New substituted pyrrolidine-3,4-diol derivatives were prepared from D-(-)- and L-(+)-phenyl glycinol. The influence of the configuration and the substitution of the lateral side chain of these derivatives on the inhibition of 25 commercial glycosidases were determined. (2R,3R,4S)-2-({[(1R)-2-Hydroxy-1-phenylethyl]amino}methyl)pyrrolidine-3,4-diol ((+)-7a) was a potent and selective inhibitor of jack bean alpha-mannosidase (Ki = 135 nM). However, when evaluated on human tumor cells, 7a, and the reference compound swainsonine, did not efficiently inhibit the growth of glioblastoma cells. Further derivatization of the hydroxyl group with lipophilic groups to increase bioavailability improved their growth inhibitory properties for human glioblastoma and melanoma cells. In particular the 4-bromobenzoyl derivative 26 demonstrated high efficacy for human tumor cells whereas primary human fibroblasts were less sensitive to 26. Therefore functionalized pyrrolidines have the potential to inhibit the growth of tumor cells and display selectivity for tumor cells compared to normal cells.