## 4 Synthesis of Amidoalkyl Naphthols via Multicomponent Mannich Condensation
### Heterogeneous Catalysis
#### 4.2.2 Other Types of Heterogeneous Catalysts  
The application of other heterogeneous catalysts to the multicomponent Mannich reaction leading to the formation of amidoalkyl naphths is summarized in Table 3, Refs. [93; 94; 95; 96].  
A procedure for the synthesis of amidoalkyl naphths using a biodegradable and reusable polymeric catalyst, chitosan-SO\({}_{3}\)H (CTSA), was developed by [PERSON] and colleagues. Their method operates under solvent-free conditions at 80 \({}^{\circ}\)C using a 0.02 g/1 mmol catalyst/substrate ratio and renders high yields in short reaction times [93]. Similar results have been reported for natural hydroxyapatite derived from waste bovine bone and further loaded with zinc chloride. Notably, the remarkable number of 22 amidoalkyl naphths have been prepared [94]. [PERSON] et al. reported the synthesis of a series of pyrazole-containing amidoalkyl naphths by using silica-supported sodium hydrogen sulfate (NaHSO\({}_{4}\).SiO\({}_{2}\)). Although in some instances the targeted amidoalkyl naphths were obtained in very high yields, this method requires longer reaction times and the use of acetic acid as a solvent [95]. [PERSON] et al. prepared a zeolite catalyst, zirconium silicate, and utilized it for the preparation of amidoalkyl naphths. Excellent yields have been obtained; however, this catalyst operates at higher temperature and catalyst loading [96].  
\begin{table}
\begin{tabular}{p{42.7 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Refs.** &
\begin{tabular}{} \end  
It can be seen (Table 1) that when a Bronsted acid is applied as a catalyst the reaction times are relatively long. In most cases, between 1 and 7 h are needed for the reaction to finish. The exception is ascorbic acid, with the use of which the products were obtained quite quickly (within 10 min) [61]. Furthermore, only one of the catalysts (dipicolinic acid) was recycled and reused [62]. Despite these drawbacks, the products' yields were good to excellent. The advantages of ionic liquids/deep eutectic solvents (Table 2) as catalysts are short reaction times, the possibility of recycling and reusing some of them, and obtaining the target compounds in good to excellent yields. The only drawback that  
\begin{table}
\begin{tabular}{l l c c c c c c} \hline  & & & \(\mathrm{R^{2}=Aryl}\) & & & & \\  & Nano-Fe\({}_{3}\)O\({}_{4}\)@TiO\({}_{3}\)-Fr- & & \  
could be noted about these catalysts is they are not commercially available and must be prepared beforehand.  
Almost all of the heterogeneous catalysts (Table 3) were easy to remove from the reaction mixture (separated by an external magnetic field or simple filtration) and could be involved in other reaction cycles which demonstrated their reusability. This advantage, as well as the short reaction times (typically below 1 h and in many cases within a few minutes) and the high yields of the products, make nanomaterials a preferable choice as catalysts for the synthesis of amidalkyl naphthols. Like ionic liquids/deep eutectic solvents, this type of catalysts are not commercially available and they must be prepared in-house.