A simplified and efficient route to 2 '-O, 4 '-C-methylene-linked bicyclicribonucleosides (locked nucleic acid)

A novel efficient method for the synthesis of locked nucleic acid (LNA) mon omers is described. The LNA 5 ' 3 ' -diols containing thymine, 4-N-acetyl- and 4-N-benzoylcytosine, 6-N-benzoyladenine, and 2-N-isobutyrylguanine as n ucleobases were prepared via convergent syntheses. The method is based on t he use of the common sugar intermediate 1,2-di-O-acetyl-3-O-benzyl-4-C-meth anesulfonoxy-methyl-5-O-methanesulfonyl-D-erythro-pentofuranose (8) that ea sily can be prepared from D-glucose in multigram scale. Four different nucl eobases were stereoselectively coupled to 8 using a modified Vorbruggen pro cedure to give the corresponding 4 ' -C-branched nucleoside derivatives. Su bsequent ring closing furnished the protected LNA nucleosides. The 5 ' -O-m esyl groups were efficiently displaced by nucleophilic substitution using s odium benzoate. Saponification of the 5 ' -benzoates followed by catalytic removal of the 3 ' -O-benzyl groups afforded the free LNA diols. The exocyc lic amino groups of adenosine and cytidine were selectively acylated to giv e 4-N-acetyl- or 4-N-benzoyl-LNA-C and 6-N-benzoyl-LNA-A. The isobutyryl gr oup 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 ol igonucleotides.