Structure–sweetness relationships for fructose analogs. Part III. 3-Deoxy-D-erythro-hexulose (3-deoxy-D-fructose): composition in solution and evaluation of sweetness

Abstract

As part of continuing studies on the structural features responsible for the intense sweetness of D-fructose, 3-deoxy-D-erythro-hexulose (3-deoxy-D-fructose, 1) was prepared, its solution composition was determined, and its taste was evaluated. In aqueous solution, 3-deoxy-D-fructose exists as a complex mixture of five tautomeric forms in which the β-D-pyranose form is preponderant (52.5% at 22 °C) and the α-D-pyranose form is the least abundant (5%). Quite remarkable is the behavior of the open-chain keto form of 1: its content increases from 7.5% at 22 °C, to 36% at 82 °C, and to 47% at 97 °C, making this form the preponderant one at high temperatures. 3-Deoxy-D-fructose was found to be sweet, albeit probably not as sweet as D-fructose. The hydroxyl group at C-3 is thus not an essential function of the glycophore of D-fructose. The significance of this result is discussed in relation to the evidence already available and the divergent theories that have been proposed to explain the origin of the sweet taste of D-fructose. Keywords: 3-deoxy-D-erythro-hexulose (3-deoxy-D-fructose), sweetness, solution composition.