Achieving Enantioselectivity in Difficult Cyclohexa-1,3-diene Diels-Alder Reactions with Sulfur-Stabilized Silicon Cations as Lewis Acid Catalysts.

A novel cationic silicon-sulfur Lewis pair with a chiral H8-binaphthyl backbone is reported. It catalyzes otherwise sluggish Diels-Alder reactions of cyclohexa-1,3-diene and chalcone derivatives in good yields and decent enantioselectivities (up to 81% ee). The enantioinduction is highest with a [1,1'-biphenyl]-4-yl substituent at the carbonyl carbon atom. This moiety can be later converted into a carboxyl group by Baeyer-Villiger oxidation. The same oxidant also epoxidizes the double bond in the cycloadduct, and the epoxide engages in a lactonization with the carboxylic acid. Synthetically interesting hexahydro-3,6-methanobenzofuran-2(3 H)-one skeletons are obtained in one pot.