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方法文章

通过4-硝基亚硝基苯与炔酮的环加成反应直接、区域选择性且原子经济性地合成3-酰基-N-羟基-5-硝基吲哚

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DOI:

10.3791/60201

2020年1月21日

本文内容

摘要

通过4-硝基亚硝基苯与共轭末端炔酮在一步热反应中发生环加成反应,合成了3-芳酰基-N-羟基-5-硝基吲哚。硝基芳基亚硝基化合物和炔酮的制备已有充分报道,分别通过对相应的苯胺和炔醇进行氧化反应获得。

摘要

我们介绍了一种通过硝基苯与炔基酮环合反应来合成3-取代吲哚的区域选择性高且原子经济性优良的方法。该反应无需任何催化剂,即可高效地生成吲哚,并表现出优异的区域选择性,未检测到2-甲酰基吲哚副产物。以4-硝基硝基苯为起始原料时,生成的3-甲酰基-N-羟基-5-硝基吲哚产物会从反应混合物中直接析出,通过过滤即可分离,无需进一步纯化。与相应的N-羟基-3-芳基吲哚在溶液中会自发发生脱氢二聚反应不同,N-羟基-3-甲酰基吲哚具有良好的稳定性,未观察到任何二聚化产物。

引言

芳香族C-亚硝基化合物1和炔酮2是用途广泛的反应物,常被持续深入地研究并用作制备高价值化合物的起始原料。亚硝基芳烃在有机合成中发挥着日益重要的作用,可用于多种不同目的(例如,杂原子Diels-Alder反应3,4、亚硝基-醇醛反应5,6、亚硝基-烯反应7、偶氮化合物的合成8,9,10)。最近,它们甚至被用作起始材料以制备多种不同的杂环化合物11,12,13。在过去的几十年中,共轭炔酮因其在合成多种高价值衍生物和杂环产物方面所具有的显著潜力而受到广泛关注14,15,16,17,18C-亚硝基芳香化合物可通过相应且市售可得的苯胺经氧化反应制得,常用的氧化剂包括过氧单硫酸氢钾复合物(KHSO5·0.5KHSO4·0.5K2SO4)19、Na2WO4/H2O220、Mo(VI)配合物/H2O221,22,23以及硒类衍生物24。炔酮可通过相应的炔醇氧化制备,所用氧化剂包括CrO325(即Jones试剂)或较温和的试剂如MnO226和Dess-Martin高碘烷27。而炔醇则可通过乙炔基溴化镁与市售可得的芳醛或杂芳醛直接反应获得28

吲哚可能是被研究最多的杂环化合物,其衍生物在众多科研领域具有广泛而多样的应用。无论是药物化学家还是材料科学家,均已开发出多种基于吲哚的产物,涵盖不同的功能和潜在活性。许多研究团队对吲哚类化合物进行了研究,含有吲哚骨架的天然产物和合成衍生物均表现出显著且独特的性质29,30,31,32在众多吲哚类化合物中,3-芳酰基吲哚在具有生物活性的分子中具有重要作用(图1)。不同的吲哚类产物属于多种药物候选物,有望成为潜在的新药33合成与天然存在的3-芳酰基吲哚已知具有抗菌、抗有丝分裂、镇痛、抗病毒、抗炎、抗伤害感受、抗糖尿病及抗癌作用34,35“1-羟基吲哚假说”由Somei及其同事提出,作为一种引人兴趣且富有启发性的假设,用以支持1-羟基吲哚在生物学功能中的作用 N羟基吲哚在吲哚类生物碱的生物合成与功能化中的作用36,37,38,39. 这一假设最近因观察到许多内源性 N具有相关生物活性的羟基杂环化合物,以及作为前药在多种用途中的重要作用40近年来,对新型活性药物成分的探索表明,不同的 N-羟基吲哚片段在天然产物和生物活性化合物中被检测并发现(图2): 斯蒂法菌素B41 和Coproverdine42 被称为抗肿瘤生物碱,噻唑霉素43 (A 和 D),Notoamide G44 和Nocathacins45,46,47 (I、III 和 IV)是研究较为深入的抗生素,Opacaline B48 是一种来自海鞘 Pseudodistoma opacum 的天然生物碱,而 Birnbaumin A 和 B 是两种来自 Leucocoprinus birnbaumii49. 新型且高效的 N基于羟基吲哚的LDH-A(乳酸脱氢酶A)抑制剂及其在细胞内降低葡萄糖向乳酸转化能力的研究已得到发展50,51,52,53,54,55,56其他研究人员重复发现,一些原本未表现出生物活性的吲哚类化合物,在插入一个基团后,可转变为有用的前体药物 N-羟基57.

