X-ray studies reveal that tert-butyl (6S)-6-iso­butyl-2,4-dioxo­piperidine-1-carboxyl­ate occurs in the 4-enol form, viz. tert-butyl (6S)-4-hydroxy-6-iso­butyl-2-oxo-1,2,5,6-tetra­hydropyri­dine-1-carboxyl­ate, C14H23NO4, when crystals are grown from a mixture of di­chloro­methane and pentane, and has an axial orientation of the iso­butyl side chain at the 6-position of the piperidine ring. Reduction of the keto functionality leads predominantly to the corresponding [beta]-hydroxy­lated [delta]-lactam, tert-butyl (4R,6S)-4-hydroxy-6-iso­butyl-2-oxo­piperidine-1-car­boxyl­ate, C14H25NO4, with a cis configuration of the 4-hydroxy and 6-iso­butyl groups. The two compounds show similar molecular packing driven by strong O-H...O=C hydrogen bonds, leading to infinite chains in the crystal structure.