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

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.

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.

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).

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.