New C15 Acetogenins from Two Species of Laurencia from the Aegean Sea

Abstract

The chemical diversity of the approximately 1,200 natural products isolated from red algae of the genus Laurencia, in combination with the wide range of their biological activities, have placed species of Laurencia in the spotlight of marine chemists’ attention for over 60 years. The chemical investigation of the organic (CH2Cl2/MeOH) extracts of Laurencia microcladia and Laurencia obtusa, both collected off the coasts of Tinos island in the Aegean Sea, resulted in the isolation of 32 secondary metabolites, including 23 C15 acetogenins (1–23), 7 sesquiterpenes (24–30) and 2 diterpenes (31 and 32). Among them, six new C15 acetogenins, namely 10-acetyl-sagonenyne (2), cis-sagonenyne (3), trans-thuwalenyne C (4), tinosallene A (11), tinosallene B (12) and obtusallene XI (17), were identified and their structures were elucidated by extensive analysis of their spectroscopic data. Compounds 1–3, 5–11, 13 and 15–32 were evaluated for their antibacterial activity against Staphylococcus aureus and Escherichia coli.

Antimicrobial Activity of the Secondary Metabolites Isolated from a South African Red Seaweed, Laurencia corymbosa

“…A total of three methine moieties observed at δ H 4.77 (dd, J = 13.3, 4.7 Hz), δ H 2.92 (t, J = 3.0 Hz) and δ H 4.06 (t, J = 7.7 Hz), together with four methyl singlets between δ H 1.0 and 2.0, were suggestive of a sesquiterpene skeleton. These data, together with the 13 C and HMBC data obtained, supported a chamigrane sesquiterpene structure with a spiro [5.5] undecane arrangement for metabolites typically produced by Laurencia spp. [14].…”

“…The C12 and C13 methyl protons show clear 3 J HMBC correlations (Figu S7) to the bromo-methine at C10 where the chemical shift is δC 57.6 and no suggested by Bano et al (1987), while the spiro-center at C6 in compound 1 quaternary carbon with a chemical shift of δC 44.6 and not δC 42.3 [22]. This is by unambiguous 3 J HMBC correlations from methyl moieties at C13 and C14 to 1c, the 13 C NMR chemical shift of the gem-dimethyl quaternary carbon at be δC 42.3 and not δC 44.6, as both methyl groups C12 and C13 display stron correlations to C11 (δC 42.3). Bano et al (1987) [22] and Crews et al (1984) [23] two carbon resonances at δC 62.0 (Figure 4).…”

“…of California [19], there is some conflict in the reported carbon chemical shifts ing an informed re-assignment following comprehensive analysis of the 13 C N compound 1. Unambiguous COSY (Figure S4) and HMBC (Figures S6 and S7) obtained for compound 1 in this study conflicted with those reported by Bano [22], as shown in compound 1 (Figure 4), especially regarding positions C6, C1 Although 13 C NMR data are included by the authors (Ojika et al, 1982) [21] fo epimer 1d, several discrepancies are observed (Figure 4). Crews et al (1984) [23 report the 13 C NMR data for a semi-synthetic analogue prepared by Howard (1975) [19].…”

“…Marine macroalgae (seaweeds) have generated over 3000 biologically active, new chemical structures in the last 40 years, most of which were isolated from the phylum Rhodophyta [11]. More than 1200 unique natural products, including halogenated sesquiterpenes, diterpenes, C15 acetogenins, and indoles, have been reported from the red algal genus Laurencia [12,13]. Unfortunately, factors such as high morphological variability amongst individual species make the classification and/or identification of members within this genus increasingly difficult [14].…”

Marine natural products

“…Acetogenins are common metabolites of red algae, with nine C 15 variants being reported from investigations of various Laurencia species from Japan 884 (Katsura) 351 and Greece 885 – 892 , (Tinos Island), 352 respectively. The tongalides 893– 898 are halogenated butenolides isolated from an Antarctic (Bonaparte Point) Delisea sp.…”