Regio- and Stereocontrolled Selective Debenzylation and Substitution Reactions of C<i>-</i>2 Formyl Glycals. Application in the Synthesis of Constrained β-Sugar Amino Acids

O-Benzyl-protected C-2 formyl glycals are regioselectively deprotected and acetylated first at C-3 and then at C-6 upon treatment with a ZnCl2−Ac2O−AcOH reagent combination. Treatment of the thus-obtained C-3 acetate with various nucleophiles leads to substitution at C-3 with retention of stereochemistry in the galactal series, whereas mixed results were obtained with glucal-derived compounds. Two of the azido-substituted products were converted into the corresponding β-sugar amino acids.