The preparation of carbohydrate mimics in which the endocyclic oxygen has been replaced by a guanidine-type nitrogen atom is reported. The synthetic strategy involves the furanose --> piperidine rearrangement of 5-deoxy-5-guanidino-L-idose precursors. The reaction proceeds through elimination of water to give 3-oxopiperidines, which were isolated as the corresponding hydrates. Biological evaluation of the new glycomimetics evidenced a strong influence of the nature of the substituents at the nitrogen atoms on the glycosidase inhibitory properties.