Subsequently, we applied the optimized conditions to the challenging yet highly important nucleophiles, aliphatic amines, which had previously demonstrated their difficulty as substrates in our oxetane synthesis work. As expected, no desired product was overserved when aliphatic amine 10 was employed, likely due to the strongly coordinating power of the amine, which deactivated Cu(OTf)2 catalyst (Table , entry 1). Fortunately, when refluxing ATA 5 and amine 10 in the presence of one equivalent of MsOH in MeNO2, the desired product 11 was obtained in 75% yield (Table , entry 2). The addition of MsOH as the activator proved to be crucial; omitting MsOH or replacing it with other Brønsted acids led to diminished yields (Table , entries 3-4). Investigation of solvents revealed that MeNO2 served as the optimal solvent for this reaction (Table , entries 5-7), probably due to its exceptional ability to stabilize carbocation. The choice of protecting groups proved to significantly influence the reaction yields (Table , entries 8-9). Lastly, omitting the 4Å molecular sieves, reducing the amount of MsOH, or decreasing the temperature to 80 °C , all resulted in inferior yields (Table , entries 10-12).