Application of NMR Spectroscopy and Mass Spectrometry to the Structural Elucidation of Modified Flavan‐3‐ols and Their Coupling Reaction Products*

Abstract

NMR and MS techniques were used for the unambiguous structural elucidation of synthesized modified monomeric and dimeric flavan‐3‐ols presenting different substituents on the A‐ring C8 position. The full characterization of the synthesized compounds was achieved by concerted use of NMR and ESI‐MS techniques. Assignments of proton and carbon atoms was achieved through analysis of the 1D 1H and 13C NMR spectra combined with homo‐ and heteronuclear 2D NMR experiments. In each case, HMBC correlation between proton H2 and carbon C8a was observed allowing assignment of this carbon, which represents the key for attribution of the A‐ring carbon atoms. The synthesis and structural characterization of activated monomeric and dimeric flavanols were also achieved and used as precursors for preparation of natural and modified dimeric procyanidin derivatives. The preparation of various dimeric species involving modified flavanols was explored through different coupling reactions. The structures of the compounds formed were characterized on the basis of their MS and NMR spectral analysis. Dimeric species were characterized through proton–proton and proton–carbon correlations, which distinguished between the different flavanol moieties and established their sequences.