Synthése, conformation et méthanolyse des chlorures de 2,3,4-tri-O-chlorosulfonyl-β- et α-L-fucopyranosyle

Treatment of α- L -fucose with sulfuryl chloride at low temperature gave mainly 2,3,4-tri- O -chlorosulfonyl-β- L -fucopyranosyl chloride ( 1 ) and a small proportion of the α-anomer ( 2 ). Both compounds adopt a 1 C 4 chair conformation. Methanolysis of 1 in the presence of silver carbonate and anhydrous calcium sulfate gave methyl 2,3,4-tri- O -chlorosulfonyl-α- L -fucopyranoside (the β-anomer being only present in small proportion), further converted into methyl α- L -fucopyranoside by treatment with a basic resin and a catalytic amount of sodium iodide. Methanolysis of 1 in the presence of sodium iodide gave directly methyl α- L -fucopyranoside, in a more rapid but less stereoselective way. Methanolysis of 2 in the presence of silver carbonate is very slow and gave, after removal of the chlorosulfonyl groups, methyl β- L -fucopyranoside with a rather poor stereoselectivity.