Unveiling the Chemical Diversity of the Deep-Sea Sponge Characella pachastrelloides

Sponges are at the forefront of marine natural product research. In the deep sea, extreme conditions have driven secondary metabolite pathway evolution such that we might expect deep-sea sponges to yield a broad range of unique natural products. Here, we investigate the chemodiversity of a deep-sea tetractinellid sponge, Characella pachastrelloides, collected from ~800 m depth in Irish waters. First, we analyzed the MS/MS data obtained from fractions of this sponge on the GNPS public online platform to guide our exploration of its chemodiversity. Novel glycolipopeptides named characellides were previously isolated from the sponge and herein cyanocobalamin, a manufactured form of vitamin B12, not previously found in nature, was isolated in a large amount. We also identified several poecillastrins from the molecular network, a class of polyketide known to exhibit cytotoxicity. Light sensitivity prevented the isolation and characterization of these polyketides, but their presence was confirmed by characteristic NMR and MS signals. Finally, we isolated the new betaine 6-methylhercynine, which contains a unique methylation at C-2 of the imidazole ring. This compound showed potent cytotoxicity towards against HeLa (cervical cancer) cells.

Advances in Biosynthesis of Natural Products from Marine Microorganisms

Natural products play an important role in drug development, among which marine natural products are an underexplored resource. This review summarizes recent developments in marine natural product research, with an emphasis on compound discovery and production methods. Traditionally, novel compounds with useful biological activities have been identified through the chromatographic separation of crude extracts. New genome sequencing and bioinformatics technologies have enabled the identification of natural product biosynthetic gene clusters in marine microbes that are difficult to culture. Subsequently, heterologous expression and combinatorial biosynthesis have been used to produce natural products and their analogs. This review examines recent examples of such new strategies and technologies for the development of marine natural products.

Developing a scalable synthesis of Peloruside A

<p>Peloruside A (PelA, 1) is a marine natural product isolated from a sponge Mycale hentscheli found in Pelorus Sound, New Zealand. It is a microtubule-stabilising agent, active against various cancerous cell lines at nanomolar concentrations and offers several advantages over the current drugs on the market due to its unique mode of microtubule stabilisation, its potency and its activity in multidrug resistant cells. Since large-scale isolation of the compound from the sponge is unsustainable and an attempt to grow the sponge failed due to a sea-slug infestation, devising an efficient synthesis of peloruside A that will be able to deliver larger quantities of this compound is essential in order to conduct further studies and enable the eventual manufacture of the drug. Peloruside A is also a very interesting synthetic target as a macrolide with ten stereogenic centres, an internal pyran ring and a trisubstituted Z-double bond. Our synthetic strategy combines elements from previous total and partial syntheses with novel elements with an aim to make the synthesis more efficient. The synthesis of the side-chain fragment (C12–C20) was based on Evans' methodology1 which was also utilised to couple this fragment with the C8–C11 fragments. It was envisioned to evaluate two different end-game strategies, and to this end it was necessary to synthesise two different versions of the C8–C11 fragment. However, the synthesis of the C1–C7 fragments proved to be quite challenging and required a lot of alterations to the synthetic plan and the protecting group strategy. Various routes based on previous syntheses by Ghosh, Jacobsen and Taylor were explored.2–4 Eventually, the key intermediate was synthesised using a modified Taylor methodology. Our future work will focus on optimising and establishing fragment coupling methodologies and evaluating the two end-game approaches: macrolactonisation and a ring-closing metathesis.</p>

