Z-E isomerization of .BETA.-methoxychalcones: Preferred existence of E-isomers in naturally occurring .BETA.-methoxychalcones.
作者:Fumiyuki KIUCHI、Xing CHEN、Yoshisuke TSUDA
DOI:10.1248/cpb.38.1862
日期:——
Methylation of dibenzoylmethanes with diazomethane always gave Z-β-methoxychalcones, which are the products of kinetically controlled methylation of chelated enol forms of β-hydroxychalcones. Z-β-Methoxychalcones were thermodynamically unstable and readily isomerized into more stable E-isomers on contact with silica gel, on keeping in polar solvents, or on exposure to light. The structures of both isomers of the simplest β-methoxychalcone were determined by X-ray crystal structure analysis. All β-methoxychalcones prepared were fully characterized by spectroscopic methods, revealing that the E- and Z-isomers are distinguishable in terms of ultraviolet and 13C-nuclear magnetic resonance spectra, and nuclear Overhauser effect between the β-methoxyl group and H-8 proton. The spectral data reported for natural β-methoxychalcones, methylpongamol and praecansone A, are attributable to the E-isomers.
与二氟代甲烷反应时,二苯甲烷的甲基化总是会产生Z-β-甲氧基查尔酮,这些是β-羟基查尔酮的螯合烯醇形式进行动力学控制的甲基化反应的产物。Z-β-甲氧基查尔酮在热力学上不稳定,容易在与硅胶接触、保持在极性溶剂中或暴露在光照下时异构化为更稳定的E-异构体。通过X射线晶体结构分析确定了最简单的β-甲氧基查尔酮的两种异构体的结构。所有合成的β-甲氧基查尔酮均通过光谱法进行了充分表征,结果表明E-异构体和Z-异构体在紫外光和13C核磁共振光谱中是可以区分的,以及β-甲氧基基团与H-8质子的核Overhauser效应。报告中提到的天然β-甲氧基查尔酮、甲基龟甲酮和A型普雷卡松的光谱数据均归因于E-异构体。