To a solution of (2S,3R)-2-hydroxy-3-{3-[4-(trifluoromethoxy)phenyl]propyl}butanedioic acid (5.58 g; 16.6 mmol; 1.0 eq.) in acetone (61 mL) was added 2,2-dimethoxypropane (3.5 mL; 28.2 mmol; 1.70 eq.) and copper(II) chloride (220 mg; 1.66 mmol; 0.1 eq.). The resulting reaction mixture was stirred 14 h at RT. The reaction mixture was evaporated and the residue taken up in Et2O and filtered on a pad of cellite. The liquids were evaporated again and the residue was treated with activated charcoal in Et2O and filtered on a pad of cellite to give the title product as a greenish oil used in the next step without further purification. (5.85 g; 93.7%). M+(ESI): 377.0; M−(ESI): 375.1.
