Contrasteric Diels–Alder reactions of 5-methyl-5-phenylcyclopentadiene

The frontier molecular orbital of 5-phenylcyclopentadiene was predicted on the basis of the orbital mixing rule to deform to favor the Diels–Alder reaction in a syn contrasteric manner. The prediction was substantiated experimentally by the reactions of 5-methyl-5-phenylcyclopentadiene with dienophiles to afford the syn attack products, exclusively.