Introduction of 3′-Terminal Nucleosides Having a Silyl-Type Linker into Polymer Supports without Base Protection

New 3'-terminal deoxyribonucleoside-loading reagents having a silyl-type linker were developed. They were effectively introduced into polymer supports under the conditions of Huisgen [3 + 2] cycloaddition without base protection. Moreover, four unmodified DNA oligomers d[TACCTAAATCCAX] (X = T, A, C, and A) and a base-labile modified DNA 12mer d[A*C*T*C*C*GT*C*T*A*C*G] 16 (A* = 6-N-acetyl-8-aza-7-deaza-2'-deoxyadenosine, C* = 4-N-acetyl-2'-deoxyctydine, T* = 2-thio-T) were successfully synthesized by cleavage of the silyl-type linker using Bu4NF under neutral conditions in our N-unprotected phosphoramidite method. In this paper, we also report a new reaction of chlorination of cytosine base using 1,3-dichloro-5,5-dimethylhydantoin.