N4-methylation of (3S,6S)-3-alkyl-6-benzylpiperazine-2,5-diones (S,S)-1a-c was found to change their folded conformation to an extended conformation. Conformational aspects of N1- and/or N4-methylated (S,S)-1a-c were revealed by single crystal X-ray crystallography and H-1 NMR spectroscopy. (C) 2016 Elsevier B.V. All rights reserved.