New 5-O-Caffeoylquinic Acid Derivatives in Fruit of the Wild Eggplant Relative Solanum viarum

摘要

Fruit of the cultivated eggplant species Solanum melongena, Solanum aethiopicum, and Solanum macrocarpon, and wild relatives including Solanum anguivi and Solanum incanum, have a high content of hydroxycinnamic acid conjugates with potential human health benefits. Typically, caffeoylquinic acid esters predominate, and in particular 5-O-(E)-caffeoylquinic acid. By contrast, fruit from accession PI 319855 in the USDA eggplant core collection, unambiguously identified as Solanum viarum by morphological characters, were found to include several major, closely related hydroxycinnamic acid conjugates with much longer C_(18)-HPLC retention times than those of 5-O-(E)-caffeoylquinic acid and other monocaffeoylquinic acid isomers. Four of these compounds were isolated from methanolic extracts of lyophilized fruit tissues by C_(18)-HPLC, and structurally elucidated using ~1H and ~(13)C NMR techniques and HR-TOF-MS. Isomeric compounds 1 and 2 are composed of 5-O-(E)-caffeoylquinic acid with a malonyl group on the 3- or 4-hydroxyl of quinic acid, respectively, plus a 6-O-sinapoylglucose group 1-O-β-D linked with the 4-hydroxyl on the phenyl ring of the caffeoyl moiety (1β,4β-dihydroxy-3β-carboxyacetoxy- and 1β,3β-dihydroxy-4β-carboxyacetoxy-5α-[[3-[4-[1β-(6-O-(E)-sinapoyl-β-D-glucopyranosyl)oxy]-3-hydroxyphenyl]-(E)-1-oxo-2-propenyl]oxy]cyclo-hexanecarboxylic acid). Compound 3 has the same structure as 1 and 2 without malonation of quinic acid (1β,3β,4β-trihydroxy-5α-[[3-[4-[1β-(6-O-(E)-sinapoyl-β-D-glucopyranosyl)oxy]-3-hydroxyphenyl]-(E)-1-oxo-2-propenyl]oxy]cyclohexanecarboxylic acid). Compound 4 differs from 3 by methylation of the carboxyl group on quinic acid (methyl 1β,3β,4β-trihydroxy-5α-[[3-[4-[1β-(6-O-(E)-sinapoyl-β-D-glucopyranosyl)oxy]-3-hydroxyphenyl]-(E)-1-oxo-2-propenyl]oxy]cyclohexanecarboxylate). Some features of these four new compounds, such as malonation and the specific linkages between caffeoyl, glucosyl, and sinapoyl moieties, are common in acylated and glycosylated phenylpropanoids, but have not previously been reported in complex derivatives of 5-O-(E)-caffeoylquinic acid.