A simplified and efficient route to 2′-O, 4′-C-methylene-linked bicyclic ribonucleosides (locked nucleic acid)

A novel efficient method for the synthesis of locked nucleic acid (LNA) monomers is described. The LNA 5 ' 3 ' -diols containing thymine, 4-N-acetyl- and 4-N-benzoylcytosine, 6-N-benzoyladenine, and 2-N-isobutyrylguanine as nucleobases were prepared via convergent syntheses. The method is based on the use of the common sugar intermediate 1,2-di-O-acetyl-3-O-benzyl-4-C-methanesulfonoxy-methyl-5-O-methanesulfonyl-D-erythro-pentofuranose (8) that easily can be prepared from D-glucose in multigram scale. Four different nucleobases were stereoselectively coupled to 8 using a modified Vorbruggen procedure to give the corresponding 4 ' -C-branched nucleoside derivatives. Subsequent ring closing furnished the protected LNA nucleosides. The 5 ' -O-mesyl groups were efficiently displaced by nucleophilic substitution using sodium benzoate. Saponification of the 5 ' -benzoates followed by catalytic removal of the 3 ' -O-benzyl groups afforded the free LNA diols. The exocyclic amino groups of adenosine and cytidine were selectively acylated to give 4-N-acetyl- or 4-N-benzoyl-LNA-C and 6-N-benzoyl-LNA-A. The isobutyryl group of guanine was retained during the preparation of 2-N-isobutyryl-LNA-G. The LNA-T diol and base-protected LNA diols can be directly converted into LNA-phosphoramidites for automated chemical synthesis of LNA containing oligonucleotides.