(2S,3'S)-2-isobutyl-2-[4'-(tert-butoxycarbonyl)-3'-allyl-2'-oxopiperazin-1'-yl]acetic acid

(S)-2-Benzyl-4-((S)-1-hydroxymethyl-3-methyl-butyl)-3-oxo-piperazine-1-carboxylic acid tert-butyl ester

(S)-2-Benzyl-4-((S)-1-carboxy-3-methyl-butyl)-3-oxo-piperazine-1-carboxylic acid tert-butyl ester

(S)-2-(2-Bromo-benzyl)-4-((S)-1-hydroxymethyl-3-methyl-butyl)-3-oxo-piperazine-1-carboxylic acid tert-butyl ester