Intramolecular [2+2] Photocycloaddition Reactions as an Entry to the 2‐Oxatricyclo[4.2.1.0<sup>4, 9</sup>]nonan‐3‐one Skeleton of Lactiflorin
作者:Ping Lu、Eberhardt Herdtweck、Thorsten Bach
DOI:10.1002/asia.201200295
日期:2012.8
center (5R*,2′R*), the desired straight products were obtained in regioselectivities of 74:24 to 55:45 (61–83 % yield). Following this route, the aglycon part of lactiflorin was obtained by an intramolecular [2+2] photocycloaddition and a subsequent hydrogenolysis in 53 % yield. Its further conversion into the natural product after glycosylation included a methyl addition to the lactone carbonyl group
评估了两个[2 + 2]光环加成途径,作为接近在萜烯单糖苷乳糖苷中发现的三环核心结构的可能方法。尽管通过γ-取代的环戊烯酮的第一条路线很快被放弃了,但外消旋(5 R *)-3-苄氧基-5--5-丁-3-3′-烯基-4-甲氧基羰基呋喃-2(5 H)的反应进行得很高。产率高且具有非对映选择性。然而,事实证明,区域选择性在很大程度上取决于but-3'-enyl链内的取代方式,该取代方式将末端烯烃双键连接至光激发的丁烯内酯发色团。如果链未被取代或叔丁基二甲基甲硅烷氧基位于syn的2'位与现有的立体定位中心(5 R *,2 'S *)相关,交叉产物的区域选择性为89:11至69:31。如果叔丁基二甲基甲硅烷氧基与现有的立体异构中心呈反相关关系位于2' (5 R *,2'R *),以74:24至55:45的区域选择性(61-83%的产率)获得所需的直链产物。按照这种方法,通过分子内[2 + 2]光环