1.5 g (7.18 mmol) of 2-amino-3-(4-boronophenyl)propionic acid were suspended in 50 ml DCM and 7.4 ml (43.1 mmol) DIPEA and refluxed for 20 min together with 4.5 ml (35 mmol) TMSCl. The solution was cooled in an ice bath and a solution of 1.44 g (7.9 mmol) 2,4,6-trimethylbenzoyl chloride in 5 ml DCM were added dropwise. The reaction mixture was stirred 2 h at room temperature, diluted with 100 ml DCM and washed with 2 N aqueous HCl. During this washing some product precipitated which was collected. The organic phase was extracted with 2 N NaOH twice. Upon acidifying of the combined basic phases the product precipitated, which was collected and dried together with the first precipitate. The crude product was used without further purification.
