Tetrahydroisoquinoline alkaloid N-methylnorsalsolinol from the Australian marine sponge Xestospongia SP.

Abstract

The EtOH extract from the Australian marine sponge Xestospongia sp. (Order Haplosclerida) collected during the 9th cruise of RV Akademik Oparin (Bougainville Reef, Australia) was studied during a search for antioxidants in marine organisms. A lyophilized specimen of the marine sponge (100 g) was extracted exhaustively first with CHCl3 and then EtOH (50%). The aqueous EtOH extract, which exhibited high activity for trapping DPPH radicals (2,2-diphenyl-1-picrylhydrazyl), was concentrated in vacuo. The aqueous residue was extracted with EtOAc and then n-BuOH. The aqueous and BuOH fractions showed antiradical activity. They were separated at low pressure over a column of C-18 silica gel (Fluka) using a H2O:EtOH gradient. Antiradical activity was observed in the fraction eluted by H2O during the separation of the n-BuOH fraction. It was concentrated in vacuo and crystallized from aqueous EtOH to afford 1 (132.4 mg), which was active for trapping DPPH radicals. An additional amount of 1 (8 mg) was isolated in the same manner from the aqueous fraction remaining after distribution between H2O and n-BuOH. Compound 1 (140.4 mg, 0.07% per dry sponge wt.), pale-yellow crystals (H2O:EtOH), mp 270–273°C. UV spectrum (H2O, max, nm, log ): 225, 283, 385 (3.56, 3.43, 2.54). The 13C NMR spectrum of 1 contained resonances for 10 C atoms including one methyl, three methylene, two methine, and four quaternary C atoms. The PMR spectrum also showed resonances for one methyl, three methylenes, and two singlets of aromatic protons in addition to two exchangeable protons ( 9.05 and 9.09) (Table 1). The high-resolution mass spectrum (HR-EI-MS) of 1 showed a base peak for [M – H]+ at m/z 178.0867 (C10H12NO2, 100%) that was characteristic of electron-impact decomposition of 1,2,3,4-tetrahydroisoquinolines [1]. 2D NMR spectroscopy experiments (HSQC, HMBC, COSY) enabled all resonances of 1 to be assigned. Thus, 1 was identified as 2-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (N-methylnorsalsolinol). A comparison of NMR spectra of 1 with those of 6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (norsalsolinol) [2] confirmed that 1 was the N-methyl derivative of norsalsolinol. The PMR and 13C NMR spectra of N-methylnorsalsolinol are given in Table 1 because these data have not been reported in the literature.