Application of a phase transfer reaction to the synthesis of l-fructose

l-Fructose was prepared in five steps from l-sorbose (1). 1,2-O-Isopropylidene-α-l-sorbopyranose (2), prepared from 1, was selectively tosylated using phase transfer catalysis to give 1,2-O-isopropylidene-3-O-(p-tolylsulfonyl)-α-l-sorbopyranose (3). Formation of a 3,4-anhydro ring, followed by its base-catalyzed opening to effect overall inversion of configuration at both C-3 and C-4 gave 1,2-O-isopropylidene-α-l-fructopyranose (5), which was deacetonated to yield l-fructose (6).