Catalytic Asymmetric Diels–Alder Reaction of β,β-Disubstituted Enals with Cyclopentadiene: Syntheses of (−)-Camphene, (−)-β-Santalene, and (−)-( Z )-β-Santalol

Despite the long and rich history of the Diels–Alder reaction, certain aspects of its scope still remain remarkably underexplored. Most prominently, reactions with β,β-disubstituted enals as dienophiles are scarcely represented in the literature. The first general and highly enantioselective method, enabled by a strongly acidic and confined imidodiphosphorimidate organocatalyst, is reported here. The reaction tolerates a broad range of enals, including substrates with functional groups and complex alkenyl substituents, delivering the corresponding cycloadducts in good yields and excellent enantioselectivity. The synthetic utility of the method is further highlighted by concise asymmetric total syntheses of (−)-camphene, (−)-β-santalene, and (−)-(Z)-β-santalol from inexpensive starting materials.