Hiroya Kou, Kawamoto Kei, Inamoto Kiyofumi, Doi Takayuki
Symposium on the Chemistry of Natural Products, symposium papers, (50) 635-640, Sep 1, 2008
Piperidine alkaloids are expected to be lead compounds for new medicines because of their various biological activities. In order to establish the efficient synthetic methods for a variety of functionalized piperidine alkaloids, we have developed Michael addition reaction between silyl ketene acetals and 2-pyridinone derivatives in the presence of Lewis acid. Recently we have examined the reactivity of substrates having a methoxycarbonyl group at the C3 or the C5-position on the 2-pyridinone ring for control of the regioselectivity. As a result, when the reaction was applied to the substrate having the C3-methoxycarbonyl group, C4-substituted product was selectively obtained in the presence of a catalytic amount of Lewis acid. On the other hand, the reaction of the substrate having the C5-methoxycarbonyl group provided C6-substituted product in the presence of a catalytic amount of Lewis acid and azabicyclic compound with 2 equivalents of Me_3Al. On the examination of the reaction mechanism, it was found that the active Lewis acid was not Me_3Al, but the complex produced from Me_3Al and silyl ketene acetal. These reactions were applied to the syntheses of natural products. The enantioselective synthesis of the DE ring system of camptothecin, anti-tumor natural product, was accomplished in six steps. Next, we have studied the synthesis of awajanomycin, which is a new anti-tumor natural product isolated from marine-derived fungus in 2006. Since its absolute configuration has not been determined yet, we planned the stereoselective synthesis of awajanomycin as an optically active form. To construct its azabicyclic part, we have examined introduction of functional groups to the Michael adduct.