Solvent- and Microwave-Assisted Dehydrations of Polyols to Anhydro and Dianhydro Polyols

Carbohydrate polyol (sorbitol, mannitol, and xylitol) dehydrations by both strong and weak acid catalysts were enhanced by microwave irradiation when performed in aqueous dioxane. Reaction conditions such as the percent of water, type and concentration of acid, reaction time, and temperature were varied. Reactions with different conditions were compared by using the “combined severity” parameter. An 88% yield of the dianhydro­polyol iso­sorbide was achieved by heating sorbitol in a microwave for only 20 min at 205 °C in 7% water/dioxane with H2SO4 as the catalyst, while heating for 1.5 h at 210 °C with the weak acid salt NaHSO4 gave 74% iso­sorbide. An 85% yield of 1,4-anhydro­xylitol was achieved by heating xylitol in a microwave, with H2SO4 as the catalyst, for 15 min at 180 °C in 28% water/dioxane. Increasing reaction severity in the presence of the H2SO4 catalyst gave organic sulfate impurities due to solvent cleavage; in contrast, no solvent cleavage was observed with the weak acid catalyst NaHSO4.