Acid‐modified Halloysite Nanotubes as a Stereoselective Catalyst for Synthesis of 2H‐Chromene Derivatives by the Reaction of Isopulegol with Aldehydes.

Abstract: Acid‐modified halloysite nanotubes were used for the first time as a stereoselective catalyst for synthesis of oxygen‐containing heterocycles applying of allyl alcohol (−)‐isopulegol condensation with aldehydes to the octahydro‐2H‐chromenol (4R‐ and 4S‐diastereomers) as an example. The catalysts were characterized by XRF, XRD, N2‐adsorption, FTIR with pyridine and MAS NMR methods. A high ratio of 4R/4S diastereomers (7.6–14.5) under mild conditions in cyclohexane was considerably exceeding previously reported results. Unprecedented selectivity (79–83 %) to 4R isomer of thiophenyl‐substituted chromenol exhibiting high analgesic activity was achieved. An increase in stereoselectivity with a decrease in the halloysite drying temperature and catalyst acidity clearly indicates formation of 4R diastereomer on the weak Brønsted sites. This work is an example that control of the stereoselectivity of acid‐catalyzed organic reactions can be effectively carried out by varying water content on the aluminosilicate surface. Modified halloysite nanotubes can be considered as extremely promising catalysts for stereoselective synthesis of heterocyclic compounds. [ABSTRACT FROM AUTHOR]