Enantiodifferentiation of phytoalexin spirobrassinin derivatives using the chiral solvating agent (R)-(+)-1,1′-bi-2-naphthol in conjunction with molecular modeling

The enantioresolutions of chiral phytoalexins spirobrassinin, 1-methoxyspirobrassinin and cis- and trans-1-methoxyspirobrassinol methyl ethers were obtained by solution-state 1H NMR spectroscopy using the chiral solvating agents (R)-(+)-1,1′-bi-2-naphthol and, for 1-methoxyspirobrassinin, (R)-(−)-1-phenyl-2,2,2-trifluoroethanol. In addition to the evaluation of the magnitude of the chemical shift differences, several other aspects with respect to enantiodifferentiation were also examined, namely titration of the enantiomers with the chiral solvating agents, signal degradation in the presence of excess, solid binaphthol, measurement of longitudinal relaxation times (T1) and diffusion coefficients (D), conducting enantiomeric titrations of spirobrassinin under SIDA and atypical SIDA conditions, and conducting chromatographic-type examinations (‘TLC in an NMR tube’). To rationalize the observed spectroscopic and adsorption behavior, the conformations, geometries, and energies of the complexes were calculated by DFT calculations at the B3LYP/TZVP{6-31G(d,p)} level of theory with solvation simulated by IEF-PCM. Chemical shifts were assessed by making a comparison with population-weighted averaged shieldings calculated by DFT at the B3LYP/cc-pVTZ level of theory using the GIAO method. The reliability of the interpretations with respect to the assignment of the configuration was very dependent on obtaining a large calculated ΔG and/or adopting a holistic approach by evaluating several measured parameters.

Graphical abstractDifferentially degraded 1H NMR signals of spirobrassinin due to excess, solid binaphthol reflect the interaction level of each nucleus with binaphthol rather than the enantiomer to which the signal belongs.Figure optionsDownload full-size imageDownload as PowerPoint slide