DFT-supported structure determination of gymunomitr-3(15)-en-4b,9b-diol

Abstract The investigation of the relative and absolute configuration of gymnomitr-3(15)-en-4β,9β-diol (1), a metabolite isolated from the culture medium of Calcarisporium arbuscula, is described. Since the pseudosymmetry of 1 hindered its configurational elucidation based on conventional spectral analysis, we resorted to DFT (Density Functional Theory)-based 13C NMR chemical shift calculations, which allowed identifying the rel-(2S,4R,6S,7R,9R,11S)-form as the candidate isomer giving 99.6% DP4 probability among the eight possible diastereomers. However, the root mean square value of its diastereomer at C-9 was also within the allowable level (1.9 ppm), considering the accuracy of this calculation method (around 2.0 ppm). The latter diastereomer could be ruled out by comparing the calculated 3JH-9/C-7 and 3JH-9/C-11 values with the intensities of the corresponding HMBC (Heteronuclear Multiple Bond Coherence) signals. The absolute configuration was established by reproducing the ECD (Electronic Circular Dichroism) spectrum of the dibenzoyl derivative 2. The biosynthesis of 1 is also discussed.