DP4-Assisted Structure Elucidation of Isodemethylchodatin, a New Norlichexanthone Derivative Meager in H-Atoms, from the Lichen Parmotrema tsavoense.

Thuc-Huy Duong,Mehdi A Beniddir,Kim-Phi-Phung Nguyen,Joël Boustie,Pierre Le Pogam,Warinthorn Chavasiri,Guillaume Bernadat

doi:10.3390/molecules24081527

Abstract

A phytochemical investigation of the foliose lichen Parmotrema tsavoense (Krog and Swinscow) Krog and Swinscow (Parmeliaceae) resulted in the isolation of a new trichlorinated xanthone, isodemethylchodatin. The structure elucidation of this new norlichexanthone derivative proved tricky owing to proton deficiency, and to the lack of NMR data of closely related analogues. The structure of this compound was determined based on an integrated interpretation of 13C-NMR chemical shifts, MS spectra, and DP4-based computational chemistry was also performed to provide an independent and unambiguous validation of the determined structure. Isodemethylchodatin represents the first chlorinated lichexanthone/norlichexanthone derivative bearing a methoxy group at C-5.

Highlights

Xanthones represent ubiquitous polyphenolic metabolites endowed with various and significant bioactivities [1]
As a consequence of these biosynthetic processes, lichen xanthones often result in proton-deficient structures that are tricky to elucidate owing to the scarce number of NMR signals and to multiple possible regioisomers, which often co-occur within a single lichen species, having formerly led to numerous erroneous assignments related to the low H/C ratio [4,5,6]
Vietnamese lichen species are poorly studied from a chemical perspective, recent studies from our group have shed light onto new scaffolds obtained from this untapped biota

Summary

Introduction

Xanthones represent ubiquitous polyphenolic metabolites endowed with various and significant bioactivities [1] This tricyclic scaffold exclusively arises from the polyketide pathway in fungi whereas it is of mixed biosynthetic origin (shikimate/polyketide) in plants, resulting in different substitution patterns [2]. These structural differences legitimate joint efforts on plants, fungi and microbes to obtain structurally diverse molecules displaying this privileged scaffold. 1,3,6-trihydroxy-8-methylxanthone), while a rather limited number of lichen xanthones (essentially sustained by xanthone dimers), are related to ravenelin (i.e., 1,4,8-trihydroxy-3-methylxanthones) [2]. The reliability of structural assignments slowly increased over time as analytical approaches tailored to lichen xanthones arose such as standardized TLC procedures [7], and HPLC procedures based on their specific UV/Vis profile [8]

Methods

Results

Conclusion