## 3 Results and Discussion
### Diol Protection in Continuous Flow  
The efficiency of heterogeneous catalysts can be enhanced when packed-bed reactors are implemented in combination with continuous flow operation [40]. In a packed-bed reactor, the catalyst is loaded into a pressure-resistant column. Then, the reaction mixture is flowed through the bed using a pumping system. As the liquid phase moves through the catalyst particles, a significant increase in the mass transfer properties of the system is achieved, as the catalyst bed acts as a passive mixer. Moreover, the liquid fraction in contact with the catalyst at a given time encounters a very high catalyst-to-substrate ratio, resulting in very fast transformations [41]. Contact times of a few minutes or even seconds are common in continuous-flow reactors. An additional advantage of packed-bed reactors in continuous flow is that the catalytic reaction and the filtration step are performed simultaneously. Thus, an operation step (filtration) can be removed from the process. This advantage, with the good scalability of packed beds, makes this system particularly suitable for industrial implementation [42]. To demonstrate that heterogeneous catalyst **HT-S** is suitable for continuous processing in a packed-bed reactor, it was loaded into a stainless steel column (3 mm i.d., 12 cm length). The column was plugged with glass wool on both ends to avoid leaching of the catalyst during flow operation. The column, with an inner volume of 0.85 mL, could carry ca. 350 mg of catalyst. Once loaded with the catalyst, the void volume of the packed bed was 0.623 mL. This volume was used to calculate the residence time of the reaction mixture in the reactor. In a typical experiment, a 0.2 M solution of **10** was pumped through the catalyst bed using a syringe pump (Figure 13). The system was pressurized using a 7-bar backpressure regulator. The temperature of the packed-bed reactor was monitored and controlled in a hot plate (see Supplementary Materials for details, Figure S6). Figure 12: Schematic representation of the effect of cavitation intensifying bags (BuBble® bags) in heterogeneous catalysis. Adapted with permission from the literature [38]. Once the flow system was assembled, the reaction conditions were optimized. Thus, the reactor temperature and residence time were screened. During the optimization process, the solution of **10** in 2,2-dimethoxyopyran was continuously pumped through the reactor. Once the temperature and flow rate had been set and were stable, two reactor volumes were discarded (to ensure steady-state conditions), and then an aliquot of the crude reaction mixture was collected in a round bottom flask, evaporated under reduced pressure, and analyzed by \({}^{1}\)H NMR. Gratifyingly, when the reactor temperature was slightly increased from 25 \({}^{\circ}\)C to 40 \({}^{\circ}\)C, excellent conversion of the substrate was obtained within a residence time of 5 min (Table 10, entries 1 and 2). A further increase in the temperature to 60 \({}^{\circ}\)C did not have negative effects on the reaction outcome (entry 3), while at 80 \({}^{\circ}\)C, small amounts of unknown side products were observed in flow mode (entry 4). Subsequently, the residence time was optimized at 40 \({}^{\circ}\)C and 60 \({}^{\circ}\)C by increasing the pump flow rate. At 40 \({}^{\circ}\)C, the residence time could be decreased to 2 min while still obtaining a very good 97% conversion (entry 6). A further decrease to 1 min resulted in a significant amount of unreacted starting material (88% conversion, entry 7). Notably, at 60 \({}^{\circ}\)C, the flow rate could be gradually increased until the residence time of the reaction mixture in the system was only 30 s while maintaining excellent conversion of the starting material (Table 10, entries 8-11). Indeed, under a residence time of 30 s, a slight increase in conversion from 98% to 99% (no traces of starting material observed) was achieved, most likely due to an increase in mass transfer under a higher flow rate.