Chiral Anion Catalysis in the Enantioselective 1,4-Reduction of the 1-Benzopyrylium Ion as a Reactive Intermediate
作者:Masahiro Terada、Takuto Yamanaka、Yasunori Toda
DOI:10.1002/chem.201302486
日期:2013.10.4
Anionic! Novel chiral anion catalysis of the enantioselective 1,4‐reduction of the 1‐benzopyrylium ion by a chiral phosphoric acid was accomplished with a Hantzsch ester as the reducing agent. The enantioselective reduction established is composed of a two‐step consecutive transformation involving stereoablative loss of the hydroxy group from racemic 2H‐chromen‐2‐ol derivatives to generate the achiral
阴离子!以Hantzsch酯为还原剂,完成了手性磷酸对1-苯并吡啶离子对映体1,4-还原的新型手性阴离子催化。建立的对映选择性还原包括两步连续转化,其中涉及从消旋的2 H - chromen -2-ol衍生物中羟基的立体烧蚀损失,生成非手性的1-benzopyrylium离子作为反应性关键中间体。
Shedding Light on Organocatalysis-Light-Assisted Asymmetric Ion-Pair Catalysis for the Enantioselective Hydrogenation of Pyrylium Ions
A new light‐driven asymmetricion‐pair catalysis procedure for the metal‐free enantioselectivehydrogenation of in situ generated pyryliumions from readily available chalcones was developed (see scheme). The photo‐assisted Brønsted acid catalyzed procedure has broad scope and allows, for the first time, access to valuable 4H‐chromenes in good yields and with excellent enantioselectivities.
[Graphics]The tandem reaction of phenols and chalcones in refluxing TFA gave the flavylium species of 2-hydroxy-2-phenyl-2H-chromenes in moderate to good isolated yields. The reaction was proposed to involve a tandem transformation of Friedel-Crafts alkylation, dehydrative cyclization, intermolecular hydrogen transfer, and hydration. As a multifunctional catalyst, TFA mediated the processes efficiently and cleanly.
The Reaction between Lactones and the Grignard Reagent. II. On the Intermediate Stages of the Reaction