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