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 <b>1</b> or 4-hydroxy-6-methyl-2-pyrone <b>2</b> and (2) to fully characterize the dimerization products bis-coumarins <b>3</b> and bis-pyrones <b>4</b>. 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 <b>4i</b>. 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.