(R)-1-((S)-1-Benzylcarbamoyl-2-phenyl-ethyl)-2-hydroxy-5-oxo-2,5-dihydro-1H-pyrrole-2-carboxylic acid ethyl ester

2-{[(1S,2S)-1-((S)-2-tert-Butoxycarbonyl-pyrrolidine-1-carbonyl)-2-methyl-butylcarbamoyl]-ethynyl}-2-hydroxy-malonic acid diethyl ester

(R)-1-[(1S,2S)-1-((S)-2-Carboxy-pyrrolidine-1-carbonyl)-2-methyl-butyl]-5-oxo-2-triethylsilanyloxy-2,5-dihydro-1H-pyrrole-2-carboxylic acid ethyl ester

2-[((S)-1-Benzylcarbamoyl-2-phenyl-ethylcarbamoyl)-ethynyl]-2-hydroxy-malonic acid diethyl ester

2-{[(1S,2S)-1-((S)-2-Carboxy-pyrrolidine-1-carbonyl)-2-methyl-butylcarbamoyl]-ethynyl}-2-hydroxy-malonic acid diethyl ester