2‘-C-Branched Ribonucleosides: Synthesis of the Phosphoramidite Derivatives of 2‘-C-β-Methylcytidine and Their Incorporation into Oligonucleotides

We describe a strategy for the incorporation of a 2‘-C-branched ribonucleoside, 2‘-C-β-methylcytidine, into oligonucleotides via solid-phase synthesis using phosphoramidite derivatives. 4-N-Benzoyl-2‘-C-β-methylcytidine (2b) was synthesized by coupling persilylated 4-N-benzoylcytosine with 1,2,3,5-tetra-O-benzoyl-2-C-β-methyl-α-(and β)-d-ribofuranose (1) in the presence of SnCl4 in acetonitrile, followed by selective deprotection with NaOH in pyridine/methanol. The 3‘- and 5‘-hydroxyl groups were blocked as a cyclic di-tert-butylsilanediyl ether 3 by treatment with di-tert-butyldichlorosilane/AgNO3 in DMF. The 2‘-hydroxyl group was then protected as a tert-butyldimethylsilyl ether 4a by treatment with tert-butylmagnesium chloride followed by addition of tert-butyldimethylsilyl trifluoromethanesulfonate in THF. As an alternative to 2‘-silyl protection, the corresponding 2‘-O-tetrahydropyranyl ether 4b was prepared by treatment of 3 with 4,5-dihydro-2H-pyran in the presence of a catalytic amount of 10-camph...