Epoxidations with triphenylphosphine and diethyl azodicarboxylate. Part 1. Synthesis of methyl 3,4-anhydro-D-tagatofuranosides

Treatment of methyl β-D-fructofuranoside (6) with triphenylphosphine and diethyl azodicarboxylate in dimethylformamide gives a high yield of methyl 3,4-anhydro-β-D-tagatofuranoside (5); no blocking of the hydroxy-groups at C-1 and C-6 is necessary. The structure of the product was confirmed by an X-ray structural study of its 1,6-bis-O-(p-tolylsulphonyl) derivative and by an unambiguous synthesis. A similar reaction of methyl α-D-fructofuranoside yielded the 3,4-anhydro-α-D-tagatofuranoside. The reaction mechanism is discussed.