A solvent-free Diels–Alder cycloaddition of piperine on silica gel: synthesis of dimeric amide alkaloid from the roots of Piper nigrum

A solvent-free intermolecular Diels–Alder cycloaddition of piperine on silica gel is described. The dimerization of piperine in the presence of silica gel gives better yields than that under the condition of only heating. The new procedure is simple, clean, and easy to operate. It has higher yields and can be completed in short time.