Efficient ring closure of stable crystalline 3,5-di-O-triflates of pentofuranosides with amines to form azetidines allowed preliminary evaluation of four-ring iminosugars as glycosidase inhibitors; significant and specific inhibition of nonmammalian 伪-glucosidases is shown by class="smallcaps">l-xylo- and class="smallcaps">l-arabino-iminosugar azetidines.