Structure Revision of Pyranoxanthones via DFT-Assisted ¹³ C NMR Analysis and NAPROC-13 Platform: Diagnostic Markers and Discovery of Unreported Natural Products

The structures of 47 natural and synthetic pyranoxanthones, previously misassigned in the literature, were revised using the NAPROC-13 platform in combination with DFT-based ¹³C NMR calculations. Structural misassignments primarily involved incorrect prenyl group localization or misplacement of the oxygen atom involved in the pyran ring. NAPROC-13 searches frequently revealed alternative, structurally distinct compounds sharing identical ¹³C NMR data, or instances where the same compound had been reported with divergent data sets. DFT-calculated chemical shifts confirmed the revised structures, which included six previously unreported natural products. Systematic deviations between calculated and experimental values were observed for carbons adjacent to the carbonyl and for the olefinic carbons of the pyran ring. Statistical analysis of a 76-compound modeling set yielded empirical correction factors that markedly improved data concordance. This combined database-computational approach demonstrates high reliability in structural validation and is applicable to other families of natural products. Additionally, diagnostic ¹³C NMR chemical shifts were identified for carbons adjacent to the pyran ring, offering a rapid tool for determining the site of fusion on the xanthone scaffold.