BF3•SiO2 is a simple and efficient Lewis acid catalyst for the one-pot synthesis of polyfunctionalized piperidin-4-ones

A range of Lewis acid catalysts were used for a series of one-pot multi-component reactions. Of them, silica-supported boron trifluoride (BF3 center dot SiO2) was found to be an effective catalyst for the promotion of the modified Mannich condensation of ketones, aldehydes, and ammonium acetate in 1:2:1 molar ratio to afford 3,5-dialkyl-2,6-diarylpiperidin-4-ones in high yields of 80-92%. Also this simple, easily prepared, and reusable catalyst executed the condensation very effectively in a significantly shorter reaction duration (about 1-4 h) than the conventional method, which requires 1 day or more and involves a tedious work-up procedure. Further, this protocol ensures the stereospecificity; accordingly, all the synthesized piperidones adopted a chair conformation with an equatorial orientation of all substituents at C-2, C-3, C-5, and C-6.