## 3 Results and Discussion
### Screening of Heterogeneous Catalysts for Diol Protection  
Firstly, we investigated the acetalization of two model diols (**9** and **10**) against a wide variety of heterogeneous catalysts (Scheme 1). These reactions were performed under magnetic stirring at 80 \({}^{\circ}\)C in a sealed vial to minimize the evaporation of the solvent and to avoid the presence of moisture in the reaction media. The catalysts employed in this screening are listed above (Table 1). Catalyst load and acetalization reagent were optimized in a previous study [12]. Reactions were monitored by TLC analysis until the complete consumption of starting materials. Purification by column chromatography was only required when analysis of the crude reaction mixtures by \({}^{1}\)H-NMR showed the presence of impurities. Table 7 summarizes the results of this catalytic screening. The accelization reaction was not completed for catalysts **Zr** (entries 1 and 2) and **N-Zr** (entries 5 and 6). Likewise, catalyst **M** did not complete the reaction either, even after 8 days, probably due to its basic character. Interestingly, the reaction of diol **9** (entry 21) could not be completed after 9 days with hydrothermal carbon **HT**, which is strongly acidic, whereas protection of diol **10** was achieved with excellent yield (91%, entry 22) after the same reaction time. Nevertheless, and as will be demonstrated below, these findings show that the acidity of the catalysts is required but not exclusive to reach optimum catalytic activity. Protection of both model diols was feasible with zirconium-based catalyst **Zr-S**, providing excellent yields, although purification by column chromatography was required to isolate pure acetonides **11** and **12**. On the other hand, catalysts **N-Zr-S1** and **N-Zr-S2** (entries 7-10) allowed for the formation of acetonides with excellent yields, but after reaction times of one day. Similarly, the use of catalysts **M-S** (entries 13 and 14) and **M-N** (entries 15 and 16) drove excellent yields for **11** and **12**. The slightly basic catalyst **N-HT** also allowed for the isolation of ketals in good yields (entries 17 and 18), even though reaction times were longer, requiring 5 days for completion. Finally, sulfonated catalysts **N-HT-S** and **HT-S** accelerated the accelization reactions significantly. Protection of diol **9** was completed in one hour in the presence of **N-HT-S** (entry 19) and remarkably in only 20 min when using catalyst **HT-S** (entry 23), which was by far the most active catalyst found in this study. Likewise, the accelization of **10** required 3 h with **N-HT-S** (entry 20), whereas the reaction catalyzed by **HT-S** took place again in 20 min with an excellent yield (entry 24). Therefore, one can conclude that the best catalysts identified in this preliminary study were acid carbons **N-HT-S** and **HT-S** since reactions were completed in less than 3 h with \begin{table}
\begin{tabular}{c c c c c} \hline
**Entry** & **Diol** & **Catalyst (5\%) a** & **Time** & **Yield** \\ \hline  
% c \\
23 & **9** & **HT-S** & 20 min & 97\% c \\
24 & **10** & **HT-S** & 20 min & 93\% c \\ \hline \end{tabular} a
[FOOTNOTE:a]Footnote a: Percentage of catalyst with respect to the starting weight of diol. b
[FOOTNOTE:b]Footnote b: Isolated yield after purification by column chromatography. c
[ENDFOOTNOTE]  
\end{table} 
Table 7: Preliminary screening of heterogeneous catalysts for protection of diols **9** and **10**. excellent yields. Moreover, acetonides **11** and **12** were isolated with no need for purification by column chromatography. The structure of isopropylidene ketals for **11** and **12** was corroborated by analytical and spectroscopic techniques (see Section 2) and was compared to reported data [12]. The high purity of these compounds was confirmed by their correct elemental analyses and by the fact that both acetonides crystallized spontaneously after evaporation of the solvent, even though both compounds have been previously described as oils [35; 36].