Crambescidin Acid from the French Polynesian Monanchora n. sp . Marine Sponge
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CRAMBESCIDIN ACID FROM THE FRENCH POLYNESIAN Monanchora n. sp. MARINE SPONGE
A. El-Demerdash,1,2* S. Petek,3,4 C. Debitus,3,4 and A. Al-Mourabit1
Marine natural products continue to be rich reservoirs for pharmacological lead compounds [1, 2]. Marine sponges are a distinct multicellular phylum and represent the biggest class of marine invertebrates with approximately 8,900 registered species, though 15,000 species are distributed over seas and oceans [3–5]. Monanchora marine sponges are well recognized for producing a high number of structurally divergent bioactive secondary metabolites [6], including acyclic and polycyclic guanidine alkaloids [7–9], steroids [10], terpenoids [11, 12], and long chain fatty acids [13]. These compounds display a myriad of biological potentialities as antimalarial [14], anticancer [15], anti-infective [16], and antiviral [17]. As a part of our ongoing program on bioactive natural products from marine sponges [18–21], we report herein the isolation and identification of five natural products 1–5 for the first time from the n-BuOH fraction of Monanchora n. sp. sponge. Chemical investigation of the French Polynesia Monanchora sp. marine sponge led to the isolation and identification of one pentacyclic guanidine alkaloid crambescidin acid (1) along with four simple nitrogenous compounds, including one nucleoside thymidine (2) and three amino acid derivatives, namely phenylethylamine (3), tryptophan (4). and phenylalanine (5). Compound 1 (11.5 mg) was purified as a viscous pale yellow oily material. It has the molecular formula C22H33N3O4 as established from its positive HR-ESI-MS m/z 404.2461 [M + H]+ (calcd 404.2549), indicating eight degrees of unsaturation. A preliminary inspection of its 1H NMR recorded in MeOH-d4 showed characteristic signals with similar chemical shifts to those reported previously for crambescidine pentacyclic guanidine alkaloids, confirming the presence of a pentacyclic guanidinic core (vessel), which accounts for five degrees of unsaturation. A detailed analysis of the 13C NMR and HSQC spectra disclosed 22 resonances, which were attributed to two methyls at δC 10.9 (C-1) and 22.1 (C-20), two oxymethines at δC 72.3 (C-3) and 68.3 (C-19), two azamethines at δC 55.6 (C-10) and 54.9 (C-13), two olefinic methines at δC 134.5 (C-4) and 131.6 (C-5) within the seven membered ring, giving the sixth degree of unsaturation, one methine at δC 54.1 (C-14), and nine methylene groups at δC 30.4 (C-2), 24.6 (C-6), 38.4 (7), 38.7 (C-9), 31.0 (C-11), 31.0 (C-12), 33.6 (C-16), 19.7 (C-17), and 32.9 (C-18). Four quaternary resonances, including two sp3 spiro-carbons at δC 85.1 (C-8) and 82.8 (C-15) and two downfield resonances at δC 150.8 (C-21) and 181.6 (C-22), are attributed to the guanidinic and free carboxylic acid functionalities, giving the remaining two degrees of unsaturation and satisfying the eight degrees required by HR-MS. The 2D NMR spectral data interpretations, including 1H–1H COSY, and 1H–13C HMBC, enabled us to establish and confirm the total structure of 1 and s
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