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Diels–Alder cycloaddition: cyclopentadiene + maleic anhydride → endo-norbornene-2,3-dicarboxylic anhydride

Diene + Dienophile →[4+2] cyclic adduct

Textbook [4+2] cycloaddition. Electron-rich diene (cyclopentadiene, freshly cracked from its dimer) reacts with electron-poor dienophile (maleic anhydride) in a concerted, suprafacial, endo-selective addition. The endo product crystallises from cold toluene. Frontier-MO analysis predicts the rate (HOMO_diene/LUMO_dienophile gap) and the ortho/endo regio+stereo selectivity — see Houk, JACS 95, 4092 (1973).

Conditions: Run at 20–25 °C in toluene (~1 M substrate) for 1–4 h, or neat above maleic anhydride's mp (~52 °C) for 30 min. No catalyst required — the EDG/EWG-matched pair is fast at room temperature. Cyclopentadiene must be freshly cracked from dicyclopentadiene (170 °C distillation, used within hours). Workup: cool to 0 °C, filter the white crystalline endo product, wash with cold toluene, dry under vacuum.

Expected yield

80–95 %

Expected duration

1–4 h at 20–25 °C in toluene; 30 min neat above 52 °C

Mechanism reference ↗