Anthocyanins from red onion, Allium cepa, with novel aglycone

Four anthocyanins with the same novel 4-substituted aglycone, carboxypyranocyanidin, have been isolated from acidified, methanolic extracts of the edible scales as well as from the dry outer scales of red onion, Allium cepa L. The structures of 1 and 2 were identified as the 3-O-beta-glucopyranoside and 3-O-(6"-O-malonyl-beta-glucopyranoside) of 5-carboxypyranocyanidin, respectively. This aglycone, 5-carboxy-2-(3,4-dihydroxyphenyl)-3,8-dihydroxy-pyrano[4,3,2-de]-1-benzopyrylium, is with exception of the substitution pattern on the phenyl ring similar to carboxypyranomalvidin (vitisidin A) recently isolated from red wines. In addition to 1 and 2, two analogues of 2 methylated at the terminal carboxyl group of the acyl moiety (3) or at the aglycone carboxyl (4), respectively, were also identified. These latter compounds are most probably formed by esterification of 2 with the solvent (acidified methanol) during the isolation process. The structures were elucidated by 2D NMR spectroscopy and LC-MS.