38 (±)-ジベレリンA_2,A_4,A_9,A_<10>の全合成

概要

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The first total synthesis of the title compounds, four of the C_<19>-gibberellins, is discussed. The key intermediates are (±)-epigibberic acid (2), the (±)-diketo ester (3), the dienone (4) and gibberellin C (5). Catalytic reduction of the aromatic A-ring of 19 was one of the crucial steps. A model experiment employing 23 afforded 24 which showed an analogous ORD curve as that of the known 25. In the case of 19, the desired 3 was obtained in 4% yield. Conversion of 30 into 31 was successfully carried out in 12% yield by Pd-C catalyzed double bond migration. The carbomethoxylation of 35 followed by removal of the ketal protective group gave crystalline 37 in 11.4% yield after chromatographic purification. Alkaline hydrolysis of gibberellin A_4 methyl ester (42) afforded gibberellin A_4 (7) and its 2-(eq)-epimer (43).
1968-09-20