Synthetic studies towards the antimalarial fungal polyketide Codinaeopsin via an intramolecular Diels-Alder reaction and Efficient deprotection of benzyl ethers and aryl acetals using triphenylphosphine hydrobromide

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題名:	Synthetic studies towards the antimalarial fungal polyketide Codinaeopsin via an intramolecular Diels-Alder reaction and Efficient deprotection of benzyl ethers and aryl acetals using triphenylphosphine hydrobromide
作者:	馬瑞門;Ramanathan,Mani
貢獻者:	化學學系
關鍵詞:	none;acetal;benzyl ethers;BHT;Intramolecular Diels-Alder reaction;Lewis acid;Triphenylphophine hydrobromide
日期:	2013-01-04
上傳時間:	2013-03-25 15:51:40 (UTC+8)
出版者:	國立中央大學
摘要:	Codinaeopsin (1) 是一種新的色胺酸衍生物之聚酮化合物，在2008年從Costa Rica 收集的endophytic fungus中分離得到(圖一)，這化合物的結構主要是由一個多取代的trans decalin carbocycle 與一個具有indole結構的雜環相連結而成，在這一類的天然物中，Codinaeopsin 在 decalin carbocycle 上具有最多的取代基，包含四個methyl和一個 1-methyl-propenyl groups，而在這類型的天然物中大部分的α-acyl-γ-hydroxy lactam 的Cα 位置上都接有 Hydroxy group，Codinaeopsin具有抑制plasmodium falciparum的活性，Codinaeopsin對malaria做活性測試IC50=2.33μg/mL 或 4.7μM，Codinaeopsin 的複雜結構以及很好的生物活性，促使我們選擇這個結構作為我們的合成目標，其最關鍵的步驟是以E,E,E-triene經由路易士酸催化，進行分子內的Diels-Alder (IMDA) 反應，生成得到主要的decalin 骨架。主要合成策略是用可購買到的mesitol 進行Rh 催化的氫化反應，接著進行E-selective Horner-Wittig olefination得到三取代alkene，再進行one pot TPAP oxidation/Wittig olefination ，之後經由加熱反應或路易士酸催化，進行分子內的 Diels-Alder (IMDA) 反應得到racemic exo 和endo decalin cores ，產物的立體化學是由NOE 和單晶X-ray 進行鑑定。在第二章中，Triphenylphosphine hydrobromide是已知可用來切斷benzyl ethers derived from 1o, 2o, alkyl and aryl alcohols得到相對應的醇類和benzyltriphenylphosphonium bromide，並且具有好的產率，我們也利用微波反應和加熱反應，以及使用化學劑量的triphenylphosphine hydrobromide對此條件做了改進，也對其反應機構進行研究。在第三章中，Triphenylphosphine hydrobromide是已知可用來去掉aryl acetals 和 ketals 官能基得到相對應的carboyl 化合物，然後alkyl phosphonium salts在簡單的條件(50oC, microwave, 5 min.)下，產率最高可到 90%，同時這反應也可以忍受酸敏感的官能基，我們也利用具有不同官能基的起始物反應來進行反應機構的研究。Codinaeopsin (1) a new tryptophan derived polyketide was isolated in 2008 from an endophytic fungus collected in Costa Rica (Fig.1).1 The unique chemical structure of this target includes a densely substituted trans decalin carbocycle which is connected to an indole fragment through an unusual heterocyclic unit. Among the natural products found in this family, Codinaeopsin has the most substituted decalin unit with four methyl and a 1-methyl-propenyl groups attached to it. The unusual α-acyl-γ-hydroxy lactam (modified tetramic acid cores) with oxidized Cα position of tryptophan are often present in natural products from fungal sources. Codinaeopsin is having promising activity against plasmodium falciparum, the causative agent of most lethal form of malaria (IC50=2.33μg/mL) or 4.7μM. The structural complexity, together with the extraordinary biological profile, prompted us to choose this natural product as an ideal synthetic target. The synthetic plan relied on a late stage Lewis acid mediated intramolecular Diels-Alder (IMDA) reaction of E,E,E-triene with a suitable dienophile to yield the appropriately decorated decalin skeleton.The synthetic approach described here shares a number of key features, including rhodium catalyzed hydrogenation of commercially available mesitol, E-selective Horner-Wittig olefination protocol to construct the trisubstituted alkene and one pot TPAP oxidation/Wittig olefination of the sensitive polyene substrate. Syntheses of racemic exo and endo decalin cores of Codinaeopsin are accomplished via intramolecular Diels-Alder reaction under thermal and Lewis acidic conditions. The relative stereochemistry of these IMDA adducts were confirmed by NOE and single crystal X-ray techniques.In chapter 2, Triphenylphosphine hydrobromide is reported to cleave the benzyl ethers derived from 1o, 2o, alkyl and aryl alcohols to the corresponding alcohols and benzyltriphenylphosphonium bromide in good yields. The reaction is optimized both under microwave and conventional heating conditions using stoichiometric amount of triphenylphosphine hydrobromide. The mechanism of this reaction is also discussed. In chapter 3, Triphenylphosphine hydrobromide is reported to deprotect the aryl acetals and ketals to give the corresponding carbonyl compounds and alkyl phosphonium salts under convenient conditions (50 oC, microwave, 5 min. yields up to 90%). The reaction is compatible with acid sensitive functional groups and the mechanism is discussed through the substrates surveyed.
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