Synthesis of condensed tannin model compounds regioselectively labeled with a 13C-stable isotope

Condensed tannins exhibit diverse bioactivities that render them promising for applications in the food and medical fields. For the analysis and monitoring of condensed tannins, 13C-labeled derivatives would provide a useful tool. In this study, condensed tannin polymers regioselectively labeled with a stable isotope were synthesized using 13C-labeled acetic acid or 13C-labeled dimethylformamide as the starting material. The resulting synthetic polymers were labeled with 13C at the C-4 or C-2 positions. A GPC analysis revealed that all model polymers comprised mainly tetramer to hexamer structures. According to the 13C-NMR data of the synthetic tannin models and natural condensed tannin obtained from sugi bark, the chemical structure of both compounds was very similar. Furthermore, compared with the natural condensed tannin and unlabeled synthetic polymer, the labeled compound showed more intense C-4 and C-2 13C signals, indicating that the 13C labeling proceeded regioselectively. These compounds are useful for analyzing the chemical reactions of condensed tannins and monitoring structural transformation processes in vivo.