New synthetic route to γ-mercaptomethyl PNA monomers

Article abstract of DOI:10.1080/00397910802219122

Peptide nucleic acids (PNAs) are oligonucleotide mimics widely used as antisense, antigene molecules, and biotechnological tools. Recently, several microarrays and other biosensors based on PNAs have been developed. The construction of PNA molecular beacons or light-up probes for DNA detection requires the labelling of the PNA moiety. Labels are usually attached at the C or N terminal end by a flexible linker or in the middle of a PNA sequence, substituting one PNA base with an artificial base or by attaching fluorophores to a modified PNA backbone. The need to develop simple protocols to label PNAs encouraged us to design a new procedure for the synthesis of γ-mercaptomethyl-modified PNA. Here we propose a new strategy for the synthesis of modified PNAs, bearing amino acid side chains. The synthesis is straightforward and is an improvement to the procedures reported so far, as it uses stable intermediates and proceeds with better yields. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910802219122

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New Synthetic Route to γ-
Mercaptomethyl PNA Monomers
Soccorsa Pensato a , Luca Domenico D'Andrea b ,
Carlo Pedone a & Alessandra Romanelli a
a University of Naples “Federico II,” Department
of Biological Science , School of Biotechnological
Sciences , Napoli, Italy
b CNR, Institute of Biostructure and Bioimaging ,
Napoli, Italy
Published online: 09 Sep 2008.
To cite this article: Soccorsa Pensato , Luca Domenico D'Andrea , Carlo Pedone
& Alessandra Romanelli (2008) New Synthetic Route to γ-Mercaptomethyl
PNA Monomers, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 38:15, 2499-2506, DOI:
10.1080/00397910802219122
To link to this article: http://dx.doi.org/10.1080/00397910802219122
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Synthetic Communications1, 38: 2499–2506, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802219122
New Synthetic Route to c-Mercaptomethyl PNA
Monomers
Soccorsa Pensato,1 Luca Domenico D’Andrea,2 Carlo Pedone,1
and Alessandra Romanelli1
1University of Naples ‘‘Federico II,’’ Department of Biological Science,
School of Biotechnological Sciences, Napoli, Italy
2CNR, Institute of Biostructure and Bioimaging, Napoli, Italy
Abstract: Peptide nucleic acids (PNAs) are oligonucleotide mimics widely used
as antisense, antigene molecules, and biotechnological tools. Recently, several
microarrays and other biosensors based on PNAs have been developed. The con-
struction of PNA molecular beacons or light-up probes for DNA detection
requires the labelling of the PNA moiety. Labels are usually attached at the C
or N terminal end by a flexible linker or in the middle of a PNA sequence,
substituting one PNA base with an artificial base or by attaching fluorophores to
a modified PNA backbone. The need to develop simple protocols to label PNAs
encouraged us to design a new procedure for the synthesis of c-mercaptomethyl-
modified PNA. Here we propose a new strategy for the synthesis of modified
PNAs, bearing amino acid side chains. The synthesis is straightforward and is
an improvement to the procedures reported so far, as it uses stable intermediates
and proceeds with better yields.
Keywords: Amino acid, PNA, synthesis
Peptide nucleic acids (PNAs) are oligonucleotide mimics widely used as
tools for regulating gene expression. In fact, thanks to their ability to
hybridize with high affinity and specificity with complementary DNA
and RNA and because of their high chemical and enzymatic resistance
they have found several applications, such as antisense, antigene mole-
cules and biotechnological tools.[1–5] Recently several microarrays and
Received in the U.K. July 18, 2007
Address correspondence to Alessandra Romanelli, University of Naples
‘‘Federico II,’’ Department of Biological Science, School of Biotechnological
Sciences, via Mezzocannone 16, 80134 Napoli, Italy. E-mail: romanelli@unina.it
2499

2500
S. Pensato et al.
other biosensors based on PNAs were developed; relative to DNA, PNAs
are advantageous for the DNA survey in complex mixtures as cell
extracts because the hybridization event is not influenced by the ionic
strength of the media and by DNA binding proteins.[6] The construction
of PNA molecular beacons (MBs) for DNA detection requires the label-
ling of the PNA moiety at the C and N terminal end with a fluorophore
and a fluorescence quencher. Recently, quencher-free DNA-based MBs
were synthesized that utilize fluorophores quenched by nucleobases. This
strategy allows for the inclusion of fluorophores at various points in the
oligonucleotide sequence, not just at the termini, and allows in principle
for the incorporation of multiple fluorophores.
