Synthesis of Hexane-Tetrols and -Triols with Fixed Hydroxyl Group Positions and Stereochemistry from Methyl Glycosides over Supported Metal Catalysts

Abstract

Carbohydrates are a renewable feedstock for the production of partially reduced polyols, but typical hydrogenolysis processes are unselective toward C-O bond cleavage at different positions and erase the stereocenters present in the feedstock. In this study, we demonstrate the synthesis of new types of acyclic polyols from methyl glycosides with fixed hydroxyl group positions and stereochemistry. Products include (2R,3S)-1,2,3,6-hexanetetrol, (2R,5S)-1,2,5,6-hexanetetrol, (2S,5S)-1,2,5-hexanetriol, and (4R,5S)-1,4,5-hexanetriol. Methyl glycosides are first selectively deoxydehydrated and hydrogenated to methyl dideoxy-glycosides as reported in previous work. These methyl dideoxy-glycosides are then converted to hexane-triols and -tetrols over Pt-based catalysts in water in 80-95% yield via methoxy bond hydrolysis and hydrogenation. This route largely preserves the stereocenters of the remaining hydroxyl groups (>92% stereopurity). The nature of the intermediates formed depends on the structure of the glycoside feedstock. 3,4-Dideoxy-glycosides can undergo inversion of the C2-OH stereocenter because of an aldose-ketose isomerization reaction, which can be mitigated by using a bifunctional metal-acid catalyst to facilitate the reaction at lower temperature. By demonstrating a new route to produce renewable polyols with fixed hydroxyl group positions and stereochemistry, this report lays the groundwork for further research into the applications of these molecules in the chemical industry.