3,4-Anhydro-1,2-O-isopropylidene-β-d-tagatopyranose and 4,5-anhydro-1,2-O-isopropylidene-β-d-fructopyranose

3,4-Anhydro-1,2-O-isopropylidene-β-d-tagatopyranose (8) and 4,5-anhydro-1,2-O-isopropylidene-β-d-fructopyranose (10) have been prepared by treatment of 3,5-di-O-acetyl-1,2-O-isopropylidene-4-O-toluene-p-sulfonyl-β-d-fructopyranose with methanolic sodium methoxide. The structures of 8 and 10 were assigned by 1H and 13C NMR spectroscopy and that of 10 by X-ray crystallography; both exist in half-chair conformations. Compounds 8 and 10 interconvert in aqueous sodium hydroxide, giving a ratio of 1:2 at equilibrium. The monoacetates of 8 and 10 (5-O-acetyl-3,4-anhydro-1,2-O-isopropylidene-β-d-tagaopyranose and 3-O-acetyl-4,5-anhydro-1,2-O-isopropylidene-β-d-fructopyranose) undergo stereospecific epoxide ring opening in 80% acetic acid to give mainly the axial monoacetates 5-O-acetyl-1,2-O-isopropylidene-β-d-fructopyranose and 4-O-acetyl-1,2-O-isopropylidene-β-d-tagatopyranose, respectively.