共轭三烯及Galbanolenes的立体选择性合成

Stereoselective Syntheses of Conjugated Hexatrienes——Application to the Synthesis of Galbanolenes by a Modified Ramberg - Backlund Reaction

通过修饰改造的:Ramberg-B(?)cklund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O℃时用CH_2Cl_2作溶剂可达到良好的(E)-选择性,有些时候在-78℃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.

(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 CBr2F2 in the presence of alumina- supported KOH. The stereoselectivity of the reaction depends on the solvent and temperature. Good (E) - selectivity ( > 95 %) can usually beobtained by conducting the reaction at 0℃ in CH2Cl2 but in some circumstances better (E) - stereoselectivitycan be achieved when the reaction is performed at - 78℃ in V( t - BuOH) : V(CBr2F2) = 1:1 mixture. The stereoselective syntheses of galbanolenes (7m) and (7n) using this newly developed method are exemplified.