The introduction of a label in the middle of a PNA sequence was
realized by substituting one PNA base with an artificial base or by
attaching fluorophores to a modified PNA backbone. The universal
base thiazole orange was successfully inserted into a PNA oligomer
by divergent solid-phase synthesis; a doubly labelled PNA conjugate
containing one dansyl unit at the C-terminus and one dabsyl unit con-
nected through a linker to the exocyclic amine of an adenine in the mid-
dle of a PNA oligomer were obtained.[7,8] A fluorene fluorescent probe
was attached at the c-Lys-modified PNA by solid-phase synthesis.[9]
The need to develop simple protocols to label PNA encouraged us to
design a new procedure for the synthesis of c-mercaptomethyl-modified
PNA. The nucleophilic SH group, in fact, allows for the selective label-
ling of unprotected molecules.[10] Molecular dynamic simulations on
PNA=DNA duplexes containing L-c-hydroxymethyl PNA demon-
strated that the side chain in the c position points toward the outer part
of the duplex.[11] As is reasonable in a PNA=DNA duplex in which the
oligomer contains a Cys-modified PNA monomer, the L-c-Cys PNA
monomer side chain will also be directed to the outer part of the
duplex; this will allow for the modification of the PNA by a fluoro-
phore, without hampering the formation of Watson–Crick hydrogen
bonds between complementary bases. A cysteine-like PNA monomer
at the N-terminus of a chain can be used for chemical ligation purposes,
for obtaining either long PNA or PNA-peptide conjugates.[3,12] Further-
more, the presence of Cys-like residues in complementary strands
might allow for the stabilization of duplex structures upon disulfide
bond formation.
Here we propose a new strategy, an alternative to those reported in
the literature, for the synthesis of modified PNA bearing amino acid side
chains, starting from amino alcohols.[13,14]. The main advantage of this
strategy is represented by the use of more stable intermediates (azides
and amines) as compared to those used in the protocols reported so
far. Today all the synthetic procedures for Cys-modified PNA monomers

c-Mercaptomethyl PNA Monomers
2501
Scheme 1. Regents and condition: a) (Boc)2O, TEA, 98%; b) PPh3, NaN3
CCL4=DMF, 72%; c) H2, Pd=C, 100%; d) BrCH2COOCH3, TEA, 55%.
involve the formation of an aldehyde intermediate, which cannot
be purified nor stored as it easily racemizes.[15,16] The present protocol
overcomes the racemization issue and the need to operate in extremely
dry conditions (as required for the N-protected a amino aldheydes
synthesis).
The synthesis of the modified monomer was carried out by reactions
that do not involve the chiral carbon. Synthesis started out from commer-
cially available S-benzyl cysteinol. The amino alcohol was synthesized
following procedures reported in the literature.[17] After protection of
the terminal amino group with Boc, the alcohol (2) was transformed into
the corresponding azide derivative (3) by treatment with PPh3=NaN3 in
CCl4=DMF (Scheme 1).[18] This reaction is usually employed for the
synthesis of azides and amines from alcohols. The reaction proceeds
through an intermediate chloride derivative, which is easily transformed
into the azide, thanks to the high nucleofilicity of the azide group.
Triphenylphosphine is used both for the halogenation reaction and for
the reduction of the azide. Attempts to obtain the amine from the
alcohol, using either large amounts of phosphines or various phosphine
derivatives, were not successful. Reduction of the azide (3) was carried
out by hydrogenation on palladiated charcoal. The resulting amine (4)
was then reacted with methyl bromoacetate to give the fully protected
backbone (5) (Scheme 1). The new backbone was coupled to a functiona-
lized nucleobase, synthesized following procedures reported in the
literature (Scheme 2).[19]
Scheme 2. Regents and condition: a) EDC, DMAP, 99%; b)NaOH, 87%.

