamino-20,30-dideoxyadenosine (11a)
50-O-(tert-Butyldimethylsilyl)-30-tritylamino-20,30-dideox-
yadenosine 10 (3.19 g, 5.26 mmol) dissolved in dry acetonitrile
(40 mL). To it N-benzoyl tetrazole (1.82 g, 9.89 mmol) and 4-
dimethylaminopyridine (0.604 g, 5.25 mmol) was added. The re-
action mixture is allowed to stir at 65 ꢁC for 90 min. After com-
pletion of reaction, solvent was removed in vacuo and residue
dissolved in dichloromethane. Organic layer washed with saturated
NaHCO3 solution, dried over Na2SO4 and concentrated to give crude
product. This crude was purified by column chromatography (1%
MeOH/CH2Cl2) to afford 11a (2.42 g, 65%) as a white solid. 1H NMR
(200 MHz, CDCl3): 9.15 (br s, 1H), 8.75 (s, 1H), 8.13 (s, 1H), 8.00 (d,
J¼6.70 Hz, 2H), 7.49e7.59 (m, 8H), 7.15e7.32 (m, 10H), 6.36 (dd,
J¼6.06, 5.69 Hz,1H), 3.64e3.91 (m, 3H), 3.51 (m, 1H), 2.08 (br s, 1H),
1.64e1.88 (m, 2H), 0.81 (s, 9H), ꢀ0.02 (s, 3H), ꢀ0.03 (s, 3H); 13C
NMR (50 MHz, CDCl3): 164.6, 152.4, 151.1, 149.2, 146.0, 140.9, 133.6,
132.6, 128.7, 128.5, 128.0, 127.8, 126.6, 122.8, 87.2, 83.9, 71.1, 63.4,
54.5, 41.0, 25.8, 18.3, ꢀ5.45, ꢀ5.53; MS (ESI) m/z 711 (Mþþ1, 100),
733 (MþþNa, 70).
4.10. (10R, 30S, 40R)-N6-Benzoyl-30-tritylamino-20,30-dideox-
yadenosine (11b)
Compound 11a (2.4 g, 3.38 mmol) dissolved in anhydrous THF
(15 mL) and 1 N TBAF in THF (5.07 mL, 5.07 mmol) was added.
Reaction was stirred at rt for 1 h. THF was removed in vacuo and the
residue was dissolved in dichloromethane, washed with water and
then brine. Organic layer was dried over Na2SO4, concentrated in
vacuo and purified by column chromatography (3% MeOH/CH2Cl2)
to get product 11b (1.59 g, 79%) as a white solid. 1H NMR (200 MHz,
CDCl3): 9.26 (br s, 1H), 8.61 (s, 1H), 8.03 (m, 3H), 7.49e7.56 (m, 8H),
7.20e7.33 (m, 10H), 6.24 (dd, J¼7.45, 6.31 Hz, 1H), 3.68e3.78 (m,
3H), 3.55 (d, J¼11.32 Hz, 1H), 2.30 (m, 1H), 2.04 (br s, 1H), 1.76 (m,
1H); MS (ESI) m/z 597 (Mþþ1, 20), 619 (MþþNa, 100).
4.7. General procedure for synthesis of 30-tritylamino-20, 30-
dideoxy-purine nucleosides23
Et3N (1.15 mL, 8.26 mmol) was added to a stirred solution of 30-
amino-20,30-dideoxy purine nucleoside 9a/9b (7.51 mmol) in dry
DMF (80 mL). The reaction mixture was stirred at 60 ꢁC for 30 min.
Trityl chloride (2.24 g, 8.26 mmol) was added to reaction mixture in
two portions and allowed to stir for 3 h at room temperature. The
reaction mixture poured on 120 mL cold water and the flask kept in
freezer for 2 h. The obtained precipitate was filtered and washed
thoroughly with cold water. White solid desiccated overnight to
give crude 30-tritylamino-20,30-dideoxyadenosine (3.32 g, 90%)/30-
tritylamino-20,30-dideoxyguanosine (4.13 g, 92%), which was used
as such for next reaction.
