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Systematizing chiroptical properties of lignans: implications for correct absolute configuration assignments

Abstract

Natural product chemistry deals with organic molecules of high structural and stereochemical complexity of different classes and biosynthetic origins. The main techniques used in structural elucidation/determination, mass spectrometry (MS) and magnetic nuclear resonance (NMR), are intrinsically insensitive to chirality, limiting their application in absolute stereochemistry studies. Historically, chiroptical methods such as optical rotation (OR) and electronic circular dichroism (ECD) has been the techniques of choice for stereochemical studies since they can be applied directly in solution and without requiring either crystallization or derivatizations. OR and ECD are mainly used by means of comparisons of data obtained for structurally related molecules or by the application of empirical rules. The apparent ease of application of chiroptical methods, however, may be deceiving. An increasing number of absolute configuration reassignment has been reported in the literature and many incorrect assignments are due to empirical rule exceptions. Current guidelines for interpretation of chiroptical data involve the quantum chemical simulation of theoretical spectra. Despite recent advances, such simulations are still not routine in natural product chemistry laboratories worldwide. Thus, the present project aims to systematically investigate both the experimental and theoretical chiroptical properties of furan and furofuran type lignans isolated from Aristholochia species in order to verify the applicability as well as limitations of the main empirical correlations/rules found in literature. Additionally, vibrational circular dichroism (VCD) investigations will be carried out. This way, this project will provide validated tools for the correct stereochemical analysis of this important class of secondary metabolites. (AU)

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