Pyrimidines. Part 60. Nucleosides. Part 6. New Chemical Modification of the Ribosyl Moiety in Uridines: Synthesis of 2,2′‐Anhydro‐1‐(5‐deoxy‐5‐(substituted thio)‐β‐D‐arabinofuranosyl)uracil Derivatives and Their Conversion into 3′,5′‐Epithiopyrimidine Nucleosides.