Studies on the photochemistry of 1,7-diphenyl-1,6-heptadiene-3,5-dione, a non-phenolic curcuminoid model

Abstract

The comparative photostability of curcumin 1, and two non-phenolic curcuminoids: 1,7-diphenyl-1,6-heptadiene-3,5-dione 2 (unsubstituted curcumin) and dimethylcurcumin 3 in non-degassed dilute solutions (≈3–5 × 10⁻⁵ mol l⁻¹) has been established by UV-visible absorption spectroscopy; disappearance quantum yields were measured. The similar behavior of the three studied curcuminoids is indicative of only a moderate role of phenol groups in the photodegradation process. Structural analysis of the photodegradation products of compound 2 in more concentrated solution (≈3.6 × 10⁻³ mol l⁻¹) shows formation of benzaldehyde, cinnamaldehyde, 2′-hydroxy-5′,6′-benzochalcone 4, flavanone 5 and some other unidentified photoproducts. Flavanone 5 is formed by irradiation of chalcone 4. It represents a unique example of photochemical conversion of a diarylheptanoid molecule into a flavonoid, another very important class of natural products.