Highly Enantioselective Inverse-Electron-Demand Hetero-Diels–Alder Reactions of α,β-Unsaturated Aldehydes This work was supported by the NIH (GM-59316), with additional support from the Schweizerischer Nationalfonds zur Förderung der wissenschaftlichen Forschung (postdoctoral fellowship to K.G.), and the National Science Foundation (predoctoral fellowship to D.E.C.). We are grateful to Dr. R. Staples for carrying out the X-ray crystal structure analysis of 1.

Straightforward access to useful synthetic intermediates is provided by this new method. Simple, α,β-unsaturated aldehydes are excellent substrates in the hetero-Diels–Alder reaction with inverse electron demand, catalyzed by CrIII–Schiff base complexes (see scheme; R1, R2=alkyl or aryl) in the presence of 4-Å molecular sieves and no solvent. The resulting dihydropyrans are obtained in high enantio- (89–98 % ee) and diastereoselectivity (>95 % de) and yield (40–95 %).