A sustainable approach to phenylethanoid glycopyranosides: Study of glycosylations promoted by zinc salts

A comparative study of glycoside bond formation between different acetohaloglycopyranoses and acetylated or silylated p-hydroxyphenethyl alcohols (derivatives of tyrosol and hydroxytyrosol) with zinc oxide or basic zinc carbonate as promoters in various organic solvents under conventional or microwave heating is presented. The hexopyranosyl donors provided 1,2-trans products with excellent stereoselectivity under optimized reaction conditions. More flexible pentopyranosyl bromides reacted easily, but anomeric stereoselectivity could not be controlled very well. In particular, microwave irradiation accelerated reactions from hours to minutes and thermodynamic products (1,2-cis-glycosides) have been produced to an increased extent. A representative disaccharide - per-O-acetylated rungiosyl bromide also provided 1,2-trans hydroxytyrosol diglycoside with excellent stereoselectivity. This heterogeneous reaction arrangement did not affect disaccharide bond or the readily removable acetyl or silyl protecting groups on the aglycones and facilitated the subsequent isolation of the products. [Display omitted] •Application of easily available, cheap and harmless promoters for 1,2-trans-glycosylation.•No toxic glycosylation promoters or activators.•Utilization of microwave heating as green alternative energy.•No extraction process for isolation of crude products from reaction.•A range of phenylethanoid glycoside derivatives were synthesized.