Improved synthesis of 2′-amino-LNA

Article abstract of DOI:10.1081/NCN-120022818

2′-Amino-LNA phosphoramidite (10) was synthesised by means of a new strategy, which is convergent with the synthesis of 2′-oxy-LNA up until a late stage intermediate (1).

Full text of DOI:10.1081/NCN-120022818

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Improved Synthesis of 2′-Amino-LNA
Signe M. Christensen a d , Christoph Rosenbohm a , Mads Sørensen b , Lotte-Emilie Larsen a
Jesper Wengel c & Troels Koch a
,
a Cureon A/S , Copenhagen, Denmark
b Department of Chemistry , University of Copenhagen , Copenhagen, Denmark
c Department of Chemistry , Nucleic Acid Center , University of Southern Denmark , Odense
M, Denmark
d Cureon A/S , Fruebjergvej 3, DK-2100, Copenhagen, Denmark
Published online: 31 Aug 2006.
To cite this article: Signe M. Christensen , Christoph Rosenbohm , Mads Sørensen , Lotte-Emilie Larsen , Jesper Wengel &
Troels Koch (2003) Improved Synthesis of 2′-Amino-LNA, Nucleosides, Nucleotides and Nucleic Acids, 22:5-8, 1131-1133, DOI:
10.1081/NCN-120022818
To link to this article: http://dx.doi.org/10.1081/NCN-120022818
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NUCLEOSIDES, NUCLEOTIDES & NUCLEIC ACIDS
Vol. 22, Nos. 5–8, pp. 1131–1133, 2003
Improved Synthesis of 20-Amino-LNA
1
2
Signe M. Christensen,1, Christoph Rosenbohm, Mads Sørensen,
*
Lotte-Emilie Larsen,1 Jesper Wengel,3 and Troels Koch1
1Cureon A=S, Copenhagen, Denmark
2Department of Chemistry, University of Copenhagen,
Copenhagen, Denmark
3Department of Chemistry, Nucleic Acid Center,
University of Southern Denmark,
Odense M, Denmark
ABSTRACT
20-Amino-LNA phosphoramidite (10) was synthesised by means of a new strat-
egy, which is convergent with the synthesis of 20-oxy-LNA up until a late stage
intermediate (1).
Key Words: 20-Amino-LNA; Convergent strategy; Gram-scale synthesis.
LNA (Locked Nucleic Acid) was introduced in 1998[1–3] as a novel class of con-
formationally restricted oligonucleotide analogues: The first LNA monomer was
based on the 20-OCH2-40 bicyclic structure (LNA=20-oxy-LNA). Later similar high
affinity=specificityLNAmonomerssuchas20-NHCH2-40,20-N(CH3)CH2-40 (20-amino-
LNA)[4,5] and 20SCH2-40(20-thio-LNA)[4,6] were synthesised.
*Correspondence: Signe M. Christensen, Cureon A=S, Fruebjergvej 3, DK-2100 Copenhagen,
Denmark; Fax: +45 70 26 00 97; E-mail: smc@cureon.com.
1131
DOI: 10.1081/NCN-120022818
Copyright # 2003 by Marcel Dekker, Inc.
1525-7770 (Print); 1532-2335 (Online)
www.dekker.com

1132
Christensen et al.
Scheme 1. i) half sat. NH3 in MeOH (97%); ii) MsCl, pyridine (93%); iii) DBU, MeCN
(91%); iv) acetone, 0.1M H2SO4 (98%); v) Tf2O, DMAP, pyridine, CH2Cl2 (80%);
vi) NaN3, 15-crown-5, DMF (91%); vii) PMe3, NaOH(aq), THF (93%); viii) CH2O,
HCO2H (90%); ix) a) NaOBz, b) MeONa, DMF (98%), c) 20% Pd(OH)2=C, H2, AcOH (97%),
d) DMTCl, pyridine (92%), e) NC(CH2)2OP(N(iPr)2)2, 4,5-dicyanoimidazole, MeCN, CH2Cl2
(98%).
Scale up of the original synthesis of 20-amino-LNA proved to be difficult in our
hands, and we therefore developed a new synthetic approach, which converges the
syntheses of 20-oxy-LNA and 20-amino-LNA. The 20-O-acetyl dimesylated nucleo-
side 1 (Sch. 1), used in the improved and recently published synthesis of 20-oxy-
LNA nucleosides,[7] was deacetylated using half-saturated methanolic ammonia to
afford nucleoside 2 in quantitative yield. Subsequent reaction with mesyl chloride
in pyridine gave the trimesylate 3 in 96% on a 20 g scale. The 2,20-anhydro intermedi-
ate 4 was synthesised quantitatively by treatment of 3 with 1.1 equivalents of DBU in
anhydrous acetonitrile. Opening of the 2,20-anhydro intermediate by refluxing it in a
mixture of aqueous sulfuric acid (0.1M) and acetone (1:1, v=v) resulted in a clean
reaction giving the threo-configured nucleoside 5 in 91%. Treatment of 5 with
trifluoromethanesulfonic anhydride, pyridine and DMAP in anhydrous dichloro-
methane at 0ꢀC gave the desired triflate 6 in 80% after chromatography. Subse-
quent treatment with sodium azide afforded the 20-azido-20-deoxynucleoside 7 in
91%. Reduction of the azide using trimethylphosphine and aqueous NaOH in
THF gave the nucleoside 8 with the desired 2-oxa-5-azabicyclo [2.2.1] heptane ske-
leton in 93%. Methylation using Eschweiler-Clarke conditions gave the 20-N-methyl
derivative 9 in 90% yield. Nucleophilic replacement of the mesylate on C50 with
benzoate followed by transesterification with methoxide afforded the 50-hydroxy
nucleoside in 98% yield. Reductive debenzylation with hydrogen and 20% Pd(OH)2=C
in acetic acid yielded the deprotected nucleoside in 97%. 4,40-Dimethoxytritylation

Improved Synthesis of 20-Amino-LNA
1133
of the 50-hydroxygroup and phosphitylation of the 30-hydroxy group gave the ami-
dite 10 in 90% yield ready for the automated incorporation into oligonucleotides.
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