Reassessing the Structure of Pyranonigrin

Abstract

Fermentation extracts of the marine fungus Aspergillus niger LL-LV3020 were found to have relevant activity in a number of assays. Chemical screening of the extracts revealed that this organism produced numerous secondary metabolites in addition to its principal metabolite, citric acid. The compound with the most significant UV peak was isolated and its structure elucidated. Physical data suggested that this compound is identical with pyranonigrin A (1); however, our structure elucidation led to a different assignment than previously reported. On the basis of analysis of all data, we propose a correction to the structure of pyranonigrin A. Its absolute configuration was determined by electronic circular dichroism measurements in comparison with theoretical values calculated via ab initio time-dependent density functional theory and assigned as (7R)-3,7-dihydroxy-2-[(1E)-prop-1-enyl]-6,7-dihydropyrano[2,3-c]pyrrole-4,5-dione.

Survey of marine natural product structure revisions: A synergy of spectroscopy and chemical synthesis

“…In some instances, such as aspernigrin A, 26, 27 a single HMBC correlation may theoretically differentiate two isomers. However, in cases such as pyranonigrin, 26, 28 spiroleucettadine 29, 30 and almazole D (Table 3), 31, 32 careful consideration of the chemical shift values in the assigned structure may have provided the only clue to the structural misassignment.…”

Isolation of the Antioxidant Pyranonigrin-A from Rice Mold Starters Used in the Manufacturing Process of Fermented Foods

“…3). 24) Pyranonigrin-A has reportedly been isolated from fermentation extract of marine fungus Aspergillus niger (culture LL-LV3020). This study found pyranonigrin-A in the rice mold starter, which is safe and practical for use in food processing, for the first time.…”

Structural Revisions of Natural Products with the Aid of the Structure Elucidator System

“…All structures were elucidated on the basis of extensive 1D and 2D NMR spectroscopic studies ( 1 H, 13 C, COSY, HMQC, HMBC, NOE difference spectra) and MS analysis. For the two chiral pyranonigrin molecules, particularly for pyranonigrin A (C 9 H 10 NO 5 , DBE ¼ 7) 12.34, the absolute configurations were 50 isolated from the marine fungus Aspergillus niger a compound of molecular formula C 9 H 10 NO 5 whose physical data were identical to those published by Hiort et al 49 for pyranonigrin A. Interpretation of the NMR data did not permit the authors 50 to assign structure 12.34a to pyranonigrin A.…”