2‘-C-Branched Ribonucleosides. 2. Synthesis of 2‘-C-β-Trifluoromethyl Pyrimidine Ribonucleosides¹

The first synthesis of 2‘-C-β-trifluoromethyl pyrimidine ribonucleosides is described. 1,2,3,5-Tetra-O-benzoyl-2-C-β-trifluoromethyl-α-d-ribofuranose (3) is prepared from 1,3,5-tri-O-benzoyl-α-d-ribofuranose (1) in three steps and converted to 3,5-di-O-benzoyl-2-C-β-trifluoromethyl-α-d-1-ribofuranosyl bromide (5). The 1-bromo derivative (5) is found to be a powerful reaction intermediate for the synthesis of ribonucleosides. The reaction of silylated pyrimidines with (5) in the presence of HgO/HgBr₂ affords exclusively the β-anomers (6−8). Deprotection of (6−8) with ammonia in methanol yields the 2‘-C-β-trifluoromethyl nucleosides (9−11).