Synthesis of Oxa-B-Ring Analogs of Colchicine through Rh-Catalyzed Intramolecular [5+2] Cycloaddition
作者:Andreas Ole Termath、Stefanie Ritter、Marcel König、Darius Paul Kranz、Jörg-Martin Neudörfl、Aram Prokop、Hans-Günther Schmalz
DOI:10.1002/ejoc.201200677
日期:2012.8
A synthetic approach towards (5R)-5-methyl-6-oxa-desacetamido colchicine as a conformationally defined non-natural colchicine analog with a modified B-ring was undertaken. The synthetic strategy was based on a Rh-catalyzed cascade reaction involving a [5+2] cycloaddition of a carbonyl ylide intermediate as a key step, in which both seven-membered rings of the polycyclic framework are formed in a single
对 (5R)-5-甲基-6-氧杂-去乙酰氨基秋水仙碱作为构象定义的非天然秋水仙碱类似物进行了合成方法,该类似物具有修饰的 B 环。该合成策略基于 Rh 催化的级联反应,包括羰基叶立德中间体的 [5+2] 环加成反应作为关键步骤,其中多环骨架的两个七元环在一次操作中形成。从 2-碘-3,4,5-三甲氧基苯乙酮开始,通过对映选择性 CBS 还原(高达 75% ee)和炔丙基化构建上侧链,而下侧琥珀酰侧链通过碘-镁连接– 铜交换和随后与 4-氯-4-氧代丁酸甲酯反应,或通过 Pd 催化的 Stille 与 2-三丁基甲锡烷基-5-甲氧基呋喃交叉偶联,然后水解呋喃开环反应。用 Rh2(OAc)4 (3 mol-%) 处理 α-重氮酮中间体引发非对映选择性关键环化级联反应 (≥97:3 dr)。用 Et2AlCl 处理环加合物 3 提供了有趣的 11,12-二氢秋水仙碱类似物 24,然而,它不能被氧化成相应的托酚酮。通过