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Unusual ent-Labdane Diterpenoid Dimers and their Selective Activation of TRPV Channels.

Five unusual dimers of ent-labdane diterpenoids (1-5) were isolated and identified from a famous medicinal plant, Andrographis paniculata. Bisandrographolide E (1) represents the first example of labdane dimer possessing an unprecedent tricyclic system that comprised a spiroketal moiety fused with a ketal-γ-lactone unit in its skeleton. Its biosynthetically related intermediates, all the four stereoisomers at C-12 and C-15', bisandrographolides F (2, a new compound) and A-C (3-5) were obtained at the same time. The steric configurations of the newly formed asymmetric carbons in 1-5 were first solved by single-crystal X-ray diffraction of diacetone derivatives of 2-4, and ECD and NMR calculations of 1. More importantly, bisandrographolides 1-5 with different chemical structures or absolute configurations at C-12 and C-15' selectively activated different TRPV1~4 channels and protected cardiomyocytes from hypoxia-reoxygenation injury. Among them, 5 with 12R/15'S configuration activated TRPV1 most effectively and displayed the best cardiomyocyte protection.