Developing a scalable synthesis of Peloruside A

<p>Peloruside A (PelA, 1) is a marine natural product isolated from a sponge Mycale hentscheli found in Pelorus Sound, New Zealand. It is a microtubule-stabilising agent, active against various cancerous cell lines at nanomolar concentrations and offers several advantages over the current drugs on the market due to its unique mode of microtubule stabilisation, its potency and its activity in multidrug resistant cells. Since large-scale isolation of the compound from the sponge is unsustainable and an attempt to grow the sponge failed due to a sea-slug infestation, devising an efficient synthesis of peloruside A that will be able to deliver larger quantities of this compound is essential in order to conduct further studies and enable the eventual manufacture of the drug. Peloruside A is also a very interesting synthetic target as a macrolide with ten stereogenic centres, an internal pyran ring and a trisubstituted Z-double bond. Our synthetic strategy combines elements from previous total and partial syntheses with novel elements with an aim to make the synthesis more efficient. The synthesis of the side-chain fragment (C12–C20) was based on Evans' methodology1 which was also utilised to couple this fragment with the C8–C11 fragments. It was envisioned to evaluate two different end-game strategies, and to this end it was necessary to synthesise two different versions of the C8–C11 fragment. However, the synthesis of the C1–C7 fragments proved to be quite challenging and required a lot of alterations to the synthetic plan and the protecting group strategy. Various routes based on previous syntheses by Ghosh, Jacobsen and Taylor were explored.2–4 Eventually, the key intermediate was synthesised using a modified Taylor methodology. Our future work will focus on optimising and establishing fragment coupling methodologies and evaluating the two end-game approaches: macrolactonisation and a ring-closing metathesis.</p>

New Halogenated Secondary Metabolites from Red Algae of the South Pacific

<p>An NMR- and MS-directed study led to the isolation and structure elucidation of several halogenated secondary metabolites from a New Zealand and a Tongan red alga. An extensive investigation was carried out on the New Zealand red alga Rhodophyllis membranacea following mass spectrometric evidence for an unusual tetrahalogenated indole with the exceptionally rare inclusion of bromine, chlorine and iodine within a fraction of a semi-purified extract. Due to the difficulty associated with the structure elucidation of proton deficient molecules, a strategic isolation and structure elucidation of several polyhalogenated indoles was employed in order to unequivocally assign the halogen positions on the indolic core. This resulted in the isolation and characterisation of 11 new tetrahalogenated indoles (123–133), four of which contain bromine, chlorine and iodine (124 and 129–131) and represent the first isolation of such compounds. Additionally, four new pentahalogenated indoles (134–137) and an uncharacterised tribromotrichloroindole were isolated. The synthetically known compound 4-chloroisatin (138) was isolated as a new marine natural product, while 4-chloro-3-hydroxyl-3-(2-oxopropyl)-2-oxindole (139) was established to be an artefact of isolation. Several compounds were found to exhibit antifungal properties against Saccharomyces cerevisiae. A detailed examination of the Tongan alga Callophycus serratus led to the isolation of six new meroditerpenoids: callophycol C (227), iodocallophycols E (228) and F (229), iodocallophycoic acid B (230), deiodocallophycoic B (231) and callophycoic acid I (232). The relative configurations in compounds 228–231 are proposed to differ from closely related compounds in the literature. Iodocallophycol E (228) exhibited moderate cytotoxicity against the HL-60 cell line with an IC50 value of 6.0 μM.</p>

New Halogenated Secondary Metabolites from Red Algae of the South Pacific

<p>An NMR- and MS-directed study led to the isolation and structure elucidation of several halogenated secondary metabolites from a New Zealand and a Tongan red alga. An extensive investigation was carried out on the New Zealand red alga Rhodophyllis membranacea following mass spectrometric evidence for an unusual tetrahalogenated indole with the exceptionally rare inclusion of bromine, chlorine and iodine within a fraction of a semi-purified extract. Due to the difficulty associated with the structure elucidation of proton deficient molecules, a strategic isolation and structure elucidation of several polyhalogenated indoles was employed in order to unequivocally assign the halogen positions on the indolic core. This resulted in the isolation and characterisation of 11 new tetrahalogenated indoles (123–133), four of which contain bromine, chlorine and iodine (124 and 129–131) and represent the first isolation of such compounds. Additionally, four new pentahalogenated indoles (134–137) and an uncharacterised tribromotrichloroindole were isolated. The synthetically known compound 4-chloroisatin (138) was isolated as a new marine natural product, while 4-chloro-3-hydroxyl-3-(2-oxopropyl)-2-oxindole (139) was established to be an artefact of isolation. Several compounds were found to exhibit antifungal properties against Saccharomyces cerevisiae. A detailed examination of the Tongan alga Callophycus serratus led to the isolation of six new meroditerpenoids: callophycol C (227), iodocallophycols E (228) and F (229), iodocallophycoic acid B (230), deiodocallophycoic B (231) and callophycoic acid I (232). The relative configurations in compounds 228–231 are proposed to differ from closely related compounds in the literature. Iodocallophycol E (228) exhibited moderate cytotoxicity against the HL-60 cell line with an IC50 value of 6.0 μM.</p>

