Efficient Synthesis of a Tricyclic BCD Analogue of Ouabain: Lewis Acid Catalyzed Diels–Alder Reactions of Sterically Hindered Systems

The right mix! A 10:1 mixture of AlBr3 and AlMe3 promotes the Diels–Alder reaction of hindered dienophiles. For example, the Diels–Alder reaction of an acetyldiene with a tert-butyldimethylsilyl (TBS) enol ether gave mainly the exo cycloadduct in 77 % yield [Eq. (1); X=COMe, Y=OTBS]. Oxidation, hydrolysis, reduction, and desilylation of this cycloadduct afforded a BCD ring analogue of ouabain.