Applications of chiral norbornadiene bisoxazoline ligands in the asymmetric nitroaldol (Henry) reaction

In recent years, among the asymmetric synthesis methods the most common one is the use of chiral catalysts. In this study, chiral bisoxazoline ligands having a norbornadiene backbone, forming seven-membered chelates with metals were synthesized with 4588% yields and they were used as chiral ligands in the asymmetric nitroaldol (Henry) reaction. After the synthesis of the ligands, the best reaction conditions giving the highest enantioselectivity for the asymmetric Henry reaction were investigated. As a result, bisoxazoline ligand having sec-Bu group as the ligand, Cu(OAc)2 as the copper salt, 5 mol % as the catalyst loading and isopropanol as the solvent were determined to be the best reaction conditions. Finally, under the optimized reaction conditions, the addition of nitromethane to different aldehydes resulted in the formation of nitroaldol products with up to 98% yields and 67% enantiomeric excess.