Synthesis of Selectively ¹⁵N‐Labeled 2′‐O‐{[(Triisopropylsilyl)oxy]methyl}(=tom)‐Protected Ribonucleoside Phosphoramidites and Their Incorporation into a Bistable 32Mer RNA Sequence

We present optimized reaction conditions for the conversion of 2′‐ O ‐{[(triisopropylsilyl)oxy]methyl}(=tom) protected uridine and adenosine nucleosides into the corresponding protected (3‐ 15 N)‐labeled uridine and cytidine and (1‐ 15 N)‐labeled adenosine and guanosine nucleosides 4, 6, 12 , and 18 , respectively ( Schemes 1 – 4 ). On a DNA synthesizer, the resulting 15 N‐labeled 2′‐ O ‐tom‐protected phosphoramidite building blocks 19 – 22 were efficiently incorporated into five selected positions of a bistable 32mer RNA sequence 23 (known to adopt two different structures) ( Fig. 1 ). By 2D‐HSQC and HNN‐COSY experiments in H 2 O/D 2 O 9 : 1, the 15 N‐signals of all base‐paired 15 N‐labeled nucleotides could be identified and attributed to one of the two coexisting structures of 23 .