5-(Benzylmercapto)-1H-tetrazole as activator for 2′-O-TBDMS phosphoramidite building blocks in RNA synthesis

An improved method for the preparation of 5-(benzylmercapto)-1H-tetrazol as activator in RNA synthesis is described. Reaction of benzylthiocyanate with azide delivers the corresponding tetrazole with 72% yield under optimised conditions. We demonstrate that 5-(benzylmercapto)-1H-tetrazol is a superior activator of 2′-O-TBDMS phosphoramidite building blocks. Compared to routinely used 1H-tetrazol, application of a 0.25 M 5-(benzylmercapto)-1H-tetrazol solution in acetonitrile allows for higher coupling yields (>99%), lower coupling times (3 min) and reduced excess of phosphoramidites in solution over the solid-phase nucleotides (8-fold).