Four New Bioactive Manzamine-Type Alkaloids from the Philippine Marine Sponge Xestospongia ashmorica

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Abstract
Analysis of the Philippine marine sponge Xestospongia ashmorica afforded four new manzamine congeners 1−4 and four known compounds 5 and 7−9. Compound 1 is the 6-deoxy derivative of manzamine X, while 2−4 are the N-oxides of manzamine J (5), 3,4-dihydromanzamine A (6), and manzamine A (7), respectively. The structures of the new compounds were unambiguously established on the basis of NMR spectroscopic (1H, 13C, COSY, 1H-detected direct, and long-range 13C−1H correlations) and mass spectrometric (EI, FAB-MS, and electrospray ionization) data. Alkaloid N-oxide structures were confirmed by conversion to the corresponding tertiary bases by reduction with Zn/HCl. This is the first report of the occurrence of bioactive manzamine N-oxides in marine sponges. Compound 7 exhibited insecticidal activity toward neonate larvae of the polyphagous pest insect Spodoptera littoralis (with an ED50 of 35 ppm) when incorporated in artificial diet and offered to larvae in a chronic feeding bioassay. Compound 7 was also active against the Gram-positive bacteria Bacillus subtilis and Staphylococcus aureus. Cytotoxicity was studied in vitro using L1578y mouse lymphoma cells. From the alkaloids studied, the N-oxides 3 and 4 were the most active (ED50 = 1.6 μg/mL) followed by compound 7 (ED50 = 1.8 μg/mL).