2-Bromo-1-ethylpyridinium Tetrafluoroborate

Experimental procedure³⁾: To a solution of N-Boc-N-methyl-L-leucine methyl ester (3.00 g, 12.0 mmol) in CH₂Cl₂ (50 mL), trifluoroacetic acid (9.2 mL, 120 mmol) is added at -20 °C. The mixture is warmed up to rt and stirred for 2 h. The mixture is diluted with CH₂Cl₂ and neutralized carefully with sat. aq. NaHCO₃. The aqueous layer is extracted with CH₂Cl₂ (2x) and the combined organic layers are dried over Na₂SO₄. After removal of the solvent in vacuo, the free amine is dissolved in dry CH₂Cl₂ (50 mL) and N-Cbz-(L)-valine (3.32 g, 13.2 mmol) and DIEA (4.1 mL, 24.0 mmol) are added. Then, BEP (3.61 g, 13.2 mmol) is added in small portions at rt, what resulted in a yellow colored solution. The mixture is stirred overnight, diluted with CH₂Cl₂ and washed with 1 mol/L aq. KHSO₄, sat. aq. NaHCO₃ and brine subsequently. The solvent is removed after drying (Na₂SO₄) and the residue is purified by column chromatography (silica, PE:EtOAc 8:2). The dipeptide 1 (4.08 g, 10.4 mmol, 87 %) is obtained as a colorless resin.