Reaction of the trimethylsilylated derivative of 1,4-thiazin-3-one with 1-O-acetyl-2,3,5-tri-O-benzoyl-D-ribofuranose in the presence of SnCl4 gave, after deblocking, 4-beta-ribofuranosyl-1,4-thiazin-3-one (8). Treatment of 1,4-thiazin-3-one with 1-chloro-2-deoxy-3,5-di-O-p-toluoyl-alpha-D-erythro-pentofuranose in the presence of sodium hydride provided, after deblocking, the corresponding 2-deoxy-beta-D-ribofuranosyl derivatives (19). Oxidation of 4-(2,3,5-tri-O-benzoyl-beta-D-ribofuranosyl)-1,4-thiazin-3-one (7) with 1 equiv of m-chloroperbenzoic acid resulted in 4-(2,3,5-tri-O-benzoyl-beta-D-ribofuranosyl)-1,4-thiazine-2,3-dione (9) and 4-(2,3,5-tri-O-benzoyl-beta-D-ribofuranosyl)-1,4-thiazin-3-one 1-oxide (10). Evidence is presented that indicates that the oxidation of the thiazine at the 2-position is due to a Pummerere rearrangement. The new compounds failed to show significant activity against tumor cell lines in culture, L1210 cells in vivo, virus cytotoxicity in cell culture, or cytidine deaminase.