共轭三烯及Galbanolenes的立体选择性合成

通过修饰改造的Ramberg-Backlund反应,(E,E)-,(E,Z)-,(Z,Z)-二烯丙基砜(6)在CBr~2F~2存在下,用KOH/Al~2O~3处理,选择性地生成(E,E,E)-,(E,E,Z)-,(Z,E,Z)-1,3,5-己三烯(7),反应的立体选择性依赖于溶剂和温度,通常在0℃时用CH~2Cl~2作溶剂可达到良好的(E)-选择性,有些时候在-78℃以下,以V(t-BuOH):V(CBr~2F~2)=1:1作为混合溶剂时(E)-选择性更好,该新方法被用于天然产物Galbanolenes(7m)和(7n)的合成中。

Stereoselective syntheses of conjugated hexatrienes: Application to the synthesis of Galbanolenes by a modified Ramberg-Backlund reaction

(E, E)-, (E, Z)- and (Z, Z)-diallylic sulfones (6) undergo stereospecific Ramberg-Backlund reaction to give (E, E, E)-, (E, E, Z)- and (Z, E, Z)-1, 3, 5-hexatrienes (7), respectively, upon treatment with CBr~2F~2 in the presence of alumina-supported KOH. The stereoselectivity of the reaction depends on the solvent and temperature. Good (E)-selectivity (>95%) can usually be obtained by conducting the reaction at 0℃ in CH~2Cl~2 but in some circumstances better (E)-stereoselectivity can be achieved when the reaction is performed at -78℃ in V(t-BuOH):V(CBr~2F~2)=1:1 mixture. The can be achieved when the reaction is performed at -78℃ in V(t-BuOH):V(CBr~2F~2)=1:1 mixture. The stereoselective syntheses of galbanolenes (7m) and (7n) using this newly developed method are exemplified.