To a solution of 20 (100 mg, 0.12 mmol, 1.00 eq) in anhydrous DCE (3.5 mL) was added a solution of 4-chloro-3-nitrobenazaldehyde (27 mg, 0.18 mmol, 1.50 eq) in anhydrous DCE (3.5 mL) and acetic acid (10 μL, 0.18 mmol, 1.50 eq).