Corrected structure of the reaction product of gibberellin A3 with acetic anhydride and zinc and its crystalline structure. Rare case of the diels-alder reaction

The correct structure has been established for the compound obtained on the treatment of the methyl ester of gibberellin A₃ with zinc in boiling acetic anhydride, to which the erroneous structure (II) was previously assigned. A mechanism of the formation of compound (II) is suggested which includes an intramolecular diene condensation between an acetate carbonyl group and the conjugated diene system in the intermediate mixed anhydride (V).