Reinvestigation of the chemical structure of bitter-tasting quinizolate and homoquinizolate and studies on their Maillard-type formation pathways using suitable 13C-labeling experiments

Very recently, application of taste dilution analysis to heated xylose/alanine solutions led to the isolation of two bitter-tasting compounds exhibiting extraordinarily low detection thresholds of 0.00025 and 0.001 mmol/kg of water, respectively. On the basis of LC-MS and NMR spectroscopy, the structures of these compounds, named quinizolate and homoquinizolate, were proposed as 1-oxo-1H,4H-quinolizinium-7-olates, Since recent experiments in our laboratory shed some doubt on the entire correctness of their structures, labeling experiments with mixtures of multiply C-13-labeled and nonlabeled pentoses were performed to follow the joint transfer of several C-13 atoms en bloc into the bitter compounds by LC-MS and NMR isotopomer diagnosis. The site-specific visualization of the mosaics assembled from C-13-labeled. C-12-labeled carbon modules in both bitter compounds demonstrated the structures of quinizolate and homoquinizolate to be the previously unknown (2E)-7-(2-furylmethyl)-2-(2-furylmethylidene)-3-(hydroxymethyl)- and (2E)-7-(2-furylmethyl)-2-(2-furylmethylidene)-3,8-bis(hydroxymethyl)-1-oxo-2,3-dihydro-1H-indolizinium-6-olate.