(S)-malic, (S)-citramalic and (S,R)-thiomalic acid, new types of 4,4-difluoro-substituted a-ami- no-, a-hydroxy- and a-mercaptopentanoic acids have been synthe- sized, applying hexafluoroacetone as protecting and activatingreagent. The new partially fluorinated a-functionalized carboxylic acids represent interesting monomers for peptide and depsipeptidemodification and for rational design and elucidation