(2R,4S)-2-((R)-carboxy((R)-2-(4-ethyl-2,3-dioxopiperazine-1-carboxamido)-2-phenylacetamido)methyl)-3-((2R,4S)-2-((R)-carboxy((R)-2-(4-ethyl-2,3-dioxopiperazine-1-carboxamido)-2-phenylacetamido)methyl)-5,5-dimethylthiazolidine-4-carbonyl)-5,5-dimethylthiazolidine-4-carboxylic acid