Selective Cyclisation of 2,3,4,5-Tetra-O-Acetylgalactaric Acid Bis[Alkylthio(Thiocarbonyl)]Hydrazides to Saccharide 1,3,4-Oxadiazole, Thiadiazole, and Thiadiazoline Derivatives

The syntheses of galactaric acid acetate bis[alkylthio-(thiocarbonyl)]hydrazides (1,2) are described. Selective cyclisation of both hydrazide 1 and 2 was investigated. Using phosphorous oxychloride as a cyclising agent, loss of water produced 1,2,3,4-tetra-O-acetyl-1,4-bis(5-S-methyl or benzyl)-1,3,4-thiadiazol-2-yl)galacto-tetritol (3) or (4). Use of thionyl chloride lead to dehydrosulfurization and gave 1,2,3,4-tetra-O-acetyl-1,4-bis(5-S-methyl or benzyl)-1,3,4-oxadiazol-2-yl)galacto-tetritol (5) or (6). Finally, with triethyl orthoformate as the cyclising agent, compound 1 or 2 gave 3,3′-(2,3,4,5-tetra-O-acetylgalactar-1,6-dioyl)-bis-[(2-ethoxy-2,3-dihydro-5-S-methyl or benzyl)-1,3,4-thia-diazole] (7) or (8).