Fig. 3: Optimization of the conditions^a.

^aReaction conditions: To a Schlenk tube was added additive (0 or 250 mg), 2a (1.25 mmol), CH₂Cl₂ (4 mL) and Tf₂O (0.5 mmol) successively. After stirring at specified temperature for 0.5 h, a solution of 1a (0.5 mmol) in CH₂Cl₂ (4 mL) was added. The reaction mixture slowly increased to room temperature, and then continued to stir for 2 h. ^bIsolated yield. ^c1a (0.55 mmol) was used. ^d2a (0.5 mmol) and Et₃N (1.5 eq.) were used. ^e2a (0.5 mmol) and NaHCO₃ (1.5 eq.) were used.