争论的一个主题是稳定性 N羟基吲哚类化合物中,部分物质可轻易发生脱氢二聚反应,生成一类新型化合物,后被重新命名为卡布烷类(kabutanes)58,59,60,61,通过形成一个新的C-C键和两个新的C-O键。由于稳定 N羟基吲哚类化合物的合成方法研究因其易于制备而成为一个基础性课题。在我们先前的一项研究中,报道了以硝基苯乙烯和硝基芪为起始原料,通过Cadogan-Sundberg型反应实现分子内环化的方法。62近几十年来,我们开发了一种新型的分子间环加成反应,即硝基苯和亚硝基苯与不同炔烃之间的环加成,可高效生成吲哚, N-羟基- 和 N-烷氧基吲哚作为主要产物(图3).

最初,使用芳香族和脂肪族炔烃63,64,65,66,67时,反应需在炔烃大量过量(10 或 12 倍)的条件下进行,有时还需在烷基化条件下进行,以避免生成卡布坦类(kabutanes)副产物。通过该方法,可区域选择性地以中等至良好收率获得3-取代吲哚产物。当采用缺电子炔烃(如4-乙炔基嘧啶类衍生物)作为优势底物时,我们能够以1:1的亚硝基芳烃/炔烃摩尔比实现该一锅法合成反应68。利用此方法,成功合成了包括默里丁类(meridianins)在内的一类重要的激酶抑制剂;默里丁类是一类从Aplidium meridianum中分离得到的海洋生物碱69,该合成策略通过吲哚环化过程提供了一种全新的默里丁类化合物合成途径(图468。迄今为止,默里丁类化合物的合成普遍依赖于预先构建好的吲哚原料。据我们所知,此前仅有少数几种方法通过吲哚环化策略实现了默里丁类或其衍生物的全合成68,70

在近期关于缺电子炔烃应用的研究进展中,值得尝试将末端炔酮用作吲哚化反应的底物。这促使我们报道了一种分子间合成方法,用于制备3-芳酰基-N羟基吲哚产物71,72类似于研究用于制备meridianins的过程,使用末端芳基炔酮化合物时采用1/1的Ar-N=O与Ar-(C=O)-C≡CH摩尔比(图5)。以炔酮作为优势起始原料,通过使用不同反应物进行广泛的底物考察,同时改变硝基芳烃和芳香炔酮上取代基的性质,实现了通用的吲哚合成。吸电子基团位于芳环上时 C芳香族亚硝基化合物的使用使我们在反应时间和产物收率方面均观察到显著改善。一种能够便捷地获得这些化合物稳定库的合成策略具有重要意义;经过初步研究,我们优化了合成方案,采用炔酮与4-硝基亚硝基苯之间的化学计量反应,以获得稳定的3-芳酰基产物。N羟基-5-硝基吲哚。基本上,这种易于获得的 N羟基吲哚类化合物的研究使我们认识到,硝基芳烃与炔酮之间的环加成反应是一种原子经济性极高的过程。

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方案

1. 琼斯试剂的初步制备

  1. 使用刮勺将25 g(0.25 mol)三氧化铬加入一个含有磁力搅拌子的500 mL烧杯中。
  2. 加入75 mL水,并持续磁力搅拌。
  3. 在冰水浴中边小心搅拌边缓慢加入25 mL浓硫酸。
    注意:此时溶液已配制完成,稳定且可用于多种氧化反应;本方法制备的溶液浓度为2.5 M。

2. 1-苯基-2-丙炔-1-酮的合成

  1. 在装有磁力搅拌子的敞口圆底烧瓶中加入 75 mL 丙酮。
  2. 通过玻璃巴斯德吸管加入 2.0 g(15.13 mmol)1-苯基-2-丙炔-1-醇。
  3. 将反应混合物保持在 0 °C,并持续磁力搅拌。
  4. 逐滴加入琼斯试剂溶液,直至出现持久的橙色。
  5. 逐滴加入异丙醇,直至过量的 Cr(VI) 试剂被消耗至溶液呈绿色。
  6. 将溶液通过硅藻土垫过滤。
  7. 通过旋转蒸发浓缩洗涤液,得到油状物。
  8. 将油状物溶于 100 mL 二氯甲烷(CH2Cl2)中,并转移至分液漏斗。
  9. 用饱和碳酸氢钠(NaHCO3)溶液洗涤该有机相(2 × 125 mL)。
  10. 用饱和食盐水(125 mL)洗涤有机层。
  11. 用无水硫酸钠(Na2SO4)干燥有机溶液,然后过滤。
  12. 蒸发溶液,得到 1.77 g 黄色固体 1-苯基-2-丙炔-1-酮(定量收率)。
  13. 将固体在真空下干燥。
  14. 通过 1H-NMR 进行分析和表征。

