Cepaic acid, a novel yellow xanthylium pigment from the dried outer scales of the yellow onion Allium cepa

Cepaic acid was isolated as a novel xanthylium yellow pigment from the dried outer scales of the yellow onion Allium cepa Linne. Its structure was elucidated on the basis of ESI-MS and 2D NMR spectroscopy as a 9-carboxy-1,3,6,8-tetrahydroxyxanthylium, which suggests that cepaic acid and other yellow pigments in the dried outer skin of onion were formed by the nucleophilic reaction of phloroglucinol derived from quercetin, a flavonol in onion scales, by autoxidation to glyoxylic acid. To our knowledge, this is the first report of such a pigment in yellow onion.