2502
S. Pensato et al.
Reaction of 5 with 1N-carboxymethylthymine afforded in high yield
the mercaptomethyl-derivatized PNA monomer (6). The same reaction
can be carried out with all the DNA bases, functionalized with a carbox-
ymethylene. Deprotection of the ester 6 by treatment with sodium
hydroxide afforded the carboxylic acid 7, which can be used for the
solid-phase synthesis of modified PNA oligomers.
All products were characterized by NMR (1H and 13C) and mass
spectrometry (ESI). To confirm the chirality of the stereogenic center
was maintained, a measurements were carried out on the backbone (4).
In conclusion, a new procedure for the synthesis of mercaptomethyl-
derivatized PNA monomer was developed starting from a commercially
available compound. The synthesis is straightforward and represents
an improvement to the procedures reported so far, as it uses stable
intermediates and proceeds with better overall yields.
MATERIALS AND METHODS
All chemicals were purchased at Fluka at the highest purity available and
used without further purification. Thin-layer chromatography (TLC) was
run on Alugram Sil G=UV254 plates developed with ninhydrin or UV
visualized. Column chromatography was performed on Fluka silica gel
1
60 (size: 0.04–0.063 mm). H and 13C NMR spectra were recorded on a
Varian Innova instrument (600 MHz) at room temperature. The signals
of the residual protonated solvents (CDCl3 or DMSO d6) were used as
reference signals. High-resolution mass spectra were measured on a
Finnigan MSQ mass spectrometer by electrospray ionization.
EXPERIMENTAL
N-Tert-butyloxycarbonyl-S-benzyl-(R)-cysteinol (2)
Triethylamine (2.56 mmol, 359 mL) was added to a solution of S-benzyl-
(R)-cysteinol (2.56 mmol, 505 mg) in THF (7.50 mL). After 10 min of stir-
ring, di-tert-butyldicarbonate (2.56 mmol, 558 mg) was added at 0 ꢀC. The
reaction mixture was stirred for 2 h at room temperature. The solvent was
then evaporated under reduced pressure; the residue was dissolved in
ethyl acetate and washed with water. The organic layer was separated,
dried with Na2SO4, filtered, and evaporated under reduced pressure to
yield a colorless oil (744 mg, 98%). Rf: 0.60 (petroleum ether=ethyl acet-
ate 6=4 v=v). Mass spectrum: m=z (ESI), [M þ H]þ calcd. 298.14; found
1
298.12. H NMR (CDCl3; d in ppm): 1.45 (9H, s, CH3Boc), 2.36 (1H, s,
OH), 2.61 (2H, m, CH2-S), 3.71–3.80 (5H, m, CH2-OH, Ph-CH2, Ca),

c-Mercaptomethyl PNA Monomers
2503
7.25–7.34 (5H, m, Ph). 13C NMR (CDCl3; d in ppm): 28.85 (CH3Boc),
33.09 (SCH2), 37.04 (PhCH2), 51.99 (Ca), 64.73 (CH2OH), 127.7,
129.08, 129.4, 138.5 (Ph).
N-Tert-butyloxycarbonyl-2-azido-1-(benzylthiomethyl)ethylamine (3)
A mixture of NH-Boc-S-benzyl-(R)-cysteinol (2.50 mmol, 744 mg), sodium
azide (3.00 mmol, 195 mg), and PPh3 (2.50 mmol, 656 mg) in 28.0 mL of
CCl4=DMF (1=4 v=v) was warmed at 90 ꢀC with stirring. The starting
material was completely consumed (TLC monitoring) within 1 h. The reac-
tion mixture was then cooled down to room temperature, diluted with
water, and extracted with ether. The organic layer was dried over Na2SO4,
concentrated on a rotary evaporator, and dried under vacuum to give a
crude colorless oil. The residue was purified by flash-column chromato-
graphy (petroleum ether=ethyl acetate 95=5 v=v) to give 579 mg (72%) of
pure azide. Rf: 0.52 (petroleum ether=ethyl acetate 7=3 v=v). Mass spec-
trum: m=z (ESI), [M þ H]þ calcd. 323.15; found 323.10. 1H NMR (CDCl3;
d in ppm): 1.45 (9H, s, CH3Boc), 2.57 (2H, m, CH2S), 3.47– 3.86 (5H, m,
5H, m, CH2N3, PhCH2, Ca), 7.25–7.34 (5H, m, Ph). 13C NMR (CDCl3; d in
ppm): 28.82 (CH3Boc), 33.59 (SCH2), 36.95 (PhCH2), 49.76 (Ca), 53.51
(CH2N3), 127.7, 129.1, 129.4, 138.3 (Ph), 156.3 (C ¼ O).
