Controlling 6-endo-selectivity in oxidation/bromocyclization cascades for synthesis of aplysiapyranoids and other 2,2,6,6-substituted tetrahydropyrans
作者:Oliver Brücher、Uwe Bergsträßer、Harald Kelm、Jens Hartung、Marco Greb、Ingrid Svoboda、Hartmut Fuess
DOI:10.1016/j.tet.2012.05.013
日期:2012.8
alkoxy substituent anti-selectively across the double bond, indicating a bromonium ion-mechanism for the ring closure. 6-endo-control of the alkenol cyclization thereby arises from the polar effect of the aryl substituent. Two methyl substituents bound to the alkene terminus are not similarly able to favor 6-endo-cyclization, because strain arising from methyl group repulsion, as the bromonium-activated
级联,构成(I)氧钒(V)由溴化的催化的氧化叔丁基过氧化氢和(ii)的立体选择性6-内-bromocyclization,得到3-溴-2-芳基-2,6,6- trimethyltetrahydropyrans从合成有用的苯乙烯型叔烯醇。(E)-链烯醇跨双键抗选择地添加溴-和烷氧基取代基,表明用于闭环的溴离子机理。烯醇环化的6-内-控制因此来自芳基取代基的极性效应。绑定到烯烃末端的两个甲基取代基不能同样地偏向于6-内-环化反应是由于甲基排斥反应引起的应变(如溴活化的π键和羟基氧途径)将叔异戊二烯型底物的溴环化作用导向了四氢呋喃的形成。由氧化/溴环化级联反应制备的六取代的溴四氢吡喃用作自由基和极性官能团相互转化的顺序,用于合成外消旋的氨苄基吡喃类化合物A的起始原料。