Typical procedure for the synthesis of 4aa from 1a and 2a. In a reaction vial equipped with a magnetic stir bar, 1a (31.2 mg, 0.15 mmol, 1.50 equiv) and 2a (28.4 mg, 0.10 mmol, 1.00 equiv) were added into CH2Cl2 (1.0 mL). B(C6F5)3 (10.2 mg, 0.02 mmol, 20 mol%) was then added. The reaction vial was sealed, and the resulting mixture was stirred at 40 ºC for 2 h. The reaction was cooled down to room temperature, concentrated under reduced pressure and purified by flash column chromatography (eluent: