5-thio- l-rhamnose

Abstract

Two routes for the synthesis of methyl 5- S-acetyl-6-deoxy-2,3- O-isopropylidene-5-thio- l-mannofuranoside (8) have been examined. Reaction of l-rhamnose with methanol in the presence of the cation-exchange resin gives methyl 6-deoxy-α- l-mannofuranoside (2), which on conventional acetonation yields methyl 6-deoxy-2,3- O- isopropylidene-α- l-mannofuranosides (3). Compounds 3 is also obtained by acetonation of l-rhamnose followed by treatment with a mixture of methanol, acetonation, Amberlite IR-120(H +) resin. Chlorination of 3 with triphenylphosphine-carbon tetrachloride gives methyl 5-chloro-5,6-dideoxy-2,3- O-isopropylidene-β- d-gulofuranoside (7), which reacts with potassium thioacetate to give 8. Alternatively, 3 is iodized with ruthenium tetraoxide to methyl 6-deoxy-2,3- O-isopropylidene-α- l- lyxo-hexofuranosid-5-ulose (9), which reduced by sodium borohydride mainly to methyl 6-deoxy-2,3- O-isopropylidene-β- d-gulofuranoside (10). The O-tosyl derivative of 10 reacts with potassium thioacetate to produced 8. Hydrolysis of 8 with 90% aqueous triflouroacetic acid, followed by acetolysis with a solution of acetic acid, acetic anhydride, and sulfuric acids gives an anomeric mixture of 1,2,3,4,-tetra- O-acetyl-6-deoxy-5-thio- l-mannopyranoses (12), together with a small proportion of 1,2,3,-tri- O-acetyl-5- S-acetyl-6-deoxy-5-thio-β- l-mannofuranose (13). Deacetylation of 12 or 13 gives 5-thio- l-rhamnose (6), from which crystalline 1,2,3,4-tetra- O-( p-nitrobenzoyl)-5-thio-β- l-rhamnopyranose (14) is obtained.