Unique Reactivity of α-Alkoxy Ginkgolide Lactones to Nucleophilic Reagents: Preparation of New Lactol Derivatives
作者:Katsunori Tanaka、Marc Pimentel、Nina Berova、Koji Nakanishi
DOI:10.1246/bcsj.78.1843
日期:2005.10
It has been found that nucleophilic reagents, i.e., NaBH4, Grignard reagents, and alkyl lithium, uniquely react with the α-alkoxy- or acyloxy-lactone moieties in ginkgolide and F-seco-ginkgolides to give rise to lactol derivatives. The reaction is rapid and stops at the lactol stage; the strong coordination of Na, Mg, and Li metals to the conformationally rigid cage structure is involved in both the initiation and termination stages. The NaBH4 reduction of F-seco-ginkgolides gives rise to an equilibrium mixture of α- and β-lactols, the separation of which becomes only possible after acylation by p-phenylbenzoic acid. The resulting acyl-lactol stereogenic centers were elucidated by both NOE and the CD/FDCD exciton chirality method utilizing the sterically hindered 7-hydroxyl. On the other hand, the alkylation of ginkgolide B derivatives proceeds regio- and stereoselectively at the C-11 lactone group, resulting from the approach of Grignard and alkyl lithium reagents to the convex face of the cage-shaped ginkgolide molecule. The additional new stereogenic centers of the quaternary lactol hydroxyls have been determined by NOE. This facile alkylation protocol gives rise to a deep-seated skeletal transformation of ginkgolides, resulting in a new class of ball-shaped heptacyclic ginkgolide derivatives via “olefin/olefin” and “olefin/alkyne” ring-closing metathesis.
已经发现,亲核试剂,即NaBH4、Grignard试剂和烷基锂,与银杏内酯和F-seco-银杏内酯中的α-烷氧基或酰氧基内酯部分独特地反应,生成内酯醇衍生物。反应迅速且停止在内酯醇阶段;Na、Mg和Li金属对构象刚性的笼状结构的强烈配位作用涉及反应的引发和终止阶段。F-seco-银杏内酯的NaBH4还原反应生成α-和β-内酯醇的平衡混合物,只有在经过对-苯基苯甲酸酰化后才能分离。得到的酰基内酯醇手性中心的阐明是通过NOE和利用具有空间位阻的7-羟基的CD/FDCD激子手性方法进行的。另一方面,银杏内酯B衍生物的烷基化反应在C-11内酯部分区域选择性和立体选择性地进行,这是由于Grignard和烷基锂试剂接近笼状银杏内酯分子的凸面。新的手性中心,即季碳内酯醇羟基,已通过NOE确定。这种简便的烷基化方案导致银杏内酯的深层次骨架转化,通过“烯烃/烯烃”和“烯烃/炔烃”环闭合复分解反应生成一类新的球形七环银杏内酯衍生物。