Stereospecific Molybdic Acid‐Catalyzed Isomerization of 2‐Hexuloses to Branched‐Chain Aldoses.

Abstract 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- 13 C)fructose afforded d -(1- 13 C)hamamelose, thus confirming C-3C-4 bond cleavage with concomitant C-2C-3 transposition suggested from recent studies with d -(2- 13 C)fructose.