Unprecedented mild acid-catalyzed desilylation of the 2'-O-tert-butyldimethylsilyl group from chemically synthesized oligoribonucleotide intermediates via neighboring group participation of the internucleotidic phosphate residue

Hydrolytic removal of the 2'-tert-butyldimethylsilyl (TBDMS) group from a 2'-O-TBDMS protected UpU dimer [U(2'-Si)pU] (1) (Si = TBDMS) and related derivatives under various acidic conditions was studied in detail. First, desilylation of 1 by use of acetic acid was examined. Consequently, we made the unprecedented discovery that cleavage of the 2'-silyl ether linkage occurred fastest at a very low concentration of acetic acid within the range of 5-10%, depending on the temperature, Formic acid could cleave the silyl ether much faster than acetic acid, but the relationship between the reaction rate and the concentration of acid was different from that of acetic acid. The use of 20-40% formic acid resulted in very effective elimination of the 2'-TBDMS group, Moreover, diluted HCl solution (pH 2.0) could cleave the Si-O bond faster than acetic acid at 30 degrees C. In contrast, the 2'-silyl group of the corresponding methylphosphonate derivative [U(2'-Si)p(Me)U] (3) was much more stable than that of 1. In the case of a diastereomeric mixture of the phosphorothioate dimer [U(2'-Si)psU] (2), a big difference in reaction rate between the Rp- and Sp-diastereomers was observed. These results strongly suggest that neighboring group participation of the 3'-5' phosphorodiester group is involved in the present acid-catalyzed 2'-desilylation. These conditions were successfully applied to the deprotection of the 2'-TBDMS group of an RNA intermediate which was chemically synthesized by the conventional phosphoramidite approach on a CPG gel.