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

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.

Salazinic Acid-Derived Depsidones and Diphenylethers with α-Glucosidase Inhibitory Activity from the Lichen Parmotrema dilatatum

“…The minute amounts of the newly reported diphenylethers 4-6 prevented the acquisition of a 13 C NMR spectrum of satisfactory quality so that structure elucidation relied on inverse-detection heteronuclear NMR spectra. Nevertheless, these structures are meager in H atoms, with Crews scores below 1, and therefore exhibit silent fragments [35], a common situation in lichen specialized metabolites that are often highly substituted, aromatic structures [36]. Fortunately, the missing signals affected constant parts of all the compounds described in this publication (C-1′, C-5′, and C-6′) and did not cast any doubt regarding the soundness of the proposed structure elucidations.…”

α-Glucosidase Inhibitory Depsidones from the Lichen Parmotrema tsavoense

“…Several studies have reported that lichen metabolites possess potent α-glucosidase inhibitory activity [3][4][5]. In previous studies on the Vietnamese lichen Parmotrema tsavoense, we described several novel depsidones and diphenyl ethers [6][7][8]. However, there has been no investigation on the presence of α-glucosidase inhibitors in this lichen.…”

Crews' rule—Still useful but often misquoted

“…[2][3][4][5][6][7] Unfortunately, the first easily located publication [8] using the term Crews' rule gives the critical H/C atom ratio as 2 rather than the value of 1 given in the original paper. This error has propagated through the literature [9][10][11][12][13][14][15][16][17][18] and continues to do so to this day. [19][20][21] This is rather surprising as Crews' rule is normally cited in the context of solving the structures of protondeficient molecules.…”