X-ray Diffraction, Electronic Circular Dichroism, and Quantum Mechanics (TD-DFT) Investigations on 4-Dehydroxyaltersolanol A, a Secondary Metabolite from Endophytic Fungus Nigrospora oryzae

Background and objectives: A tetrahydro anthraquinone derivative, 4-dehydroxyaltersolanol A, has been obtained from Nigrospora oryzae, which was isolated from Uncaria borneensis Havil as an endophytic fungus. This is a recently described compound whose stereochemistry was assumed from biogenetic considerations. However, using ECD spectral analysis in combination with TD-DFT calculations, its stereochemistry could be determined unambiguously. Method: In the current research, the selected TH1P45 culture was analysed using semi-preparative HPLC, which led to the isolation of six secondary metabolites, including 4-dehydroxyaltersolanol A (1). We have further presented full evidence of the stereochemistry of compound 1. With the help of quantum calculations, we also determined the mechanism by which this compound degrades in solution. Results: The analysis of TH1P45 culture led to the isolation of six secondary metabolites, including 4-dehydroxyaltersolanol A, three anthraquinone derivatives (macrosporin, bostrycin and altersolanol B), and two pyrones (pestalopyrone and hydroxypestalopyrone). Conclusion: A full evidence of the stereochemistry of compound 1 with the help of the combination of X-ray crystallography, ECD, and TD-DFT quantum calculations, allowed unambiguously assigning the absolute stereochemistry of 4 dehydroxyaltersolanol A as 1S,2R,3S as correctly assumed by Proksh and collaborators from biogenetic considerations.