Multicomponent Green Synthesis Involving Aryl Aldehydes and Trapped Enols: Dimerization over Cyclization

This report serves two main purposes: (1) to correct the literature in the area of multicomponent synthesis involving aryl aldehydes and trapped enols 4-hydroxycoumarin 1 or 4-hydroxy-6-methyl-2-pyrone 2 and (2) to fully characterize the dimerization products bis-coumarins 3 and bis-pyrones 4. There have been many reports of cyclizations occurring with these species and various catalysts; however, many products have been mis-characterized and are, in fact, dimers. We successfully synthesized these dimers using a green, one-pot reaction in water that avoids hazardous organic solvents, uses a catalytic amount of acid, does not require chromatography for purification, and has strong green chemistry metrics. Our simplified procedure resulted in high yields of dimers ranging from 24 to 96% including the first report of a meta-substituted bis-pyrone 4i. Herein, we report a green method for their synthesis, along with their photophysical properties, full characterization, and potential as AChE inhibitors for anti-Alzheimer's therapy.