4.11. (10R, 30S, 40R)-50-O-(tert-Butyldimethylsilyl)-30-trityla-
mino-20,30-dideoxyguanosine (12)
30-Tritylamino-20,30-dideoxyguanosine (4.13 g, 8.12 mmol),
TBDMSCl (1.59 g, 10.56 mmol) and imidazole (1.65 g, 24.36 mmol)
was suspended in 15 mL dry DMF and the reaction was allowed to
stir overnight at rt. DMF was removed in vacuo and the residue was
dissolved in ethyl acetate (150 mL). Organic layer was washed with
water, dried over Na2SO4 and concentrated in vacuo The crude
product was purified by column chromatography (3% MeOH in
DCM) to afford 12 (4.23 g, 84%) as a white solid. 1H NMR (200 MHz,
DMSO-d6): 10.64 (br s, 1H), 7.46e7.50 (m, 7H), 7.14e7.32 (m, 9H),
6.45 (br s, 1H), 5.89e5.95 (t, J¼6.19 Hz, 1H), 3.74e3.80 (m, 1H),
3.46e3.68 (m, 2H), 3.34 (m, 1H, merged with H2O), 1.45e1.58 (m,
1H), 1.23e1.30 (m, 1H), 0.79 (s, 9H), ꢀ0.07 (s, 6H); 13C NMR
(50 MHz, CDCl3): 159.1, 153.3, 151.3, 146.2, 135.4, 128.6, 128.0, 126.7,
117.0, 87.0, 83.0, 71.1, 63.5, 54.6, 40.3, 25.9, 18.4, ꢀ5.40, ꢀ5.52; MS
4.8. (10R, 30S, 40R)-50-O-(tert-Butyldimethylsilyl)-30-trityla-
mino-20,30-dideoxyadenosine (10)
30-Tritylamino-20,30-dideoxyadenosine (3.32 g, 6.74 mmol),
TBDMSCl (1.32 g, 8.77 mmol) and imidazole (1.37 g, 20.22 mmol)
was suspended in 15 mL dry DMF and the reaction was allowed to
stir overnight at rt. DMF was removed in vacuo and the residue was
redissolved in ethyl acetate (150 mL). Organic layer was washed
with water followed by brine, dried over Na2SO4 and concentrated

S. Bagmare et al. / Tetrahedron 69 (2013) 1210e1216
1215
(ESI) m/z 623 (Mþþ1, 10), 645 (MþþNa, 100), 242 (30); HRMS (ESI):
4.16. (10R, 30S, 40S)-20,30-Dideoxy-30-tritylamino-N4-benzoyl-5-
calcd for C35H42N6O3Si: 623.3160, found: 623.3145.
methylcytidin-40-carboxylic acid (8)
4.12. Synthesis of 50-O-(tert-butyldimethylsilyl)-N2-iso-
This compound is obtained as a white solid (0.54 g, 90%) starting
from compound 7 by general procedure described above. Rf (10%
butyryl-30-tritylamino-20,30-dideoxyguanosine (13a)
MeOH/CH2Cl2) 0.59; mp: 204e207 ꢁC; [
a]
26 þ115.9 (c 1.07, MeOH);
D
1H NMR (400 MHz, DMSO-d6þdrop of D2O): 8.84 (br s,1H, NH), 8.12
(s, 2H, aromatic), 7.57 (m,1H, aromatic), 7.43e7.47 (m, 9H, aromatic),
7.23e7.27 (m, 6H, aromatic), 7.13e7.16 (m, 3H, aromatic), 5.96 (t,
J¼4.77 Hz,1H, H10), 4.22 (d, J¼5.77 Hz,1H, H40), 3.05 (m,1H, H30),1.89
(s, 3H, CH3), 1.40 (m, 1H, H200), 1.06 (m, 1H, H20); 13C NMR (100 MHz,
DMSO-d6þdrop of D2O): 174.4, 173.7, 160.4, 147.9, 146.6, 142.7, 136.6,
132.7,129.4,128.9,128.0,127.0,126.4,108.7, 87.3, 86.5, 70.8, 56.5, 39.1,
13.5;IR (nmax, cmꢀ1)(CHCl3):3619, 3318, 3018, 2976, 2897,1703,1652,
1215; HRMS(ESI): calcd for C36H32N4O5: 601.2451, found: 601.2452.