N-Tert-butyloxycarbonyl-2-(benzylthiomethyl)ethylendiamine (4)
To a solution of 3 (3.11 mmol, 1.00 g) in dry CH2Cl2 (20.0 mL) at room tem-
perature was added Pd on charcoal (100 mg); after stirring for 60 min under
an atmosphere of hydrogen, the reaction was complete as judged by TLC.
The catalyst was filtered off, and the filtrate was concentrated. The product
was used in the next step without further purification. Rf: 0.52 (ethyl aceta-
te=methanol 8=2 v=v). Mass spectrum: m=z (ESI), [M þ H]þ calcd. 297.16;
1
found 297.10. H NMR (CDCl3; d in ppm): 1.45 (9H, s, CH3Boc), 2.50–
2.65 (2H, m, CH2S), 2.80 (2H, bs, CH2NH2), 3.60–3.73 (3H, bs, CH2Ph,
Ca), 7.25–7.34 (5H, m, Ph). 13C NMR (CDCl3; d in ppm): 28.88 (CH3Boc),
34.27 (SCH2), 37.15 (PhCH2), 44.71 (CH2NH2), 52.21 (Ca ), 80.01 (CBoc),
127.6, 129.0, 129.5, 138.6 (Ph), 156.2 (C ¼ O).
N-Tert-butyloxycarbonyl-1-(benzylthiomethyl)-N0-
methoxycarbonylmethyl-ethylendiamine (5)
To a solution of 4 (3.04 mmol, 900 mg) and Et3N (3.34 mmol, 470 mL)
in dry CH2Cl2 (10.0 mL), methyl bromoacetate (3.04 mmol, 279 mL)

2504
S. Pensato et al.
in CH2Cl2 (200 mL) was added dropwise. The reaction was stirred over-
night, and subsequently aqueous NaHCO3 was added. The organic layer
was dried over Na2SO4, filtered, and concentrated to dryness. Purifi-
cation of the crude oil by silica-gel column chromatography (diethyl
ether= petroleum ether 6=4 to 9=1 v=v) afforded pure 5 as a yellow oil.
Yield: 612 mg (55%). Rf: 0.3 (diethyl ether=petroleum ether 85=5 v=v).
Mass spectrum: m=z (ESI), [M þ H]þ calcd. 369.18; found 369.10.
1H NMR (CDCl3; d in ppm): 1.45 (9H, s, CH3Boc), 2.52–2.78 (4H, m,
CH2S and CH2N), 3.37 (2H, dd, NCH2CO), 3.72 (3H, s, OCH3), 3.73
(2H, s, CH2Ph), 3.8 (1H, bs, Ca), 7.25–7.34 (5H, m, Ph).13C NMR
(CDCl3; d in ppm): 28.88 (CH3Boc), 34.66 (SCH2), 37.26 (PhCH2),
50.21 (CH2NH), 51.27 (CH2C ¼ O), 51.86 (OCH3), 52.27 (Ca), 79.97
(CBoc), 127.5, 129.0, 129.5, 138.7 (Ph), 156.0 (NHC ¼ O).
N-Tert-butyloxycarbonyl-1-(benzylthiomethyl)-N0-methoxycarbonylmethyl-
N0-[(thymin-1-yl)acetyl]-ethylendiamine (6)
To a solution of 5 (1.33 mmol, 490 mg) in dry DMF (10.0 mL), thymin-
1-ylacetic acid (1.93 mmol, 355 mg), and 4-(dimethylamino)pyridine
(DMAP) (0.13 mmol, 15.9 mg) were added, and the mixture was stirred
until most of the acid dissolved. Next, N-(3-dimethylaminoproply)-N0-
ethylcarbodimide hydrochloride (EDC) (2.93 mmol, 561 mg) was added
and the mixture was stirred overnight. The solution was concentrated in
vacuo, and the residue was partitioned between ethyl acetate and satu-
rated aqueous NaHCO3. The aqueous layer was back-extracted with
ethyl acetate, and the combined organics were washed with brine and
dried (Na2SO4). The organic layer was then concentrated on a rotary
evaporator and dried under vacuum.
