2 '-C-branched ribonucleosides. 2. Synthesis of 2 '-C-beta-trifluoromethylpyrimidine ribonucleosides

The first synthesis of 2'-C-beta -trifluoromethyl pyrimidine ribonucleoside s is described. 1,2,3,5-Tetra-O-benzoyl-2-C-beta -trifluoromethyl (3) is pr epared from 1,3,5-tri-O-benzoyl-alpha-(D)-ribofuranose (1) in three steps a nd converted to 3,5-di-O-benzoyl-2- C-beta -trifluoromethyl-alpha-(D)-1-rib ofuranosyl bromide (5). The 1-bromo derivative (5) is found to be a powerfu l reaction intermediate for the synthesis of ribonucleosides, The reaction of silylated pyrimidines with (5) in the presence of HgO/HgBr2 affords excl usively the beta -anomers (6-8). Deprotection of (6-8) with ammonia in meth anol yields the 2'-C-beta -trifluoromethyl nucleosides (9-11).