Abstract
The circular dichroism (CD) exciton chirality method (ECM) is a very popular approach for assigning the absolute configuration (AC) of natural products, thanks to its immediacy and ease of application. The sign of an exciton couplet (two electronic CD bands with opposite sign and similar intensity) can be directly correlated with the molecular stereochemistry, including the AC. However, a correct application of the ECM necessitates several prerequisites: knowledge of the molecular conformation; knowledge of transition moment direction; and preeminence of the exciton coupling mechanism with respect to other sources of CD signals. In recent years, by using quantum-chemical CD calculations, we have demonstrated that some previous applications of ECM were wrong or based on incorrect assumptions. In a recent publication of this journal (Mar. Drugs, 2017, 15(4), 121), the ECM was employed to assign the AC of a marine metabolite, laucysteinamide A. This is a further case of incorrect application of the method, where none of the aforementioned prerequisites is fully met. Using this example, we will discuss the criteria required for a correct application of the ECM.
Highlights
The very large majority of known natural products is chiral and contains multiple centers of chirality [1]
C-2 chirality center was established by recording the its structural elucidation, the absolute configuration (AC) at the C-2 chirality center was established by recording circular dichroism (CD)
CD spectrum, and electronic properties of the truncated model 2 in detail, we shall examine the major pitfalls and inconsistencies found in the reported application of the exciton chirality method (ECM) to laucysteinamide A (1)
Summary
The very large majority of known natural products is chiral and contains multiple centers of chirality [1]. A complete stereochemical elucidation is a common requirement in natural products discovery. A novel chiral natural compound cannot be said to be fully characterized until its absolute configuration (AC) is determined. Electronic circular dichroism (ECD, or CD) remains the most popular one in the field of natural products discovery [7], vibrational CD (VCD) and Raman optical activity are gaining progressive interest [8,9]. The most common approach to assign AC by ECD or VCD analysis consists in the comparison between experimental and quantum-chemical calculated spectra [10]. There are several other means for interpreting ECD spectra to be used for AC determination [7,11]
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