Intermediate VIII (Scheme 3), e.g., 3-(biphenyl-4-ylsulfonyl)-1,3-thiazolidine-2-carboxylic acid (1.747 g, 5 mmol) was dissolved in DCM (20 mL). The resulting solution was cooled down to −10° C. and oxalyl chloride (0.645 mL, 7.5 mmol) was added slowly. DMF (0.1 mL) was added carefully. The reaction mixture was allowed to warm to RT over 1 h and stirred an additional hour at RT. As the reaction was complete, the solvents were evaporated. Toluene was added and evaporated to remove residual oxalyl chloride. This process was repeated twice, affording intermediate XXX, e.g., 3-(biphenyl-4-ylsulfonyl)-1,3-thiazolidine-2-carbonyl chloride as a yellow solid (1.839 g, quantitative yield). α-Aminophenylacetic acid (831 mg, 5.5 mmol) was dissolved in water (20 mL). TEA (2.77 mL, 20 mmol) was added carefully. THF (25 mL) was added to reaction mixture. The reaction mixture was cooled to 0° C. Acid chloride previously prepared XXX, e.g. 3-(biphenyl-4-ylsulfonyl)-1,3-thiazolidine-2-carbonyl chloride dissolved in THF (25 mL) was added dropwise. The reaction mixture was stirred 15 min at 0° C. and overnight at RT. Solvents were concentrated and the resulting aqueous fraction was acidified with HCl 5N and extracted with 3 portions of EtOAc. Combined organic phases were dried over MgSO4, filtrated and evaporated. The crude product was recrystallized in acetone/Et2O mixture, affording carboxylic acid Ia, e.g., ({[3-(biphenyl-4-ylsulfonyl)-1,3-thiazolidin-2-yl]carbonyl}amino)(phenyl)acetic acid in 97% purity by HPLC (943 mg, 39% yield).
