(4S)-3-{[(2R,4S)-2-[(4R)-1-carboxy-6-(4-ethyl-2,3-dioxopiperazin-1-yl)-3,6-dioxo-4-phenyl-2,5-diazahex-1-yl]-5,5-dimethyl-1,3-thiazolidin-4-yl]carbonyl}-2-[(4R)-1-carboxy-6-(4-ethyl-2,3-dioxopiperazin-1-yl)-3,6-dioxo-4-phenyl-2,5-diazahex-1-yl]-5,5-dimethyl-1,3-thiazolidine-4-carboxylic acid