To solution of compound 12 (4.23 g, 6.80 mmol) in anhydrous
pyridine (20 mL), triethylamine (0.95 mL, 6.80 mmol) was added
and the mixture was stirred for 15 min. To this reaction mixture iso-
butyryl chloride (1.43 mL, 13.6 mmol) was added dropwise and
reaction was allowed to stir for 1 h. Pyridine was removed in vacuo
and reaction mixture dissolved in ethyl acetate, washed with sat-
urated NaHCO3 solution, then water. Organic layer dried over
Na2SO4 and concentrated to give crude product. The product was
purified by column chromatography (1% MeOH/CH2Cl2) to afford
13a (3.53 g, 75%) as a white solid. 1H NMR (200 MHz, CDCl3): 7.58 (s,
1H), 7.52 (d, J¼7.93 Hz, 6H), 7.28 (t, J¼7.93 Hz, 6H), 7.20 (t,
J¼7.32 Hz, 3H), 5.97 (dd, J¼6.72, 4.88 Hz, 1H), 3.83 (m, 1H),
3.67e3.77 (ABX, JAB¼11.30 Hz, 2H), 3.47 (m, 1H), 2.57e2.59 (septet,
J¼6.41 Hz, 1H), 1.68e1.71 (m, 1H), 1.54e1.57 (m, 1H), 1.21e1.25 (m,
6H) 0.80 (s, 9H), ꢀ0.05 (s, 3H), ꢀ0.06 (s, 3H); 13C NMR (50 MHz,
CDCl3): 178.1, 155.4, 147.7, 147.2, 146.1, 136.5, 128.6, 128.1, 127.9,
126.7, 121.1, 87.1, 83.0, 71.1, 63.4, 54.6, 40.4, 36.5, 25.9, 18.9, 18.4,
ꢀ5.45, ꢀ5.55; MS (ESI) m/z 693 (Mþþ1, 10), 715 (MþþNa, 100).
4.17. (10R, 30S, 40S)-20,30-Dideoxy-30-tritylamino-N6-benzoyl-
adenin-40-carboxylic acid (14)
This compound is obtained as a white solid (0.46 g, 76%) starting
from compound 11b by general procedure described above. Rf (10%
26
MeOH/CH2Cl2) 0.52; mp: 262e270 ꢁC; [
a
]
þ5.1 (c 0.79, MeOH);
D
1H NMR (400 MHz, DMSO-d6þdrop of D2O): 8.63 (s, 1H, C2eH),
8.46 (s, 1H, C8eH), 7.97 (d, J¼7.28 Hz, 2H, aromatic), 7.66 (m, 1H,
aromatic), 7.51e7.55 (m, 2H, aromatic), 7.44e7.46 (m, 6H, aro-
matic), 7.24 (m, 6H, aromatic) 7.07e7.16 (m, 3H, aromatic),
6.35e6.38 (dd, J¼6.27, 4.52 Hz,1H, H10), 4.30 (d, J¼6.27 Hz,1H, H40),
3.47 (dd, J¼13.05, 6.53 Hz, 1H, H30), 1.62e1.69 (m, 1H, H200),
1.31e1.34 (m,1H, H20); 13C NMR (100 MHz, DMSO-d6þdrop of D2O):
173.0, 165.5, 151.7, 151.5, 150.1, 146.2, 142.4, 133.3, 132.4, 128.58,
4.13. (10R, 30S, 40R)-N2-Isobutyryl-30-tritylamino-20,30-dideox-
yguanosine (13b)
