The presence of water improves reductive openings of benzylidene acetals with trimethylaminoborane and aluminium chloride Original Research Article

The acidic reagent formed in situ from anhydrous AlCl3 and H2O in 3:1 ratio is much more efficient for the reductive openings of the cyclic benzylidene acetals with Me3N·BH3 in tetrahydrofurane than the AlCl3 alone. Under proposed conditions, the dioxane-type 4,6-O-bezylidene acetals of hexopyranosides give regioselectively the corresponding 4-hydroxy,6-O-benzyl derivatives in excellent yields. Reductive openings of the dioxolane-type 3,4-O-benzylidene acetals of galactopyranoside are also very efficient and regioselective and give either 3-O-benzyl derivative (from 3,4-O-exo-benzylidene acetal) or 4-O-benzyl derivative (from 3,4-O-endo-benzylidene acetal) depending on the configuration of the acetal carbon atom.