Synthesis and Characterization of Oligodeoxyribonucleotides Modified with 2'-amino-α-L-LNA Adenine Monomers: High-affinity Targeting of Single-stranded DNA

The development of conformationally restricted nucleotide building blocks continues to attract considerable interest because of their successful use within antisense, antigene, and other gene-targeting strategies. Locked nucleic acid (LNA) and its diastereomer α-L-LNA are two interesting examples thereof. Oligonucleotides modified with these units display greatly increased affinity toward nucleic acid targets, improved binding specificity, and enhanced enzymatic stability relative to unmodified strands. Here we present the synthesis and biophysical characterization of oligodeoxyribonucleotides (ONs) modified with 2'-amino-α-L-LNA adenine monomers W-Z. The synthesis of the target phosphoramidites 1-4 is initiated from pentafuranose 5, which upon Vorbrüggen glycosylation, O2'-deacylation, O2'-activation and C2'-azide introduction yields nucleoside 8. A one-pot tandem Staudinger/intramolecular nucleophilic substitution converts 8 into 2'-amino-α-L-LNA adenine intermediate 9, which after a series of nontrivial protecting-group manipulations affords key intermediate 15. Subsequent chemoselective N2'-functionalization and O3'-phosphitylation give targets 1-4 in ~1-3% overall yield over 11 steps from 5. ONs modified with pyrene-functionalized 2'-amino-α-L-LNA adenine monomers X-Z display greatly increased affinity toward DNA targets (ΔTm/modification up to +14 °C). Results from absorption and fluorescence spectroscopy suggest that the duplex stabilization is a result of pyrene intercalation. These characteristics render N2'-pyrene-functionalized 2'-amino-α-L-LNAs of considerable interest for DNA-targeting applications.