Toward the Synthesis of Rimarikiamide A

<p>Rimarikiamide A is a linear diterpenoid marine natural product featuring an unusual taurine structural moiety. Rimarikiamide A was isolated from the sea sponge Latrunculia brevis found in the Rimariki Islands in northern New Zealand, and was shown to elicit low μM cytotoxicity against HL-60 cells. Unfortunately, only 400 μg of rimarikiamide A was isolated from 700 g of sea sponge, making the characterisation of the two chiral centres impossible by spectroscopic means. To explore the full potential of rimarikiamide A as a therapeutic agent, the molecule must be synthesised in a stereoselective manner or from a starting material of known stereochemistry, fully characterised, and tested for biological activity. A Barbier coupling of two terpene units is required early in the proposed synthesis of Rimarikiamide A, and previous attempts have failed to generate the desired linear product. In this work, the suitability of a titanium-mediated Barbier coupling was investigated, and proved successful for the generation of the desired linear product.</p>

Toward the Synthesis of Rimarikiamide A

<p>Rimarikiamide A is a linear diterpenoid marine natural product featuring an unusual taurine structural moiety. Rimarikiamide A was isolated from the sea sponge Latrunculia brevis found in the Rimariki Islands in northern New Zealand, and was shown to elicit low μM cytotoxicity against HL-60 cells. Unfortunately, only 400 μg of rimarikiamide A was isolated from 700 g of sea sponge, making the characterisation of the two chiral centres impossible by spectroscopic means. To explore the full potential of rimarikiamide A as a therapeutic agent, the molecule must be synthesised in a stereoselective manner or from a starting material of known stereochemistry, fully characterised, and tested for biological activity. A Barbier coupling of two terpene units is required early in the proposed synthesis of Rimarikiamide A, and previous attempts have failed to generate the desired linear product. In this work, the suitability of a titanium-mediated Barbier coupling was investigated, and proved successful for the generation of the desired linear product.</p>

Studies on the Synthesis of Marine Natural Product (-)-Zampanolide and its Analogues

<p>(-)-Zampanolide is a microtubule-stabilising marine natural product, with promise as a cancer drug candidate. The potential therapeutic application of zampanolide has fuelled worldwide interest in its total synthesis, but few analogue studies have been reported. Analogues afford the possibility of examining the structure-activity relationships with a view to optimising for potency and medicinal viability. This project seeks to devise a new route to zampanolide and generate a series of analogues for bioactivity evaluation. The initial approach to zampanolide and a number of designed analogues was through disconnections at C20 by an N-aldol reaction, at C1 by Yamaguchi esterification, at C8-C9 by metathesis and at C15-C16 by alkynylation. During the development of fragment syntheses, problems were encountered with protection of the secondary hydroxyl group at C19 and establishment of an aldehyde at C15. Useful natural and analogue fragments were generated during this exploratory phase. The order of connections was revised, and effort has been put towards the improvement of the synthetic efficiency. A three-component reaction involving (triphenylphosphoranylidene)-ketene, also known as Bestmann ylide, as a linchpin was envisaged to provide the dienoate of zampanolide. This is an expanded application of Bestmann ylide and therefore the scope of this linchpin reaction was investigated using simple alcohols and aldehydes. Success in the scoping study fortified this approach, and the coupling of the C3-C8 and C16-C20 fragments of zampanolide proceeded with good yields and stereoselectivity of the E,Z-geometry. The planned late stage connections were tested on model substrates. The side arm attachment by a chiral boron reagent-promoted aza-aldol reaction failed to produce desired product on a simple model. However, model substrates that better account for the functionality of the zampanolide macrocycle are proposed for subsequent studies. In case these also do not succeed, reliable alternative methods described in the literature would be used. Several methods were scanned for the asymmetric alkynylation required for the C15-C16 bond connection. That involving ProPhenol and diethylzinc produced an excellent yield with a model alkyne. Although the stereoselectivity of the alkynylation is yet to be optimized, it was also tested on the full zampanolide fragment generated from the Bestmann ylide reaction. A small amount of the desired product was isolated, establishing 16 out of the 18 carbons of the macrocycle. Formation of a macrocycle is close at hand.</p>