P-47 IRスペクトルによるキラル誘導体の優位配座の確認を取り入れた信頼性の高いNMRによる絶対配置決定法(ポスター発表の部)

概要

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Chiral derivatizing agents (CDAs) are frequently used for the NMR determination of the absolute configurations of chiral secondary alcohols and chiral primary amines. In the determination, the validity of the assignments of the absolute configurations strongly depends on the actual conformations of the chiral derivatives in solutions. Here we propose a direct method for verification of the preferred conformation of CFTA esters (CFTA = α-cyano-α-fluoro-p-tolylacetic acid) in solution by observing the C=O stretching absorption in IR spectra. Each IR spectrum of the CFTA ester exhibited two C=O stretching absorption bands conceivably due to two conformers. In all the esters, the absorptions of higher wavenumbers were stronger than that of lower ones. By the ab initio calculations, two stable conformers, the sp and the ap were found for each diastereomeric ester. The calculated wavenumbers of C=O stretching for the sp and the ap conformers well agreed with the higher and the lower wavenumbers of the absorptions experimentally observed, respectively. These results indicate that the predominance of the sp conformers in solutions which was previously deduced from ^1H chemical shift differences of the diastereomeric CFTA esters. Thus, the verification of the preferred conformation in solution is possible by simple IR measurement and this must greatly increase the reliability of the CFTA method.
天然有機化合物討論会の論文
2007-08-24