Purification by flash chromatography (ethyl acetate=petroleum
ether 95=5 v=v) gave 6 (670 mg, 94%) as a white solid. Rf: 0.55 (diethyl
ether=petroleum ether 7=3 v=v). [a]20 ¼ ꢁ7.4 (c ¼ 0.02 in CHCl3).
D
Mass spectrum: m=z (ESI), [M þ H]þ calcd. 535.21; found 535.29. H
1
NMR (CDCl3; d in ppm): two rotamers; major product 1.41 (9H, s,
CH3Boc), 1.66 (1H, s, NH), 1.91 (3H, s, CH3Thy), 2.48–2.78 (4H, m,
CH2S and CH2N), 3.43–3.46 (2H, dd, NCH2CO), 3.72 (3H, s,
OCH3), 3.76 (2H, s, N1ThyCH2CO), 3.8 (2H, s, CH2Ph), 4.12–4.16
(1H, m, Ca), 6.98 (1H, s, H6), 7.25–7.32 (5H, m, Ph), 8.35 (1H, s,
NHThy). 13C NMR (CDCl3; d in ppm): two rotamers; major product
12.81 (CH3Thy), 28.81 (CH3Boc), 37.41 (SCH2), 41.61 (PhCH2), 48.00
(CH2COOMe), 49.42 (Ca), 51.08 (CH2N1Thy), 52.83 (COOMe),
53.40 (Ca CH2N), 111.1 (C5Thy), 127.9, 129.2, 129.4, 138.5 (Ph),
151.3 (C(2) ¼ OThy), 164.3 (C(4) ¼ OThy), 169.9 (COOMe).

c-Mercaptomethyl PNA Monomers
2505
N-Tert-butyloxycarbonyl-1-(benzylthiomethyl)-N0-carboxymethyl-N0-
[(thymin-1-yl)acetyl]-ethylendiamine (7)
Compound 6 (46. 8 mmol, 25. 0 mg) was dissolved in dioxane (500 mL)
and 2 M NaOH (117 mL) was added. After stirring for 20 min, the reac-
tion was complete as shown by TLC. The mixture was acidified by adding
1 N HCl to pH 2–3. The aqueous phase was extracted with ethyl acetate.
The combined organic phases were dried with Na2SO4, and the solvent
was evaporated under reduced pressure. Purification of an analytical
sample (9.7 mg) by reverse-phase HPLC afforded 7 (8.2 mg, 87%) as a
white powder. Rf: 0.70 (ethyl acetate=methanol=formic acid 85=10=5
v=v=v). Mass spectrum: m=z (ESI), [M þ H]þ calcd. 521.21; found
521.27. 1H NMR (CDCl3; d in ppm): two rotamers; major product
1.42 (9H, s, CH3Boc), 1.88 (3H, s, CH3Thy), 2.48–2.68 (4H, m, CH2S
and CH2N), 3.48–4.58 (1H, m, Ca), 3.71 (2H, s, NCH2CO), 3.75 (4H, s,
CH2Ph and N1ThyCH2CO), 6.97 (1H, s, H6Thy), 7.23–7.32 (5H, m, Ph),
10.1 (1H, s, COOH). 13C NMR (DMSO; d in ppm): two rotamers; major
product 15.85 (CH3Thy), 32.22 (CH3Boc), 37.98 (SCH2), 38.70 (PhCH2),
51.97, 52.43, 54.98, 81.98 (CBoc), 111.9 (C5Thy), 130.6, 132.2, 132.8,
142.6 (Ph), 146.1, 155.0, 159.3, 168.4 (COOH).
ACKNOWLEDGMENTS
`
This work was supported by the Ministero dell’Universita e della Ricerca,
Programmi di ricerca di Rilevante Interesse Nazionale (MUIR PRIN05)
grant. The authors are grateful to G. Perretta and L. Zona for technical
assistance.
REFERENCES
1. Nielsen, P. E.; Egholm, M.; Berg, R. H.; Buchardt, O. Sequence-selective
recognition of DNA by strand displacement with a thymine-substituted
polyamide Science. Science 1991, 254, 1497.
2. Uhlmann, E. Peptide nucleic acids (PNA) and PNA-DNA chimeras: from
high binding affinity towards biological function. Biol. Chem. 1998, 379, 1045.
