Innovative 2′‑O‑Imino-2-propanoate-Protecting Group for Effective Solid-Phase Synthesis and 2′‑O‑Deprotection of RNA Sequences

The implementation of protecting groups for the 2′-hydroxyl function of ribonucleosides is still challenging, particularly when RNA sequences must be of the highest purity for therapeutic applications as nucleic acid-based drugs. A 2′-hydroxyl-protecting group should optimally (i) be easy to install; (ii) allow rapid and efficient incorporation of the 2′-O-protected ribonucleosides into RNA sequences to minimize, to the greatest extent possible, the formation of process-related impurities (e.g., shorter than full-length sequences) during solid-phase synthesis; and (iii) be completely cleaved from RNA sequences without the production of alkylating side products and/or formation of mutagenic nucleobase adducts. The reaction of 2′-O-aminoribonucleosides with ethyl pyruvate results in the formation of stable 2′-O-imino-2-methyl propanoic acid ethyl esters and, subsequently, of the fully protected ribonucleoside phosphoramidite monomers, which are required for the solid-phase synthesis of two chimeric RNA sequences (20-mers) containing the four canonical ribonucleosides. Upon treatment of the RNA sequences with a solution of sodium hydroxide, the 2′-O-imino-2-methyl propanoic acid ethyl ester-protecting groups are saponified to their sodium salts, which after ion exchange underwent quantitative intramolecular decarboxylation under neutral conditions at 65 °C to provide fully deprotected RNA sequences in marginally better yields than those obtained from commercial 2′-O-tert-butyldimethylsilyl ribonucleoside phosphoramidites under highly similar conditions.