Toward the Valorization of Cotton Wastes: Application of a Gossypol-Derived Ligand to an Asymmetric Diels-Alder Reaction

Gossypol, a major metabolite of cotton plants, has been used for the first time as a source material for the preparation of derivatives employed in asymmetric catalysis. Specifically, the (R)-enantiomer of the 6,6',7,7'-O-tetramethyl derivative of apogossypol has been synthesized in five steps and 47% total yield from enantiopure (-)-(R)-gossypol. This axially chiral diol was used as a ligand in the enantioselective boron-catalyzed Diels-Alder reactions of 2'-hydroxychalcones and dienes, achieving high yields and moderate enantioselectivities. Considering the potentially high amounts of gossypol isolable from agricultural wastes of cotton crops, these results open the way for the introduction of a promising new class of biomass-derived asymmetric catalysts.