Heating opens the four-membered ring by the thermally allowed conrotatory electrocyclic pathway, generating an ortho-quinodimethane. The starting trans phenyl relationship is converted into the diene geometry that gives cis phenyl groups in the subsequent Diels–Alder adduct. Endo-selective trapping by methyl acrylate fixes the ester relationship. The resulting stereochemistry is the all-dashed arrangement in **D**.
