Title
Solution-state conformations of natural products from chiroptical spectroscopy: the case of isocorilagin
Author
R. F. Sprenger, S. S. Thomasi, A. G. Ferreira, Q. B. Cass, J. M. Batista Junior
Year
2016
Journal
Organic and Biomolecular Chemistry
Abstract
Isocorilagin, the α-anomer of the ellagitannin corilagin, has been frequently reported in the literature as a constituent of various plant species. Its identification is based mainly on the smaller value for the coupling constant of its anomeric proton when compared to that of corilagin. A careful investigation of the corilagin structure in both methanol and DMSO solutions using NMR, electronic and vibrational CD, and DFT and MD calculations confirmed that isocorilagin is the result of a solvent-induced conformational transition of corilagin, rather than its diastereoisomer. Corilagin changes from B1,4 and oS5 conformations of the β-glucose core in DMSO-d6 to an inverted 1C4 conformation in methanol-d4, which accounts for NMR observables attributed to the alleged α-anomer. This misassignment reinforces the risks of relying upon a single technique for structural elucidation and stereochemical analysis of complex natural products, especially those containing saccharide moieties.
Instrument
J-815
Keywords
Circular dichroism, Stereochemistry, Natural products