Fluorocarbohydrates

The action of potassium hydrogen fluoride (KHF 2 ) in ethane-1,2-diol on methyl 2,3-anhydro-4- O -benzyl-β- D -ribopyranoside ( 1 ) and methyl 2,3-anhydro-5- O -benzyl-α- D -lyxofuranoside ( 7 ) has been examined. In both cases the expected trans scission of the epoxide ring occurs to yield methyl 4- O -benzyl-3-deoxy-3-fluoro-β- D -xylopyranoside ( 2 ) and methyl 5- O -benzyl-3-deoxy-3-fluoro-α- D -arabinofuranoside ( 8 ) respectively. The structure of compound ( 2 ) was established by catalytic hydrogenation, which removed the benzyl group, to yield a crystalline glycoside ( 3 ) which did not consume periodate. Acid hydrolysis of methyl 3-deoxy-3-fluoro-β- D -xylopyranoside ( 3 ) yielded syrupy 3-deoxy-3-fluoro-αβ- D -xylose ( 5 ) as a syrup characterized as the 2,5-dichlorophenylhydrazone. Acid hydrolysis of compound ( 8 ) yielded a reducing sugar ( 9 ), characterised as the phenylhydrazone. Compound ( 9 ) consumed 1 mol. of periodate and liberated formic acid which is consistent with the structure 5- O -benzyl-3-deoxy-3-fluoro-αβ- D -arabinose. Catalytic hydrogenation of the fluorohydrin ( 8 ) removed the benzyl group to give methyl 3-deoxy-3-fluoro-α- D -arabinofuranoside ( 10 ) which, on acid hydrolysis, yielded crystalline 3-deoxy-3-fluoro-β- D -arabinose ( 13 ). The β-configuration was assigned to compound ( 13 ) on the basis of its mutarotation. This method of introducing fluorine into carbohydrates affords reasonable yields of uncontaminated fluorohydrins and is considered to have general applicability.