Q8(e). Preparation of five compounds (pericyclic routes)

Reading of the question: (v) is printed as a perspective drawing; it is read as the 7-oxabicyclo[2.2.1]heptane whose C-2 and C-3 each carry CO₂Me and are fused to a cyclohexene ring.

Introduction. Most of these targets are the products of cycloaddition reactions; (iv) is made by anti-dihydroxylation.

1. (i) Cyclohexene: Diels–Alder of buta-1,3-diene + ethene (about 200°C, pressure), [4πₛ+2πₛ]; in the laboratory, acid dehydration of cyclohexanol (H₃PO₄, distil).
2. (ii) Cyclobutane: [2πₛ+2πₛ] cycloaddition of ethene is thermally forbidden but photochemically allowed; practically, hydrogenation of cyclobutene (H₂/Ni) or ring closure of 1,4-dibromobutane with Na or Zn (low yield).
3. (iii): anthracene + maleic anhydride, reflux in xylene: Diels–Alder across C-9 and C-10 (the central ring reacts because two benzene rings are kept intact) → 9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic anhydride.
4. (iv) trans-Cyclohexane-1,2-diol: cyclohexene + mCPBA (or HCO₃H) → epoxide; H₃O⁺ opens it by back-side attack of water: anti addition, trans-diol (racemic).
5. (v): (a) furan + dimethyl acetylenedicarboxylate (DMAD), Diels–Alder → 7-oxanorbornadiene-2,3-diester; (b) H₂/Pd–C (one equivalent) reduces the unsubstituted, strained C5=C6 selectively; (c) Diels–Alder with buta-1,3-diene on the electron-poor C2=C3 (approach from the less hindered exo face) builds the cyclohexene ring.

Scheme: Two Diels–Alder steps with a selective hydrogenation between them.

Conclusion. (i) Butadiene + ethene (DA) or cyclohexanol/H₃PO₄; (ii) photochemical [2+2] of ethene or hydrogenation of cyclobutene; (iii) anthracene + maleic anhydride (DA); (iv) epoxidation then acid hydrolysis (anti); (v) furan + DMAD, selective H₂, then DA with butadiene.

Application. Furan Diels–Alder adducts (7-oxanorbornenes) are starting materials for cantharidin and for shape-defined polymers by ring-opening metathesis.