2 VCD励起子キラリティー法 : 絶対立体化学の新規非経験的決定法(口頭発表の部)

抄録

The determination of absolute configuration of chiral molecules is an essential but difficult step in natural product chemistry. Chiroptical spectroscopy is the sole technique that can nonempirically determine molecular chirality without need of crystallization. One of the most widely used is the exciton chirality method in electronic circular dichroism (ECD), developed by Harada and Nakanishi, but the requirement for two or more appropriate UV-vis chromophores restricts its applicability. Meanwhile, vibrational circular dichroism (VCD) spectroscopy using theoretical calculation has been a recent trend for its reliability and convenience; however, this approach has been hampered by the low sensitivity of vibrational absorption and the computational demand. In exploration of a more universal, sensitive method, we envisioned the potential of an exciton coupling approach in VCD. To test its feasibility, we synthesized various mono- and biscarbonyl chiral compounds and examined their VCD spectra. All the biscarbonyl compounds exhibited a bisignate VCD coupling whose intensity was 〜20 times higher than that of monocarbonyl compounds as a result of an interaction of two carbonyl groups. Furthermore, the signs of these couplets are consistent with the absolute twist of the two C=O bonds, which proved the reliability of "the VCD exciton chirality method" for determination of absolute configuration. Not only can the VCD exciton chirality method be used as conveniently as the ECD exciton chirality method, but also it can analyze molecules that are outside of the coverage of ECD and other spectroscopic techniques. In this study, we demonstrated the applicability of the VCD approach to various types of molecules such as spirobiscarbonyl compounds, α-amidelactam, α-acyloxyketone, and α-hydroxyketone. Furthermore, we recently revealed that this method can elucidate the chirality of diacylglycerolipids, major components of cell membrane. There have been few methods to analyze the absolute configuration of glycerolipids, but our current approach should provide new insight into evolution, and also clarify the detailed relationship between their chirality and biological functions. Last, the VCD exciton approach can be used as a signal intensifier. In some cases, less than 20 μg of analyte was sufficient to observe a VCD couplet, by which one can determine its absolute configuration. This method should find various usages in future, e.g., analysis of minuscule molecules with or without using theoretical calculation or time-resolved VCD measurement.