This compound is obtained in 91% yield via desilylation of 13a as
described in the synthesis of 11b. Compound was purified by col-
umn chromatography (using 3% methanol in DCM) to afford 13b as
white solid. 1H NMR (200 MHz, CDCl3): 7.60 (s, 1H), 7.52 (d,
J¼7.93 Hz, 6H), 7.18e7.38 (m, 9H), 5.98 (dd, J¼6.44, 6.57 Hz, 1H),
3.85e3.96 (m, 2H), 3.53e3.64 (m, 2H), 2.68 (septet, J¼6.95 Hz, 1H),
2.06 (br s, 1H), 1.83 (m, 1H), 1.52 (m, 1H), 1.20 (dd, J¼6.82 and
3.28 Hz, 6H); MS (ESI) m/z 579 (Mþþ1, 10), 601 (MþþNa, 100).
128.50, 128.4, 127.8, 126.3, 125.5, 84.6, 83.5, 70.7, 58.1, 37.3; IR (nmax
,
cmꢀ1
)
(CHCl3): 3619, 3399, 3018, 2976, 2896, 1707, 1216;
HRMS(ESI): calcd for C36H30N6O4: 611.2407, found: 611.2404.
4.18. (10R, 30S, 40S)-20,30-Dideoxy-30-tritylamino-N2-isobutyryl-
guanosin-40-carboxylic acid (15)
Analogous method was used for the synthesis of this compound
starting from compound 13b. In this case, after completion of the
reaction mixture was directly diluted with ethyl acetate and
extracted with water. The organic phase was dried over anhydrous
Na2SO4 and concentrated. The resulting residue was triturated se-
quentially with diethyl ether and acetone, filtered and dried in
4.14. General procedure for synthesis of nucleoside-
acids
b-amino
(Diacetoxyiodo)benzene (2.2 mmol) and TEMPO (0.25 mmol),
were added to the solution of protected 20,30-deoxy-30-tritylamino
nucleosides 3/7/11b/13b (1 mmol) in 5 mL CH3CN/H2O mixture
(1:1). The reaction mixture was stirred at 25 ꢁC for 3 h. Acetonitrile
was removed completely in vacuo. The crude product was extracted
with ethyl acetate. The organic phase was dried over anhydrous
Na2SO4 and concentrated. The resulting residue was triturated se-
quentially with diethyl ether and acetone, filtered and dried in
vacuo.
vacuo to get white solid (0.44 g, 75%). Rf (10% MeOH/CH2Cl2) 0.51;
26
mp: 235e238 ꢁC; [
a]
þ33.0 (c 1.75, MeOH); 1H NMR (400 MHz,
D
DMSO-d6þdrop of D2O): 8.49 (s, 1H, C8eH), 7.40 (d, J¼7.78 Hz, 6H,
aromatic), 7.10e7.23 (m, 10H, aromatic), 5.96e5.99 (dd, J¼5.52,
3.76 Hz, 1H, H10), 4.17 (d, J¼6.77 Hz, 1H, H40), 3.11e3.16 (m, 1H, H30),
2.72e2.75 (m, 1H, iBu-CH), 1.50e1.53 (m, 1H, H200), 1.08e1.12 (2d,
6H, iBu-CH3), 0.99 (m,1H, H20); 13C NMR (100 MHz, DMSO-d6þdrop
of D2O): 180.4, 174.0, 155.2, 147.8, 146.6, 138.83, 128.9, 128.1, 127.9,
4.15. (10R, 30S, 40S)-30-Deoxy-30-tritylaminothymidine-40-car-
boxylic acid (4)
126.6, 120.3, 85.2, 84.8, 70.7, 57.0, 39.5, 35.0, 19.2, 19.1; IR (nmax,
cmꢀ1) (CHCl3): 3393, 3016, 2975, 2895, 1685, 1609, 1215; HRMS
(ESI): calcd for C33H32N6O5: 593.2512, found: 593.2510.
