Convenient Chemo-Enzymatic Synthesis ofd-Tagatose

The ketohexose d-tagatose is a rare sugar that is of interest as a potent noncaloric sweetener. The synthesis of d-tagatose has been accomplished by microbiological conversion of dulcitol1,2 as well as by chemical syntheses in low yields originating from d-galactose3 or d-fructose.4 Recently, some patents concerning the synthesis of d-tagatose have been published.5,6,7 We here present an alternative approach to the preparation of d-tagatose by a combined chemoenzymatic synthesis starting from d-galactose. Enzymatic oxidation of d-galactose (1) leads to the 2-oxidised product, d-galactosone, which in turn is reduced chemically to d-tagatose (2). The target sugar is thus available in a one-pot / two-step procedure in a yield of 30 %.