Phosphoric Acid Catalyzed Aldehyde Addition to in Situ Generated o-Quinone Methides: An Enantio- and Diastereoselective Entry toward cis-3,4-Diaryl Dihydrocoumarins

A highly stereoselective, phosphoric acid catalyzed synthesis of cis-3,4-diarylchromanols through reaction of o-hydroxybenzhydryl alcohols and aryl acetaldehydes is reported. The products can be further manipulated to 3,4-dihydrocoumarins, 4H-chromenes, and chromanes with good overall yields and very good diastereo- and enantiocontrol. This reaction is based upon the concept of enol catalysis and comprises the in situ generation of hydrogen-bonded o-quinone methides and their formal [4 + 2]-cycloaddition with aldehyde enols.