NUCLEOPHILIC REPLACEMENT REACTIONS OF SULPHONATES .2. SYNTHESIS OF DERIVATIVES OF 4,6-DITHIO-D-GALACTOSE AND -D-GLUCOSE AND THEIR CONVERSION INTO 4,6-DIDEOXY-D-XYLO-HEXOSE

The replacement reactions of the 2,3-diacetate and 2,3-dibenzoate of methyl 4,6-di-O-mesyl-α-D-glucopyranoside with potassium thiocyanate in N,N-dimethyl-formamide have been studied. They proceed mainly via an SN2 mechanism to give the corresponding 4,6-dideoxy-4,6-dithiocyanato-D-galactopyranosides. In the case of the diacetate, however, a small proportion (≈1%) of the 4,6-dideoxy-4,6-dithio-cyanato-D-glocopyranoside was isolated, which arose by participation of the neighbouring 3-acetoxy substituent. This D-glucopyranoside was also prepared from methyl 2,3-di-O-acetyl-4,6-di-O-toluene-p-sulphonyl-α-D-galactopyranoside by treatment with potassium thiocyanate in N,N-dimethylformamide. The 4,6-dideoxy-4,6-dithiocyanato-D-galactoside undergoes base-catalysed hydrolysis to give, presumably, the 4,6-dithio-D-galactopyranoside which spontaneously oxidises with air to give the intramolecular 4,6-disulphide. Desulphurisation of these 4,6-dithiocyanates and the 4,6-disulphide with Raney nickel, followed by acid hydrolysis, gale 4,6-dideoxy-D-xylo-hexopyranose. © 1968.