Synthesis of pyridine, pyrimidine and pyridinone C-nucleoside phosphoramidites for probing cytosine function in RNA.

In the structures of the HDV ribozyme a cytosine nucleobase resides at the active site poised to participate directly in catalysis. Defining the functional role of the nucleobase requires nucleoside analogues that perturb the functional groups in a strategic manner. Herein, we have developed efficient methods for the synthesis of five C-nucleoside phosphoramidite derivatives that, when used in combination, provide strategies for probing the potential functional role of cytosine's keto group and imino nitrogen. Phosphoramidites 15a and 15b were synthesized in 11 steps starting from 2-amino-5-bromopyrimidine (1a) and 2-amino-5-bromopyridine (1b), respectively, with overall yields of 10.8% and 6.6%, respectively. Phosphoramidite 21 was prepared from intermediate 11b in seven steps with an overall yield of 33.7%. Phosphoramidites 23 and 25 were prepared from 2,4-diamino-5-(beta-d-ribofuranosyl)-1,3-pyrimidine (22) and pseudoisocytidine (24), respectively, with an overall yield of 15.9% (six steps) and 37.9% (four steps), respectively. These phosphoramidites were incorporated into oligonucleotides by solid-phase synthesis.