Prins cyclization of (-)-isopulegol with benzaldehyde for production of chromenols over organosulfonic clays

Halloysite Nanotubes as Bimodal Lewis/Brønsted Acid Heterogeneous Catalysts for the Synthesis of Heterocyclic Compounds

Halloysite nanotubes can be used for the preparation of solid catalysts. Owing to their natural availability at low-cost as well as to their large and easy-to-functionalize surface, they can be conveniently activated with mineral acids or derivatized with acidic groups. Nevertheless, the use of HNTs as catalysts in complex transformations is still limited. Herein, we report two strategies to utilize HNT-based materials as solid acidic catalysts for the Biginelli reaction. To this aim, two methods for increasing the number of acidic sites on the HNTs were explored: (i) the treatment with piranha solution (Pir-HNTs) and (ii) the functionalization with phenylboronic acid (in particular with benzene-1,4-diboronic acid: the sample is denoted as HNT-BOA). Interestingly, both strategies enhance the performance of the multicomponent reaction. Pir-HNTs and HNT-BOA show an increased reactivity (72% and 89% yield, respectively) in comparison with pristine HNTs (52%). Additionally, Pir-HNTs can be reused up to five times without significant performance loss. Moreover, the method also displays good reaction scope, as demonstrated by the preparation of 12 different 3,4-dihydropyrimidinones in up to 71% yield. Therefore, the described strategies are promising for enhancing the acidity of the HNTs as catalysts for the organic reaction.

Halloysite Nanotubes: Smart Nanomaterials in Catalysis

The use of clay minerals as catalyst is renowned since ancient times. Among the different clays used for catalytic purposes, halloysite nanotubes (HNTs) represent valuable resources for industrial applications. This special tubular clay possesses high stability and biocompatibility, resistance against organic solvents, and most importantly be available in large amounts at a low cost. Therefore, HNTs can be efficiently used as catalysts themselves or supports for metal nanoparticles in several catalytic processes. This review reports a comprehensive overview of the relevant advances in the use of halloysite in catalysis, focusing the attention on the last five years.

Enantiomeric Isopulegol as the Chiral Pool in the Total Synthesis of Bioactive Agents

Isopulegol, a pool of abundant chiral terpene, has long served as the starting material for the total synthesis of isopulegol-based drugs. As an inexpensive and versatile starting material, this compound continues to serve modern synthetic chemistry. This review highlights the total syntheses of terpenoids in the period from 1980 to 2020 in which with isopulegol applied as a building block.

Catalytic synthesis of bioactive 2H-chromene alcohols from (−)-isopulegol and acetone on sulfonated clays

Condensation of (−)-isopulegol with acetone was investigated at 25–40 °C over clay based materials (K10, cloisite Na+ and halloysite nanotubes) modified with sulfonic acid groups by one-pot sulfonation with chlorosulfonic acid or via organosilylation with 2-(4-chlrosulfonylphenyl)-ethyltrimethoxysilane. The target product was the R diastereomer of dimethyl-substituted octahydro-2H-chromen-4-ol with exhibits antiviral activity. The catalysts were characterized by a range of physico-chemical methods. The effect of the initial isopulegol concentrations on the yield of ketone-derived chromenols and the R/S isomers ratio was studied for the first time. The best catalyst was a highly acidic, large pore K10-clay modified by sulfonic acid. Varying the initial concentration of the reagents allows to find a balance between dehydration by-products and a side etherification reaction. A high yield (73%) of the desired chromenols was reached using 0.52 mol/l initial (−)-isopulegol concentration after 240 min at 87% substrate conversion and reaction temperature 25 °C. The yield exceeded that of other previously reported in the open literature catalysts.