In RNA synthesis without base protection, a new method for O-selective condensation with more than 99% selectivity was developed by 6-nitro-HOBt-mediated cleavage of undesired P(III)-N bonds on nucleobase moieties. Moreover, we for the first time succeeded in synthesizing oligoRNAs without base protection.

Amidine-type protecting groups, i.e., N,N-dimethylformamidine (dmf) and N,N-dibutylformamidine (dbf) groups, introduced into nucleobases were rapidly removed under mild acidic conditions using imidazolium triflate (IMT) or 1-hydroxybenztriazole (HOBt). This new deprotection strategy allowed a 2'-O-methyl-RNA derivative bearing a base-labile group to be efficiently synthesized using a silyl-type linker. It was also found that our new method could be applied to the synthesis of an unmodified RNA oligomer.

In the previous study, we developed a method for O-selective phosphorylation, i.e., "the activated phosphite method" for DNA synthesis without base protection. However, the O-selectivity was low in RNA synthesis when the activated phosphite method was used. In this paper, we developed two new methods for synthesis of RNA oligomers having base-labile functional groups on polymer supports. One is the N-unprotected phosphoramidite method involving P(III)-N bond cleavage. The selectivity of the phosphorylation in RNA synthesis increased to more than 99% by posttreatment of the undesired P(III)-N bonds with 6-nitoro-HOBt and was independent of the kind of protecting groups at the 2' position. Another method is the RNA synthesis involving deprotection of protecting groups of the nucleobases under acidic conditions. It was found that an N,N-dialkylaminomethlene (DAF) group could be easily removed by heat-induced deprotection using HOBt as an acidic promoter.