期刊
JOURNAL OF ORGANIC CHEMISTRY
卷 74, 期 7, 页码 2771-2779出版社
AMER CHEMICAL SOC
DOI: 10.1021/jo8028168
关键词
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资金
- NIDA [Y1-DA6002]
- UW-Whitewater
- UW-Milwaukee
- The Lynde and Harry Bradley Foundation
The present study was undertaken to shed light on the mechanism of the epimerization of cis-1,2,3-trisubstituted tetrahydro-beta-carbolines into the trans isomers via a potential carbocationic intermediate at C(1). In order to study the pathway involved in C(1)-N(2) bond cleavage, the electronic character of the carbon atom at C-I was altered by substitution of electron-rich and electron-poor phenyl rings at this position. This provided direct evidence of the effects of charge at the proposed site of the carbocationic intermediate. In this regard, a diverse set of 1-(phenyl substituted)-2-benzyl-3-ethoxycarbonyl-1,2,3,4-tetrahydro-beta-carbolines has been synthesized via the Pictet-Spengler reaction by condensation of L-tryptophan derivatives with electron-poor and electron-rich aromatic aldehydes. The epimers involved in the isomerization mechanism were investigated by dynamic H-1 and C-13 NMR spectroscopic and X-ray crystallographic analyses. The kinetic studies, which involved conversion of cis diastercomers into their trans counterparts, were carried out in dilute TFA/CH2Cl2. The 1-(4-methoxyphenyl) cis diastereomer epimerized at a much faster rate into the corresponding trans diastereomer than the related 1-(4-nitrophenyl) cis diastereomer epimerized. These observations provide support for the carbocationic intermediate in the C(1)-N(2) scission process. The understanding of this epimerization process is of importance when Pictet-Spengler reactions are carried out under acidic conditions during the synthesis of indole alkaloids.
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