AbstractChanges in sinapic acid when exposed to aqueous alkaline conditions were elucidated. Sinapic acid was exposed to a volatile buffer (pH 8.5) for 24 h, lyophilized, acidified, extracted, and characterized using nuclear magnetic resonance and mass spectroscopy. The product obtained was identified as the lignan thomasidioic acid. This identification was confirmed by comparison with a synthesized authentic sample of thomasidioic acid. Conversion of sinapic acid to thomasidioic acid under alkaline conditions previously has not been reported. Thomasidioic acid was present after exposure of sinapic acid to pH 8.5 for as few as 6 h. Thomasidioic acid also was formed at pH 7.