Aminocyclopentitol analogs of alpha -L-fucose were synthesized stereoselectively from D-ribose. Alkyl substituents were attached at the NH2 group to mimic the glycosidic leaving group. The resulting (alkylamino)cyclopentitols inhibited alpha -L-fucosidases selectively with inhibition constants in the range of K-i = 10(-7) m. Comparisons with stereoisomers and acyclic analogs showed that this inhibition only occurs with N-alkyl substitution and proper configuration at the cyclopentane, as expected for transition-state-analog-type inhibition. These observations were supported by molecular-modeling comparisons between inhibitor and transition state.