The calculation of NMR parameters for natural products was pioneered by Bifulco and coworkers in 2002. Since then, modelling 1H and 13C chemical shifts and spin-spin coupling constants for this...

Comprehensive work suggesting the amendment of helianane to the open ring curcudiol by means of computational spectroscopy and thermodynamic reasons.

No abstract

Recently, structural elucidation of natural products has undergone a revolution. The combined use of different modern spectroscopic methods has allowed obtaining a complete structural assignment of natural products using small amounts of sample. However, despite the extraordinary ongoing advances in spectroscopy, the mischaracterization of natural products has been and remains a recurrent problem, especially when the substance presents several stereogenic centers. The misinterpretation of nuclear magnetic resonance (NMR) data has resulted in frequent reports addressing structural reassignment. In this context, a great effort has been devoted to developing quantum chemical calculations that simulate NMR parameters accurately, allowing to achieve a more precise spectral interpretation. In this work, we employed a protocol for theoretical calculations of 1H NMR chemical shifts and coupling constants using density functional theory (DFT), followed by the application of the DP4+ method to revisit the structure of Heliannuol L, a member of the Heliannuol class, isolated from Helianthus annuus. Our results indicate that the originally proposed structure of Heliannuol L needs a stereochemical reassignment, placing the hydroxyl bonded to C10 in the opposite side of the methyl and hydroxyl groups bonded to C7 and C8, respectively.

Herein, the isolation of secondary metabolites from the aerial parts of Justicia aequilabris guided by HPLC-MS n and molecular networking analyses is reported. Twenty-two known compounds were dereplicated. Three new lignans (aequilabrines A− C (1−3)) and three known compounds (lariciresinol-4′-O-β-glucose (4), roseoside (5), and allantoin (6)) were obtained. The antiinflammatory activity of compounds 1−3 was evaluated in vitro by inhibiting the nitric oxide production (NO) and proinflammatory activity on the cytokine IL-1β. Compounds 2 and 3 showed significant inhibitory activity against NO production, with IC 50 values of 9.1 and 7.3 μM, respectively. The maximum inhibition of IL-1β production was 23.5% (1), 27.3% (2), and 32.5% (3).

The combination of computational methods and experimental data from Nuclear Magnetic Resonance (NMR) is a considerably valuable tool in the elucidation of new natural product structures and, also, in the structural revision of previously reported compounds. Until recently, only classical statistical parameters were used, for example, linear correlation coefficient (R 2 ), mean absolute error (MAE), or root mean square deviation (RMSD), as a way to statistically "validate" the structure pointed out by experimental NMR spectra. Regarding the resolution of the relative configuration of organic molecules, novel tools were available in the last few years to assist in the NMR elucidation process. The most relevant are DP4+, which is based on a Bayesian probabil-

No abstract

Nuclear magnetic resonance (NMR) spectroscopy can provide detailed information about the chemical structure and stereochemistry of a molecule. However, this technique alone may not always be sufficient to assign with...