Rhodospermals A–G: an Extension to the Chemical Diversity of Compounds Possessing a Unique Spiroketal with a Fused α,β-Unsaturated δ‑Lactone Isolated from the Leaves of Cryptocarya rhodosperma (Lauraceae)

Phytochemical investigation of the Australian rainforest plant Cryptocarya rhodosperma B. Hyland (Lauraceae) led to the isolation and structural elucidation of seven new unique spiroketals fused with an α,β-unsaturated δ-lactone, namely, rhodospermals A–G (1 – 7), and a new alkylbenzodioxole α,β-unsaturated-δ-lactone rhodopyrone A (8) along with the known compounds EBC-23 (9) and EBC-73 (10). The chemical structures of these compounds were established by NMR spectroscopic data analysis, relative stereochemistry confirmed by distinct J coupling, NOESY correlations, and a close comparison of the NMR data and optical rotation of the previously reported compounds (9, 10). X-ray crystallography analysis and Mosher ester analysis of 1 confirmed the absolute stereochemistry of the fused spiroketal with an α,β-unsaturated δ-lactone. A plausible identical stereochemistry of the common fused spiroketal with an α,β-unsaturated δ-lactone observed in the remaining analogues (2–7) was proposed. The absolute stereochemistry of 8 was determined by performing a Mosher ester analysis and distinct J coupling. Compounds 1–10 exhibited cytotoxic activity in several cancer cell lines, highlighting their potential use in the treatment of human tumors.