Preparative isolation and NMR characterization of carboxypyranoanthocyanins

Three pyranoanthocyanins, the 3-glucosides of 5-carboxypyranodelphinidin (2), 5-carboxypyranopetunidin (4), and 5-carboxypyranomalvidin (6), were produced by nucleophilic addition of pyruvic acid to a purified extract of black beans (Phaseolus vulgaris) containing a mixture of the 3-glucosides of delphinidin (1), petunidin (3), and malvidin (5). A Sephadex LH-20 column was used for preparative separation of both pyranoanthocyanins and anthocyanins. The optimum solvent used for isocratic separation was 20% methanol containing 0.5% trifluoroacetic acid. The chromatographic method applied on the pigment mixture yielded in a one-step separation pigments 1-6, with purities up to 98, 89, 99, 87, 55, and 81%, respectively. The structures of 2 and 4, which previously have been tentatively identified mainly by mass spectrometric data acquired from pigment mixtures in wine samples or modified blueberry extract, were confirmed in the present study by two-dimensional nuclear magnetic resonance spectroscopy.