Structure, absolute configuration, and variable-temperature 1H-NMR study of (±)-versiorcinols A–C, three racemates of diorcinol monoethers from the sponge-associated fungus Aspergillus versicolor 16F-11

Abstract

Three racemates of diorcinol monoethers, (±)-versiorcinols A–C (1–3), were initially isolated from the fungus Aspergillus versicolor 16F-11. The structures of versiorcinols B and C were elucidated by comprehensive spectroscopic analysis, while the structure of versiorcinol A was elucidated on the basis of quantum-chemical NMR chemical shifts, supported by DP4 probability analysis, and DU8 proton–proton spin–spin coupling constants (SSCCs) calculations in consortium with extensive experimental NMR analysis. The absolute configurations of the enantiomers (+)-1 and (−)-1, (+)-2 and (−)-2, and (+)-3 and (−)-3 were assigned from ECD calculations. At very low temperatures, the ¹H-NMR spectra of 1–3 revealed doubling of signals, which suggested the presence of two conformers in solution. Variable-temperature (VT) ¹H-NMR studies supported this hypothesis.