High-quality oligo-RNA synthesis using the new 2′-O-TEM protecting group by selectively quenching the addition of p-tolyl vinyl sulphone to exocyclic amino functions

During the F--promoted deprotection of the oligo-RNA, synthesized using our 2'-O-(4-tolylsulfonyl)ethoxymethyl (2'-O-TEM) group [Org. Biomol. Chem. 5, 333 (2007)], p-tolyl vinyl sulphone (TVS) is formed as a by-product. The TVS formed has been shown to react with the exocyclic amino functions of adenosine (A), guanosine (G), and cytidine (C) of the fully deprotected oligo-RNA to give undesirable adducts, which are then purified by HPLC and unambiguously characterized by H-1, C-13 Heteronuclear Multiple Bond Correlation (HMBC) NMR and mass spectroscopic analysis. The relative nucleophilic reactivities of the nucleobases toward TVS have been found to be the following: N-6-A > N-4-C > N-2-G > > N-3-U. This reactivity of TVS toward RNA nucleobases to give various Michael adducts could, however, be suppressed by using various amines as scavengers. Among all these amines, morpholine and piperidine are the most efficient scavenger for TVS, which gave highly pure oligo-RNA even in the crude form and can be used directly in RNA chemical biology studies.