Chemicals from Biomass: Synthesis of biologically active furanochalcones by Claisen-Schmidt condensation of biomass-derived 5-hydroxymethylfurfural (HMF) with acetophenones

Abstract

Furanochalcones have been synthesized trough the Claisen-Schmidt condensation of acetophenones and 5-hydroxymethylfurfural (HMF) using different solid base catalysts such as MgO, Al/Mg mixed oxide (HTc) with Lewis basic sites, and a hydrated Al/Mg mixed oxide (HTr) with Bronsted basic sites. The three catalysts provide high selectivity in absence of solvent or in the presence of polar solvents such as ethanol or acetonitrile, however catalysts become rapidly deactivated due to the strong adsorption of HMF and the furanochalcone obtained on the catalyst surface. A further increase in solvent polarity by using a mixture ethanol-water allows obtaining high conversion and high selectivity to furanochalcone using HTc and HTr as catalysts. However, MgO becomes rapidly deactivated which was mainly attributed to structural changes on MgO that is in situ rehydrated into Mg(OH)2 with low activity for aldol condensations. When the reaction was performed using the homogeneous NaOH catalyst, it was found that their activity is higher than that of the solid catalyst, but the selectivity of the later is clearly better. The results indicate that the active phase of the Al/Mg mixed oxide (HTc) in the ethanol-water medium corresponds to a partially restored hydrotalcite with basic hydroxyl groups. The HTc sample could be applied to the synthesis of a variety of furanochalcones with excellent success, while the catalyst could be reused several reaction cycles without loss of activity.