## 1 Introduction  
Protection of hydroxy functional groups, especially vicinal 1,2-diols and 1,3-diols, constitutes a key strategy in synthetic pursuits, both for research and industry-oriented targets. Ideally, the protecting group can be removed after installing other functions and/or achieving a specific derivatization. Although, in line with the green chemistry principles, the development of protecting-group-free routes is desirable to minimize additional reagents and side products [1], and sometimes, there is no avoiding functional protection as the target is too complex. Figure 1 displays a series of polyhydroxylated compounds (**1-7**) whose complexity prevents straightforward access without relying on protecting groups, such as bioactive saccharides or aminoglycoses [2, 3]. Ingenol (**6**) mimics the role of diacylglycerolglycerol, the endogenous activator of protein kinase C [4]. Molnupiravir (**7**) is particularly attractive, as this modified nucleoside was developed and licensed in less than two years to treat SARS-CoV2 infection. It can be obtained from uridine in five steps, the key ones involving acetal protection of vicinal OH groups [5]. On a larger scale, glycerol, the major residue during biofuel preparation, can be upgraded by reaction with acetone. The resulting acetonide (2,2-dimethyl-1,3-dioxolan-4-yl)methanol is marketed as solketal (**8**). This bio-based green solvent has found other applications, such as a plasticizer or a safe additive for pharmaceutical formulations [6; 7].  
Selective protection of vicinal diols can be accomplished by numerous methods [8; 9; 10], albeit acetalization represents a facile and convenient protocol affording the corresponding five-membered dioxolanes, usually in high yields. Major efforts have been directed toward heterogeneous catalysis in the search for sustainable approaches that bypass the use of harsh acid conditions and allow for rapid separation and purification. Both metal-doped and metal-free catalysts have been developed [11].  
In a recent study, we have shown that a sulfonated carbocatalyst derived from an activated carbon enabled efficient protection-deprotection of 1,2-diols under heterogeneous conditions [12]. Here, we report on a systematic screening of carbocatalysts tailored for vicinal diol acetalization, which can be easily generated from readily available precursors. In particular, we show that a hydrothermal carbon (HTC) obtained from saccharose led to enhanced results relative to other heterogeneous catalysts employed. This catalyst improves the previous protocol in terms of reaction times, catalyst efficiency, catalyst stability, and sustainability [12]. Hydrothermal carbonization has emerged in recent decades as a promising alternative to other pyrolytic and carbonization methods, using milder conditions and leading to materials with suitable microporosity and structuredness for a broad range of applications [13; 14].  
Given the acid character required for acetalization, other modified catalysts have been tested, as discussed below. Sulfated zirconia (**Zr-S**) exhibiting high acidity appears to be an  
Figure 1: Natural and synthetic targets with biological or therapeutic activity decorated by numerous vicinal diol fragments (1–7) and green solvent solketal (**8**).  
appropriate material for this end [15,16], which has been used for alkane isomerization at low temperatures [17]. It combines both Bronsted and Lewis acidity due to the electron-withdrawing effect of sulfate groups bonded to the zirconia surface. Previous research evidenced that **Zr-S** had comparable or superior activity to other solid catalysts, including zeolites, ion exchange resins, and sulfated metal oxides such as titania and tin oxide, for the esterification of lauric acid with alcohols [18]. Moreover, **Zr-S** efficiently catalyzed the esterification of various saturated/unsaturated fatty acids [19,20] and also the simultaneous esterification and transesterification of oils [21,22].