Stereospecific molybdic acid-catalyzed isomerization of 2-hexuloses to branched-chain aldoses

On treatment with a catalytic amount of molybdic acid in aqueous solution, the 2-ketohexoses d-fructose, l-sorbose and d-tagatose undergo a stereospecific intramolecular rearrangement to give the corresponding 2-C-(hydroxymethyl)aldoses, 2-C-(hydroxymethyl)-d-ribose (d-hamamelose), 2-C-(hydroxymethyl)-l-lyxose, and 2-C-(hydroxymethyl)-d-xylose, respectively. At equilibrium, the ratio of 2-ketose to 2-C-(hydroxymethyl)aldose ranged from 14:1 (fructose) to 32:1 (sorbose). A similar treatment of d-psicose failed to yield a significant amount of the corresponding branched-chain aldose. The equilibria can be shifted with the addition of boric acid to the reaction mixture; under these conditions, ratios of 3:1 and 7:1 were obserwed for d-fructose and l-sorbose, respectively. A mechanistic study with d-(3-13C)fructose afforded d-(1-13C)hamamelose, thus confirming C-3C-4 bond cleavage with concomitant C-2C-3 transposition suggested from recent studies with d-(2-13C)fructose.