Nucleosides. 137. (Trifluoromethyl)sulfonyl (triflyl) migration. Synthesis of 6,3'-anhydro-3-benzyl-1-(5-chloro-5-deoxy-.beta.-D-xylofuranosyl)barbituric acid from the 2'-trifluoromethanesulfonate (triflate) of 6,5'-anhydro-3-benzyl-1-.beta.-D-ribofuranosylbarbituric acid