A series of new N-alkyl-3-hydroxypiperidine derivatives containing a five-membered hemiketal were synthesized from 1-C-acetylmethyl sugars. Based on the stereochemistry of the products obtained, a plausible mechanism is illustrated. A new approach to the hemiketal, which has undergone hydrogenation in the presence of Pd/C and beta-elimination and intramolecular cycloaddition with 1% NaOMe in methanol