Cystophloroketals A–E, Unusual Phloroglucinol–Meroterpenoid Hybrids from the Brown Alga Cystoseira tamariscifolia

Abstract

Cystophloroketals A-E (1-5), five new phloroglucinol-meroditerpenoid hybrids, have been isolated together with their putative biosynthetic precursor, the monocyclic meroditerpenoid 6, from the Mediterranean brown alga Cystoseira tamariscifolia. They represent the first examples of meroditerpenoids linked to a phloroglucinol through a 2,7-dioxabicyclo[3.2.1]octane moiety. The chemical structures, including absolute configurations, were elucidated on the basis of extensive spectroscopic analysis (HR-ESIMS, 1D and 2D NMR, and ECD) and TDDFT ECD calculations. Compounds 1-6 were tested for their antifouling activity against several marine colonizing species (bacteria, fungi, invertebrates, micro- and macroalgae). Compound 6 showed high potency for the inhibition of macrofoulers (invertebrates and macroalgae), while cystophloroketals B (2) and D (4) displayed strong inhibition of the germination of the two macroalgae tested and moderate antimicrobial activities (bacteria, microalgae, and fungi).

Marine natural products

“…Not atypically the chemistry was dominated by terpenoids and meroterpenoids with two dolastanes, 738 and 739, four xenicanes, 740-743 and two cytotoxic sterols 744 and 745 isolated from Canistrocarpus cervicornis, 519,520 Dictyota plectens, 521 and Cystoseira trinodis 522 respectively. The compounds of mixed biosynthesis was a tranche comprised of a chromene, 746 528 The proposed, unprecedented syn-cis-anti arrangement for the A/B/C ring system in the cyclic meroditerpenoid, O,C(3)-seco-9-ene-6b-taondiol 752 was based on NOESY data and supported the notion that the folding patterns of the presumed biosynthetic precursor, 2-geranylgeranyl-6methylhydroquinone, are exible during biosynthesis leading to different classes of metabolites related to the taondiol group. [528][529][530][531] The isolation and characterisation of >20 acyclic meroditerpenoids from a collection of seven Australian brown algae (and one red alga) included ve new diterpenoids (747,748,(749)(750)(751).…”

Natural 6-hydroxy-chromanols and -chromenols: structural diversity, biosynthetic pathways and health implications

“…107 So far, the biosynthesis of mediterraneols is largely unknown. 106,108 Mediterraneols have been found to inhibit the mobility of sea urchin sperm and the mitotic cell division (ED 50 values of 2 mg ml À1 ) of fertilized urchin eggs. 106 Recently, cystophloroketal E (85), a meroditerpene with a 2,7dioxabicylo[3.2.1]octane core was isolated from Cystoseira tamariscifolia.…”

“…106 Recently, cystophloroketal E (85), a meroditerpene with a 2,7dioxabicylo[3.2.1]octane core was isolated from Cystoseira tamariscifolia. 108 The authors assumed that ketal formation was preceded by a Michael addition of phloroglucinol onto the unsaturated carbonyl of 4-methoxy-bifurcarenone. The compound showed anti-bacterial, anti-microalgal and antiinvertebrate activity (Table 3).…”

Minireview: algal natural compounds and extracts as antifoulants

“…A monocyclic meroditerpenoid isolated from Cystoseira tamariscifolia has shown high potential for inhibition of common foulers like Balanus amphitrite and Mytilus edulis . Cystophloroketals A and B from the same alga have shown high inhibition activity against two species of fouling microalgae and moderate antimicrobial activity against a range of bacteria, fungi, and microalgae (Hattab et al 2015 ). Along with the reported role of pholorotannins as antifoulants from certain fucoid species, certain metabolites like fucoxanthin and dimethylsulfoniopropionate (DMSP), which were previously considered to have just primary functions, have been reported to function as regulators of the microbial density and composition over Fucus vesiculosus (Lachnit et al 2013 ; Saha et al 2011 , 2012 , 2014 ).…”