Green syntheses of new 2-C-methyl aldohexoses and 5-C-methyl ketohexoses : D-tagatose-3-epimerase (DTE)-a promiscuous enzyme

Abstract The Kiliani synthesis on the 4 readily accessible ketohexoses ( d -fructose, d -tagatose, l -sorbose, d -psicose) allows access to 4 diastereomeric 2- C -methyl-aldohexoses (2- C -methyl- d -mannose, 2- C -methyl- d -talose, 2- C -methyl- l -gulose, 2- C -methyl- d -allose) and 4 diastereomeric 2- C -methyl-alditols (2- C -methyl- d -mannitol, 2- C -methyl- d -talitol, 2- C -methyl- l -gulitol, 2- C -methyl- d -allitol). Microbial oxidation of 2- C -methyl- d -mannitol and 2- C -methyl- l -gulitol gave 5- C -methyl- d -fructose; microbial oxidation of 2- C -methyl- d -talitol afforded 5- C -methyl- d -psicose, whereas 2- C -methyl- d -allitol formed 5- C -methyl- l -psicose. Both enantiomers of 5- C -methyl-fructose were equilibrated by d -tagatose-3-epimerase (DTE) with both enantiomers of 5- C -methyl-psicose. These transformations demonstrate that polyol dehydrogenases and DTE act on branched synthetic sugars. Full NMR analyses show that 5- C -methyl- d -fructose is present as the β-pyranose and β-furanose forms in a ratio of 90:10; all pyranose and furanose forms of 5- C -methyl- d -psicose are present in solution. The combination of chemical and biological procedures allows the environmentally friendly generation of a new family of branched monosaccharides.