Synthesis of l-Rare Sugars from d-Sorbitol Using a TiO2 Photocatalyst

In recent years, l-rare sugars have been attracting attention, both as active compounds and useful building blocks for use in the pharmaceutical industry; however, these compounds have low natural abundances and cannot be extracted from natural products. Therefore, it is necessary to convert widely available substrates, such as biomass-derived compounds, to produce rare sugars. In this study, we examined the production of l-rare sugars using a TiO₂ photocatalyst. d-Sorbitol was treated with a TiO₂ photocatalyst under light irradiation, and the products were identified as glucose, gulose, fructose, sorbose, arabinose, xylose, erythrose, threose, and glyceraldehyde using high-performance liquid chromatography, liquid chromatography–mass spectrometry, and proton nuclear magnetic resonance spectroscopy. Furthermore, the production of the l-rare sugars, l-gulose and l-xylose, was confirmed by optical rotation measurements. Analysis of the product concentrations as a function of time suggested that d-sorbitol undergoes a decarbonization reaction and C–C bond cleavage when treated with a TiO₂ photocatalyst.