3. 4-硝基亚硝基苯的制备

  1. 使用刮勺将16 g过一硫酸氢钾(2KHSO5·KHSO4·K2SO4)(26 mmol)加入含有磁力搅拌子的烧杯中,敞口置于空气中。
  2. 加入150 mL水,并在磁力搅拌下将溶液保持在0 °C。
  3. 使用刮勺加入3.6 g 4-硝基苯胺(26 mmol)。
  4. 在室温下搅拌该悬浮液。
  5. 通过薄层色谱(TLC)监测反应,直至4-硝基苯胺完全转化(Rf4-硝基苯胺 = 0.44,Rf4-硝基亚硝基苯 = 0.77;以CH2Cl2为洗脱剂)。
  6. 反应48小时后,将粗反应混合物在布氏漏斗上过滤。
  7. 将所得固体转移至单颈圆底烧瓶中。
  8. 用甲醇(50 mL)对该固体进行重结晶。
  9. 使用加热枪加热悬浮液至甲醇沸点,立即趁热过滤。
  10. 弃去固体,必要时可用于另一次重结晶。
  11. 当溶液冷却至室温后,过滤锥形瓶中析出的第二次沉淀。
  12. 将固体留在布氏漏斗上真空干燥。
  13. 通过1H-NMR对固体进行表征。

4. 3-苯甲酰基-1-羟基-5-硝基吲哚的合成

  1. 将所有烘干的玻璃仪器(包括一个装有磁力搅拌子的250 mL双口圆底烧瓶、活塞、冷凝管以及用于连接真空/氮气系统的接头)连接好,并抽真空30分钟。
  2. 在室温下,经过数次抽真空-充氮气循环后,用氮气彻底置换整个系统,并保持在惰性气氛中。
  3. 在惰性气氛下加入1.52 g(10 mmol)4-硝基亚硝基苯。
  4. 加入1.30 g(10 mmol)1-苯基-2-丙炔-1-酮。
  5. 通过注射器加入80 mL甲苯,并将反应混合物在80 °C下磁力搅拌。
  6. 数分钟后,检查反应物是否完全溶解。
  7. 在80 °C反应约30–40分钟后,确认是否有橙色沉淀生成。
  8. 当橙色固体完全析出(约2.5小时)后,关闭加热,使反应体系自然冷却至室温。
  9. 过滤反应混合物,并在布氏漏斗上收集产物3-苯甲酰基-1-羟基-5-硝基吲哚,为橙色固体。
  10. 在真空条件下干燥固体。
  11. 通过1H-和13C-NMR、FT-IR以及HRMS对所得固体产物进行分析和表征。

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结果

4-硝基亚硝基苯2的制备是通过4-硝基苯胺1与过氧单硫酸钾反应氧化得到,如图6所示。产物2经甲醇重结晶(两次)后以64%的收率获得,其中含有3-5%的4,4'-二硝基氧化偶氮苯6杂质。产物2的结构通过1H-NMR(图7)确认。1H-NMR(400 MHz, CDCl3):δ = 8.53(d, J = 8.8 Hz, 2H),8.07(d, J = 8.8 Hz, 2H)。

1-苯基-2-丙炔-1-酮4的制备通过使用琼斯试剂氧化1-苯基-2-丙炔-1-醇3实现,如图8所示...

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讨论

硝基芳烃与炔酮之间合成吲哚的反应表现出极高的通用性以及广泛的应用前景。在之前的一篇报道中,我们已将该合成方法推广至多种不同的C-硝基芳香化合物与取代的末端芳基炔酮或杂芳基炔酮的应用中72。该方法涵盖了广泛的底物范围,对各类官能团具有良好的耐受性,无论是在硝基芳烃还是在炔酮上,均兼容吸电子基团和给电子基团。

一种通过4-硝基亚硝基苯与1-苯基-2-丙炔-1-酮发生环加成反应实现吲哚化的方法被报道为典型反应。经初步筛选,甲苯被确定为最佳溶剂。按照本实验方案进行反应,得到3-苯甲酰基-1-羟基-5-硝基吲哚。 5 从反应混合物中析出沉淀。通过过滤分离得到的固体中,仅含有吲哚产物,无需进一步纯化。对母液的分析使我们发现并检测到其中唯一存在的成分为4,4'-二硝基偶氮氧苯。 6 作为含氮主要副产物,以及未反应的炔酮 4 并通过柱层析分离纯化产物(Rf偶氮氧芳烃 = 0.36 和 Rf炔酮

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披露

作者无任何利益冲突需要披露。

致谢

感谢 Enrica Alberti 博士和 Marta Brucka 博士在核磁共振谱的采集与登记方面所做的工作。我们感谢 Francesco Tibiletti 博士和 Gabriella Ieronimo 博士在有益的讨论及实验协助方面提供的帮助。

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材料

本文使用的材料清单
姓名公司目录编号评论
4-硝基苯胺TCI ChemicalsN0119
丙酮TCI ChemicalsA0054
1-苯基-2-丙炔-1-醇TCI ChemicalsP0220
硅藻土 535Fluorochem44931
二氯甲烷TCI ChemicalsD3478
碳酸氢钠Sigma AldrichS5761
氯化钠Sigma Aldrich746398
无水硫酸钠Sigma Aldrich239313
过氧单硫酸氢钾复合盐TCI ChemicalsO0310
甲醇TCI ChemicalsM0628
甲苯TCI ChemicalsT0260
三氧化铬Sigma Aldrich236470
无水二氯甲烷TCI ChemicalsD3478
无水己烷TCI ChemicalsH1197

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