This compound is obtained as a white solid (0.45 g, 91%) starting
from compound 3 by general procedure described above. Rf (10%
4.19. Pre-loading of MBHA resin
25
MeOH/CH2Cl2) 0.51; mp: 214e217 ꢁC; [
a]
þ30.9 (c 1.03, MeOH);
D
1H NMR (400 MHz, DMSO-d6þdrop of D2O): 8.73 (s, 1H, H6),
7.42e7.44 (m, 6H, aromatic), 7.22e7.26 (m, 6H, aromatic), 7.12e7.16
(m, 3H, aromatic), 5.87 (dd, J¼5.52,5.02 Hz, 1H, H10), 4.08 (d,
J¼5.78 Hz, 1H, H40), 2.89 (dd, J¼11.54, 5.52 Hz, 1H, H30), 1.64 (s, 3H,
CH3), 1.21 (m, 1H, H200), 1.00 (m, 1H, H20); 13C NMR (100 MHz,
DMSO-d6þdrop of D2O): 173.9, 164.0, 150.1, 146.5, 138.5, 128.7,
MBHA resin (60 mg, 1.75 mmol/g, 0.024 mmol) was washed and
swelled in DCM for 1 h in solid phase flask. The resin was drained
and washed with DMF. To a solution of N4-benzoyl-5-methyl 30-
tritylamino-20,30-dideoxycytidine-50-succinylate20a 16 (16.19 mg,
0.024 mmol) in 500
TBTU (9.63 mg, 0.028 mmol) was added. To this solution HOBt
(3.24 mg, 0.024 mmol) in 100 L of DMF was added. This activated
mL DMF, DIPEA (12.47 mL, 0.072 mmol) and
127.6, 126.1, 107.5, 86.2, 85.6, 70.7, 56.6, 38.9, 12.3; IR (nmax, cmꢀ1
)
m
(CHCl3): 3620, 3399, 3018, 2975, 2896, 1690, 1215; HRMS (ESI):
calcd for C29H27N3O5: 520.1848, found: 520.1846.
monomer solution was added to the resin and the suspension was
allowed for gentle shaking for 12 h. Resin was drained and washed

1216
S. Bagmare et al. / Tetrahedron 69 (2013) 1210e1216
Annu. Rev. Pharmacol. Toxicol. 2010, 50, 259; (e) Kole, R.; Krainer, A. R.; Altman,
S. Nat. Rev. Drug Discovery 2012, 11, 125.
with DMF (2ꢂ), DCM (2ꢂ) and pyridine (2ꢂ). Unreacted amino
groups capped with 10% acetic anhydride in pyridine for 1 h. The
nucleoside loading on solid support was determined by spectro-
photometric determination of the concentration of trityl cation at
410 nm released after detritylation using 3% TCA in DCM. Calculated
loading value for resin using molar extinction coefficient27
2. (a) Micklefield, J. Curr. Med. Chem. 2001, 8, 1157; (b) Kurreck, J. Eur. J. Biochem.
2003, 270, 1628; (c) Turner, J. J.; Fabani, M.; Arzumanov, A. A.; Ivanova, G.; Gait,
M. J. Biochim. Biophys. Acta 2006, 1758, 290; (d) Kumar, V. A.; Ganesh, K. N. Curr.
Top. Med. Chem. 2007, 7, 715; (e) Prakash, T. P. Chem. Biodiversity 2011, 8, 1616;
(f) Kumar, V. A. Eur. J. Org. Chem. 2002, 2021.
3. (a) Dominski, Z.; Kole, R. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 8673; (b) Sazani, P.;
Kole, R. J. Clin. Invest. 2003, 112, 481.
4. (a) Tanui, P.; Kullberg, M.; Song, N.; Chivate, Y.; Rozners, E. Tetrahedron 2010, 66,
4961; (b) Rozners, E.; Katkevica, D.; Bizdena, E.; Stromberg, R. J. Am. Chem. Soc.
2003, 125, 12125; (c) Rozners, E.; Liu, Y. J. Org. Chem. 2005, 70, 9841; (d) Xu, Q.;
Katkevica, D.; Rozners, E. J. Org. Chem. 2006, 71, 5906.
