1-(9H-Fluoren-9-ylmethyl) (2S,4S)-4-[[1-(4,4-dimethyl-2,6-dioxocyclohexylidene)-3-methylbutyl]amino]-1,2-pyrrolidinedicarboxylate (ACI),(2S,4S)-1-(((9H-fluoren-9-yl)methoxy)carbonyl)-4-((1-(4,4-dimethyl-2,6-dioxocyclohexylidene)-3-methylbutyl)amino)pyrrolidine-2-carboxylic acid,-