To a solution of 20 (250 mg, 0.30 mmol, 1.00 eq) in anhydrous DCM (9 mL) was added aldehyde 21 (120 mg, 0.36 mmol, 1.20 eq) in anhydrous DCM (9 mL) and acetic acid (20 μL, 0.36 mmol, 1.20 eq).