Based on entry 3 in Table , we next focused on optimizing the reaction to increase the reaction rate (Table ). Open air was switched to an O2 balloon to increase the partial pressure of O2 (entry 1 vs 2 and 3 vs 4). At room temperature, the reaction rate did not increase although the partial pressure of O2 increased approximately five times with this change. In contrast, at 60°C, the reaction rate improved greatly with the O2 balloon, and the reaction was completed in 20 min. The proposed catalytic cycle for aerobic oxidation is shown in Scheme 1 . At room temperature, the solubility of O2 in the reaction solution is relatively high, and the catalytic cycle A, which is associated with the solubility of O2, is not the rate-determining step. Therefore, the reaction rates under open air and O2 balloon did not differ. On the other hand, increasing the temperature decreased the solubility of O2, which converted the catalytic cycle A to the rate-determining step at 60°C. When the amounts of catalysts were decreased to 0.02 equiv (entry 5), the reaction completion took 60 min. When the reaction solution was heated to 80°C, the reaction was completed within 20 min with 0.02 equiv of Fe(NO3)3/TEMPO (entry 6). When the amounts of catalysts were decreased to 0.01 equiv at 80°C, the reaction time was significantly prolonged, which is not suitable for the flow synthesis (entry 7). Based on these findings, entry 6 was considered to be the optimal reaction conditions, and the application to flow synthesis using the honeycomb reactor was tried under these conditions. (area%)/(2a (area%) + 1a (area%)×14.083)×100. 14.083: relative sensitivity coefficient on HPLC (factor).; c Quantitative yield using an authentic sample on HPLC.; d Isolated yield (1a: 3.6 mmol scale).