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 stereospec ific intramolecular rearrangement to give the corresponding 2-C-(hydroxymet hyl)aldoses, 2-C-(hydroxymethyl)-D-ribose (D-hamamelose), 2-C-(hydroxymethy l)-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 (fruc tose) to 32:1 (sorbose). A similar treatment of D-psicose failed to yield a significant amount of the corresponding branched-chain aldose. The equilib ria 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-C-13)fructose a fforded D-(1-C-13)hamamelose, thus confirming C-3-C-4 bond cleavage with co ncomitant C-2-C-3 transposition suggested from recent studies with D-(2-C-1 3)fructose. (C) 1999 Elsevier Science Ltd. All rights reserved.