Synthesis and structure elucidation of ethyliden-linked anthocyanin — Flavan-3-ol oligomers

• Synthesis of ethyliden bridged oligomers consisting of anthocyanins and flavanols
• Structure elucidation of seven dimers and one trimer by 2D NMR
• LC-MS identification of 20 reaction products induced by acetaldehyde
• Isolated compounds apparently have impact on bluish tonalities of red wine color

The reaction of grape derived anthocyanins and catechin in the presence of acetaldehyde was examined in model wine solutions. The reaction profile was monitored by HPLC-DAD and HPLC-ESI-MSn which revealed the presence of 18 different higher oligomers in addition to the ethyliden-bridged dimers as the main products of the reaction. In addition to the ethyliden bridge between the monomeric subunits, a second bridge could be observed, which may be attributed to the reaction product of two vinyl-flavanol intermediates. The evolving dimers and higher oligomers were isolated by means of high-speed countercurrent chromatography and their structure was elucidated by two dimensional NMR spectroscopy. In addition to the ethyliden-catechin dimers of the anthocyanins malvidin-3-glucoside, petunidin-3-glucoside, delphinidin-3-glucoside, and cyanidin-3-glucoside, a trimer consisting of two catechin units and a central cyanidin-3-glucoside unit could be isolated and structurally characterized. An ethyliden bridged trimer containing an anthocyanin with ethyliden bridges at positions 6 and 8 is presented here for the first time. This structure confirms the possibility of branched oligomers and oligomers with more than two anthocyanin units that may account for color stabilization of red wine. The color stability of the isolated compounds was examined in model wine solution.