To a solution of 1,4-di-tert-butyl 2-methyl piperazine-1,2,4-tricarboxylate 29 (1.0 g, 2.9 mmol) in DCM (20 mL) was added TFA (2.31 g, 20.3 mmol) at 0° C. The reaction mixture was stirred at RT for 4 h. TLC analysis indicated complete conversion of SM. Reaction was concentrated to remove excess of TFA and sticky solid so obtained was taken in DCM (20 mL). To this suspension, HATU (1.65 g, 4.34 mmol), NMM (293 mg, 2.9 mmol), 3-formylbenzoic acid (435 mg, 2.9 mmol), and DMAP (10 mg) were added. Reaction mixture was stirred at RT for 16 h. The reaction was monitored by LCMS. The reaction mixture was diluted with DCM and washed with water; organic layer was separated, dried over Na2SO4 and concentrated under reduced pressure. The sticky solid so obtained was added to a solution of NaOH (65 mg, 1.6 mmol) in MeOH (5 mL). Reaction mixture was stirred at rt for 7 h. The reaction was monitored by TLC. When SM was completely consumed, reaction mixture was concentrated under vacuum. The solid was suspended in water and acidified to pH 6 using 1 N HCl. The suspension was filtered and the precipitate was triturated with ethanol to give 766 mg (20%) of 4-(3-formylbenzoyl)piperazine-2-carboxylic acid 32 as sticky solid.
