Triplet photosensitization in cyercene A and related pyrones

Mollusks of the Sacoglossa order contain a variety of polypropionate metabolites that are characterized by a pyrone chromophore, such as cyercene A and 9,10-deoxytridachione. Most often the pyrone is a 2-methoxy-γ-pyrone but occasionally is a 4-methoxy-α-pyrone or hydropyrone. Members of this class of metabolites have been shown to undergo photochemical reactions of biosynthetic importance. An example is the photochemical conversion of 9,10-deoxytridachione to photodeoxytridachione, which has been observed in several mollusks. In this report, a series of γ-pyrones and their α-pyrone analogs were synthesized and analyzed for photosensitizing activity. In all cases studied, the γ-pyrone was a more efficient triplet sensitizer than the corresponding α-pyrone. Included in this set of molecules was the Sacoglossan metabolite cyercene A and its α-pyrone isomer. When irradiated in the presence of oxygen, cyercene A produced singlet oxygen at significantly higher rate than the corresponding α-pyrone isomer. Furthermore, the photoisomerization of cyercene A was quenched by piperylene with concommitant isomerization of the piperylene indicating that the isomerization proceeded through a triplet excited state. In contrast, the isomerization of the α-pyrone analog was not quenched. The implications of these photochemical results in terms of the biosynthesis and biological activity of Sacoglossan polypropionate metabolites are discussed.