Synthesis and Analytical Characterization of Purpurogallin: A Pharmacologically Active Constituent of Oak Galls

Purpurogallin (PPG), an orange-red crystalline compound from nutgalls and oak bark, is an exemplar of numerous, ubiquitous natural colorants, biosynthesized through oxidative dimerization-decarboxylation of phenolic compounds. It possesses antioxidant, anticancer, and anti-inflammatory effects. Herein, a robust method is presented to allow students to expediently make this interesting compound that contains a tropolone ring, whose identity initiated a paradigm shift in the understanding of aromaticity. 1D and 2D nuclear magnetic resonance (NMR) spectroscopic data (1H, 13C-DEPT-Q, HSQC, HMBC) were used to identify which protons are connected to carbon atoms. Fourier transform infrared spectroscopy (FT-IR), mass spectrometry, and X-ray diffraction were used to provide complementary information. Importantly, sufficient historical detail is given to provide a useful narrative that highlights the shift from deductive and synthetic proof of structural identity to modern methods of structural elucidation. As a minor, but important, aspect of iron gall inks, PPG links the work of important western historical documents from Da Vinci’s Vitruvian Man to the United States Declaration of Independence. In the experience of the authors, such a pedagogic approach enlivens students with practical exercises which place them within the historical arc and reasoning of notable scientists, adding a sense of scientific discovery.