Racemic and enantiomerically pure manno-configured isoquinuclidines were synthesized and tested as glycosidaseinhibitors. The racemic key isoquinuclidine intermediate was prepared in high yield by a cycloaddition (tandem Michael addition/aldolisation) of the 3-hydroxy-1-tosyl-pyridone 10 to methyl acrylate, and transformed to the racemic N-benzyl manno-isoquinuclidine 2 and the N-unsubstituted manno-isoquinuclidine