7-Deazaadenosine: Oligoribonucleotide building block synthesis and autocatalytic hydrolysis of base-modified hammerhead ribozymes

A 7‐deazaadenosine ( = tubercidin; c7A; 1) building block for solid‐phase oligoribonucleotide synthesis was prepared. The amino group of 1 was protected with the (dimethylamino)methylidene residue (→3), and the monomethoxytrityl group was introduced at OHC(5′) (→4). Protection of OHC(2′) was carried out by silylation, showing that use of the (i‐Pr)3Si group resulted in high 2′‐O‐selectivity (→5b, 80%). Reaction of 5b with PCl3 afforded the phosphonate 7 which was used in solid‐phase oligoribonucleotide synthesis. The autocatalytic hydrolysis of hammerhead ribozymes using pG‐G‐G‐A‐G‐U‐C‐A‐G‐U‐C‐C‐C‐U‐U‐C‐G‐G‐G‐G‐A‐C‐U‐C‐U‐G‐A‐A‐G‐A‐G‐G‐C‐G‐C as substrate strand (S) and modified G‐C‐G‐C‐C‐G‐A‐A‐A‐C‐U‐C‐C‐C as enzyme strand (E) was studied. When c7A replaced A13 or A14, a small decrease of catalytic activity was observed, while modification in position A15 enhanced the autocatalytic hydrolysis. The results demonstrate, that the atom N(7) of adenosine in any of these positions is not crucial for ribozyme action.