35,300 Mꢀ1 cmꢀ1 at 410 nm for trityl cation was 35.2
which was good enough for next synthesis.
mmol/g,
4.20. Solid phase synthesis of model tetramer 18
5. De Mesmaeker, A.; Lesueur, C.; Btvikrre, M. O.; Waldner, A.; Fritsch, V.; Wolf, R.
M. Angew. Chem., Int. Ed. 1996, 35, 2790.
6. (a) Pallan, P. S.; von Matt, P.; Wilds, C. J.; Altmann, K.-H.; Egli, M. Biochemistry
2006, 45, 8048; (b) Wilds, C. J.; Minasov, G.; von Matt, P.; Altmann, K. H.; Egli,
M. Nucleosides, Nucleotides Nucleic Acids. 2001, 20, 991.
Synthesis of a model tetrameric nucleoside sequence 50-C(gly)
A(gly)C(gly)T was then undertaken using protected nucleoside
b
amino acid monomeric units and trityl glycine (17) on preloaded
7. (a) Gogoi, K.; Gunjal, A. D.; Phalgune, U. D.; Kumar, V. A. Org. Lett. 2007, 9, 2697;
(b) Chakraborty, T. K.; Koley, D.; Prabhakar, S.; Ravi, R.; Kunwar, A. C. Tetrahedron
2005, 40, 9506.
MBHA resin. The following synthetic condition were used,
(i) Deprotection of trityl group using 3% trichloroacetic acid
(3 minꢂ3) (ii) Neutralization using 5% DIPEA in DCM (3minꢂ2) (iii)
coupling using 3 equiv of monomer, 9 equiv DIPEA, 3 equiv TBTU and
1.5 equiv of HOBt as activator with respect to loading value of resin.
All are premixed in DMF prior to addition to resin. This suspension
was added to the resin. Reaction time 5 h. (iv) Capping of unreacted
amino groups using 10% acetic anhydride in pyridine (10 minꢂ2).
After the synthesis was complete, the oligomer 18 was cleaved
from solid support using aqueous methanolic ammonia at 55 ꢁC for
8 h. The terminal trityl group was deprotected before cleavage
using 3% TCA in DCM or after cleavage using 80% aqueous acetic
acid. Oligomer was extracted from resin in 20% methanol in water.
Analytical purification was achieved on reverse phase C18 column
using 5% acetonitrile in 0.1 M TEAA buffer as eluent system and
monitored at 260 nm (retention time for tetramer 3.54 min) and
8. (a) Nielsen, P. E.; Egholm, M.; Berg, R. H.; Buchardt, O. Science 1991, 254, 1497;
(b) Nielsen, P. E.; Egholm, M.; Buchardt, O. Bioconjugate Chem. 1994, 5, 3; (c)
Egholm, M.; Buchardt, O.; Nielsen, P. E.; Berg, R. H. J. Am. Chem. Soc. 1992, 114,
1895; (d) Uhlmann, E.; Peyman, A.; Breipohl, G.; Will, D. W. Angew. Chem., Int.
Ed. 1998, 37, 2796; (e) Kumar, V. A.; Ganesh, K. N. Acc. Chem. Res. 2005, 38, 404;
(f) Yeh, J. I.; Shivachev, B.; Rapireddy, S.; Crawford, M. J.; Gil, R. R.; Du, S.;
Marcela, M.; Ly, D. H. J. Am. Chem. Soc. 2010, 132, 10717; (g) Corradini, R.; Sforza,
S.; Tedeschi, T.; Totsingan, F.; Manicardi, A.; Marchelli, R. Curr. Top. Med. Chem.
2011, 1535; (h) Eglund, E. A.; Appella, D. H. Angew. Chem., Int. Ed. 2007, 46, 1414;
(i) Sugiyama, T.; Imamura, Y.; Demizu, Y.; Kurihara, M.; Takano, M.; Kittaka, A.
Bioorg. Med. Chem. Lett. 2011, 21, 7317.
9. (a) Gait, M. J.; Jones, A. S.; Walker, R. T. J. Chem. Soc., Perkin Trans. 1 1974, 1, 1684;
(b) Coull, J. M.; Carlson, D. V.; Weith, H. L. Tetrahedron Lett. 1987, 28, 745; (c)
Madhuri, V.; Kumar, V. A. Org. Biomol. Chem. 2010, 8, 3734; (d) Magdalena, J.;
Fernandez, S.; Ferrero, M.; Gotor, V. Tetrahedron Lett. 1999, 40, 1787.
