Synthesis and Hybridization Studies of 2‘-Amino-α-L-LNA and Tetracyclic “Locked LNA”^†

A convergent route to a new class of locked nucleic acids, i.e., 2‘-amino-α-L-LNA, has been developed. The optimized synthetic route to the corresponding phosphoramidite building block of thymine proceeds in 4% overall yield over 15 steps from the starting diol. Crucial synthetic steps include (a) introduction of a C2-azido group prior to nucleobase coupling, (b) Vorbrüggen glycosylation primarily affording the desired α-anomer, (c) separation of α-l-ribo- and β-l-ribo-configured bicyclic nucleosides, and (d) selection of a suitable protecting group to avoid intramolecular Michael addition of the C2‘-amino group onto the C6-position. Incorporation of a 2‘-amino-α-L-LNA monomer into oligodeoxyribonucleotides results in modest changes in thermal stability with complementary DNA, whereas significant increases in thermal stability are observed with RNA complements along with excellent Watson−Crick discrimination. These results, along with the flexibility of the synthetic strategy allowing chemoselective N2‘-functionalization at a late stage, render 2‘-amino-α-L-LNA a promising building block for nucleic acid based nanobiotechnology and therapeutics. A slight modification in strategy facilitated the synthesis of the corresponding phosphoramidite building blocks of Michael adducts, which due to their tetracyclic skeletons exhibit a conformationally restricted furanose ring and glycosidic torsion angle (anti-range). Incorporation of such a “locked LNA” monomer into oligodeoxyribonucleotides results in large decreases in thermal affinity toward DNA/RNA complements.