title: Solution-state conformations of natural products from chiroptical spectroscopy: the case of isocorilagin.
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 (o)S5 conformations of the β-glucose core in DMSO-d6 to an inverted (1)C4 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.
authors.affiliation: Department of Chemistry, Federal University of São Carlos, UFSCar. Rod. Washington Luís km 235, São Carlos, SP 13565-305, Brazil. joaombj@hotmail.com quezia@pq.cnpq.br.
authors.forename: R F
authors.initials: RF
authors.lastname: Sprenger
authors.forename: S S
authors.initials: SS
authors.lastname: Thomasi
authors.forename: A G
authors.initials: AG
authors.lastname: Ferreira
authors.forename: Q B
authors.initials: QB
authors.lastname: Cass
authors.forename: J M
authors.initials: JM
authors.lastname: Batista Junior
mesh_terms: D001688:Biological Products / Q000737:chemistry*; D002942:Circular Dichroism; D004121:Dimethyl Sulfoxide / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D011789:Quantum Theory; D012996:Solutions; D013634:Tannins / Q000737:chemistry*
chemical_list: D001688:Biological Products; D012996:Solutions; D013634:Tannins; C000589055:isocorilagin; D004121:Dimethyl Sulfoxide