10. Linkletter, B. A.; Bruice, T. C. Bioorg. Med. Chem. 2000, 8, 1893.
11. Nielsen, P. E. In Peptide Nucleic Acids Protocols and Applications; Nielsen, P. E.,
Egholm, M., Eds.; Horizon Scientific: 1999.
12. Kumar, V. A.; Meena; Pallan, P. S.; Ganesh, K. N. Org. Lett. 2001, 3, 1269.
13. Worthington, R. J.; O0Rourke, P.; Morral, J. D.; Tan, T. H. S.; Mickelfield, J. Org.
Biomol. Chem. 2007, 5, 249.
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Kumar, V. A. Tetrahedron 2006, 62, 2321.
further characterized by ESI-mass spectrometry. Mass calcd for
þ1
C
46H58N20O15 1130.4391 ESI mass observed m/z for (C46H58N20
O
15
1129.7) (C46H58N20O15þ2, 565.22).
15. Vilaivan, T.; Lowe, G. J. Am. Chem. Soc. 2002, 124, 9326.
16. Suparpprom, C.; Srisuwannaket, C.; Sangvanich, P.; Vilaivan, T. Tetrahedron Lett.
Acknowledgements
2005, 46, 2833.
17. (a) Vilaivan, T.; Srisuwannaket, C. Org. Lett. 2006, 8, 1897; (b) Mansawat, W.;
S.B. thanks ICMR, New Delhi for financial support. M.V. and A.B.
thank CSIR for research fellowship. V.A.K. thanks ICMR (Indo-US
joint project, GAP 295826) for research grants.
ꢀ
ꢀ
Vilaivan, C.; Balazs, A.; Aitken, D. J.; Vilaivan, T. Org. Lett. 2012, 14, 1440.
18. (a) Gogoi, K.; Kumar, V. A. Chem. Commun. 2008, 706; (b) Efimov, A.; Buryakova, A.
A.;Chakhmakhcheva,O. G.Bioorg. Med.Chem. Lett.1998, 8,1013;(c)Chandrasekhar,
S.; Reddy, G. P. K.; Kiran, M. U.; Nagesh, Ch.; Jagadeesh, B. Tetrahedron Lett. 2008, 49,
2969; (d) Cosstick, R.; Davies, A.; Howarth, N. M.; Fisher, J.; Cosstick, R. Chem.
Commun. 2008, 585; (e) Threlfall, R.; Davies, A.; Howarth, N. M.; Cosstick, R.
Nucleosides, Nucleotides, Nucleic Acids 2006, 26, 611; (f) Cosstick, R.; Cosstick, R.
Nucleic Acids. Symp. Ser. 2008, 52, 313.
Supplementary data
1H, 13C and mass spectra of selected compounds in Schemes 1
and 2. HRMS spectra of new compounds 4, 8, 10, 12, 14 and 15.
HPLC and ESI and MALDI-TOF-TOF mass spectra of oligomer 18.
Supplementary data related to this article can be found online at
http://dx.doi.org/10.1016/j.tet.2012.11.028.
19. Bagmare, S.; D0Costa, M.; Kumar, V. A. Chem. Commun. 2009, 6646.
20. (a) Fearon, K. L.; Nelson, J. S. Curr. Protoc. Nucleic Acid Chem. 2001, 4.7.1; (b)
Divakar, K. J.; Reese, C. B. J. Chem. Soc., Perkin Trans. 1 1982, 1171.
21. Malkinson, J. P.; Falconer, R. A. Tetrahedron Lett. 2002, 43, 9549.
22. (a) Epp, J. B.; Widlanski, T. S. J. Organomet. Chem. 1999, 64, 293; (b) De Mico, A.;
Margarita, R.; Parlanti, L.; Vescovi, A.; Piancatelli, G. J. Org. Chem. 1997, 62, 6974.
23. Zielinska, D.; Pongracz, K.; Gryaznov, S. M. Tetrahedron Lett. 2006, 47, 4495.
24. Bhat, B.; Sanghvi, Y. S. Tetrahedron Lett. 1997, 38, 8811.
References and notes
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