3. Nielsen, P. E. Curr Opin Mol Ther Antisense peptide nucleic acids. Curr.
Opin. Mol. Ther. 2000, 2, 282.
4. Cogoi, S.; Codognotto, A.; Rapozzi, V.; Xodo, L. E. Antigene property of
PNA conjugated to the nuclear localization signal peptide Nucleosides
Nucleotides Nucleic Acids. Nucleosides Nucleotides Nucleic Acids 2005, 24,
971.
5. Janowski, B. A.; Kaihatsu, K.; Huffman, K. E.; Schwartz, J. C.; Ram, R.;
Hardy, D.; Mendelson, C. R.; Corey, D. R. Inhibiting transcription of

2506
S. Pensato et al.
chromosomal DNA with antigene peptide nucleic acids. Nat. Chem. Biol.
2005, 1, 210.
6. Brandt, O.; Hoheisel, J. D. Peptide nucleic acids on microarrays and other
biosensors. Trends Biotechnol. 2004, 22, 617.
7. Seitz, O.; Bergmann, F.; Heindl, D. A Convergent strategy for the modifi-
cation of peptide nucleic acids: Novel mismatch-specific PNA-hybridization
probes. Angew Chem. Int. Ed. Engl. 1999, 38, 2203.
8. Jarikote, D. V.; Kohler, O.; Socher, E.; Seitz, O. Divergent and linear solid-
phase synthesis of PNA containing thiazole orange as artificial base. Eur. J.
Org. Chem. 2005, 3187.
9. Englund, E. A.; Appella, D. H. Synthesis of gamma-substituted peptide
nucleic acids: a new place to attach fluorophores without affecting DNA
binding. Org. Lett. 2005, 7, 3465.
10. Kupihar, Z.; Schmel, Z.; Kele, Z.; Penke, B.; Kovacs, L. Synthesis and appli-
cation of a novel, crystalline phosphoramidite monomer with thiol terminus,
suitable for the synthesis of DNA conjugates. Bioorg. Med. Chem. 2001, 9,
1241.
11. Dragulescu-Andrasi, A.; Rapireddy, S.; Frezza, B. M.; Gayathri, C.; Gil, R.
R.; Ly, D. H. A simple gamma-backbone modification preorganizes peptide
nucleic acid into a helical structure. J. Am. Chem. Soc. 2006, 128, 10258.
12. Dawson, P. E.; Muir, T. W.; Clark-Lewis, I.; Kent, S. B. H. Synthesis of
proteins by native chemical ligation. Science 1994, 266, 776.
13. Dose, C.; Seitz, O. Convergent synthesis of peptide nucleic acids by native
chemical ligation. Org. Lett. 2005, 7, 4365.
14. De Koning, M.; Petersen, L.; Weterings, J. J.; Overhand, M.; van der Marel,
G. A.; Filippov, D. V. Synthesis of thiol-modified peptide nucleic acids
designed for post-assembly conjugation reactions. Tetrahedron 2006, 62,
3248.
15. Jurczak, J.; Golebiowski, A. Optically active N-protected? amino aldehydes
in organic synthesis. Chem. Rev. 1989, 89, 149.
16. Falkiewicz, B.; Kolodziejczyk, A. S.; Liberek, B.; Wisniewski, K. Synthesis of
achiral and chiral peptide nucleic acid (PNA) monomers using Mitsunobu
reaction. Tetrahedron 2001, 57, 7909.
17. Caputo, R.; Cassano, E.; Longobardo, L.; Palumbo, G. Amino acid and
peptide chemistry. 3. synthesis of enantiopure N- and C-protected homo-?-
amino acids by direct homologation of?-amino acids. Tetrahedron 1995, 51,
12337.
18. Reddy, G. V. S.; Rao, G. V.; Subramanyam, R. V. K.; Iyengar, D. S. A new
novel and practical one pot methodology for conversion of alcohols to
amines. Synth. Commun. 2000, 30, 2233.
19. Will, D. W.; Breipohl, G.; Langner, D.; Knolle, J.; Uhlmann, E. The
synthesis of polyamide nucleic acids using a novel monomethoxytrityl
protecting-group strategy. Tetrahedron 1995, 51, 12069.