Facial Selectivity in the Diels–Alder Reactions of 2,2‐Disubstituted Cyclopent‐4‐ene‐1,3‐dione Derivatives and a Computational Examination of the Facial Selectivity of the Diels–Alder Reactions of Structurally Related Dienes and Dienophiles

The facial selectivity in the Diels–Alder reactions via endo transition states of the plane‐nonsymmetric dienophiles spiro[bicyclo[2.2.2]octane‐2,1′‐[3]cyclopentene]‐2′,5′‐dione ( 6 ) and spiro[bicyclo[2.2.1]heptane‐2,1′‐[3]cyclopentene]‐2′,5′‐dione ( 7 ) has been found to be very similar to the facial selectivity of reactions of cyclopentadienes fused to bicyclo[2.2.2]octane and bicyclo[2.2.1]heptane ( 4 and 5 ). This is strong evidence for steric control in their facial selectivities. Density functional computations involving the reactions of these compounds, as well as a series of conformationally locked model addends ( 26a – c and 27a – c ), revealed geometrical and energetic properties that confirmed that the facial selectivity is governed by steric effects. Steric interactions appeared to be the reason for the low endo / exo selectivity of dienophiles 6 , 7 , 2,2‐dimethylcyclopent‐4‐ene‐1,3‐dione ( 20 ), and spiro[5.4]dec‐8‐ene‐7,10‐dione ( 23 ).