Total synthesis and structure revision of the marine metabolite palmerolide A

Bioinformatic and Mechanistic Analysis of the Palmerolide PKS-NRPS Biosynthetic Pathway From the Microbiome of an Antarctic Ascidian

Complex interactions exist between microbiomes and their hosts. Increasingly, defensive metabolites that have been attributed to host biosynthetic capability are now being recognized as products of host-associated microbes. These unique metabolites often have bioactivity targets in human disease and can be purposed as pharmaceuticals. Polyketides are a complex family of natural products that often serve as defensive metabolites for competitive or pro-survival purposes for the producing organism, while demonstrating bioactivity in human diseases as cholesterol lowering agents, anti-infectives, and anti-tumor agents. Marine invertebrates and microbes are a rich source of polyketides. Palmerolide A, a polyketide isolated from the Antarctic ascidian Synoicum adareanum, is a vacuolar-ATPase inhibitor with potent bioactivity against melanoma cell lines. The biosynthetic gene clusters (BGCs) responsible for production of secondary metabolites are encoded in the genomes of the producers as discrete genomic elements. A candidate palmerolide BGC was identified from a S. adareanum microbiome-metagenome based on a high degree of congruence with a chemical structure-based retrobiosynthetic prediction. Protein family homology analysis, conserved domain searches, active site and motif identification were used to identify and propose the function of the ∼75 kbp trans-acyltransferase (AT) polyketide synthase-non-ribosomal synthase (PKS-NRPS) domains responsible for the stepwise synthesis of palmerolide A. Though PKS systems often act in a predictable co-linear sequence, this BGC includes multiple trans-acting enzymatic domains, a non-canonical condensation termination domain, a bacterial luciferase-like monooxygenase (LLM), and is found in multiple copies within the metagenome-assembled genome (MAG). Detailed inspection of the five highly similar pal BGC copies suggests the potential for biosynthesis of other members of the palmerolide chemical family. This is the first delineation of a biosynthetic gene cluster from an Antarctic microbial species, recently proposed as Candidatus Synoicihabitans palmerolidicus. These findings have relevance for fundamental knowledge of PKS combinatorial biosynthesis and could enhance drug development efforts of palmerolide A through heterologous gene expression.

Marine-Derived Macrolides 1990-2020: An Overview of Chemical and Biological Diversity

Macrolides are a significant family of natural products with diverse structures and bioactivities. Considerable effort has been made in recent decades to isolate additional macrolides and characterize their chemical and bioactive properties. The majority of macrolides are obtained from marine organisms, including sponges, marine microorganisms and zooplankton, cnidarians, mollusks, red algae, bryozoans, and tunicates. Sponges, fungi and dinoflagellates are the main producers of macrolides. Marine macrolides possess a wide range of bioactive properties including cytotoxic, antibacterial, antifungal, antimitotic, antiviral, and other activities. Cytotoxicity is their most significant property, highlighting that marine macrolides still encompass many potential antitumor drug leads. This extensive review details the chemical and biological diversity of 505 macrolides derived from marine organisms which have been reported from 1990 to 2020.

Heck macrocyclization in natural product total synthesis

Covering: 1981-2020 Heck macrocyclization is a logical extension of the award-winning Mizoroki-Heck reaction. Through covalent linking of two otherwise discrete coupling partners, the resultant chimeric substrate is transformed into a large ring with enhanced rigidity and unique functional group disposition. Pioneered in the early 1980s, this methodology has evolved into a competent option for creating diverse macrocycles. Despite its growing influence, hitherto no systematic survey has ever appeared in the literature. The present review delineates the state-of-the-art of Heck macrocyclization in the context of natural product synthesis. Sixteen selected cases, each examined from a different perspective, coalesce into the view that the title reaction is a viable tool for synthesis-enabled macrocycle research.

Structure determination, correction, and disproof of marine macrolide natural products by chemical synthesis

Integration of chemical synthesis, NMR spectroscopy, and various analytical means is key to success in the structure elucidation of stereochemically complex marine macrolide natural products.

Development of α,α-Disubstituted Crotylboronate Reagents and Stereoselective Crotylation via Brønsted or Lewis Acid Catalysis

The development of α,α-disubstituted crotylboronate reagents is reported. Chiral Brønsted acid-catalyzed asymmetric aldehyde addition with the developed E-crotylboron reagent gave (E)-anti-1,2-oxaborinan-3-enes with excellent enantioselectivities and E-selectivities. With BF₃·OEt₂ catalysis, the stereoselectivity is reversed, and (Z)-δ-boryl-anti-homoallylic alcohols are obtained with excellent Z-selectivities from the same E-crotylboron reagent. The Z-crotylboron reagent also participates in BF₃·OEt₂-catalyzed crotylation to furnish (Z)-δ-boryl-syn-homoallylic alcohols with good Z-selectivities. DFT computations establish the origins of observed enantio- and stereoselectivities of chiral Brønsted acid-catalyzed asymmetric allylation. Stereochemical models for BF₃·OEt₂-catalyzed reactions are proposed to rationalize the Z-selective allyl additions. These reactions generate highly valuable homoallylic alcohol products with a stereodefined trisubstituted alkene unit. The synthetic utility is further demonstrated by the total syntheses of salinipyrones A and B.

Synthetic studies on palmerolide C: synthesis of an advanced intermediate towards the revised structure of palmerolide C

A stereoselective synthesis of the highly advanced intermediates towards the revised structure of palmerolide C and 10-epi-palmerolide C is described in this paper. The required key fragments C1-C6, C7-C14 and C15-C23 have been successfully assembled in a convergent manner to access the C1-C23 framework bearing all the five stereocenters present in the natural product. The synthesis involves the Julia-Kocienski reaction, Yamaguchi esterification, Takai olefination and regioselective epoxide opening as key steps. The proposed route is flexible and could also be applied to the synthesis of structurally related palmerolides.

Total Synthesis, Stereochemical Revision, and Biological Assessment of Iriomoteolide-2a

Iriomoteolide-2a is a marine macrolide metabolite isolated from a cultured broth of the benthic dinoflagellate Amphidinium sp. HYA024 strain. This naturally occurring substance was reported to show remarkable cytotoxic activity against human cancer cell lines HeLa and DG-75 and in vivo antitumor activity against murine leukemia P388 cell line. Herein, the total synthesis, stereochemical revision, and biological assessment of iriomoteolide-2a are reported in detail. Total synthesis of the proposed structure 1 of iriomoteolide-2a featured a late-stage convergent assembly of three components by a Suzuki-Miyaura coupling, an esterification, and a ring-closing metathesis. However, the NMR data of synthetic 1 were not identical to those of the natural product. Careful analysis of the NMR data of the authentic material and synthesis/NMR analysis of appropriately designed model compounds led to consideration of four possible stereoisomers 2-5 as candidates for the correct structure. Accordingly, total syntheses of 2-5 were achieved by taking advantage of the convergent strategy, and comparison of the NMR spectra of synthetic 2-5 with those of the natural product led to the conclusion that 5 shows the correct relative configuration of iriomoteolide-2a. The absolute configuration of this natural product was finally established through chiral HPLC analysis of synthetic 5/ent-5 with the authentic sample. The antiproliferative activity of the synthetic compounds was assessed against HeLa and A549 cells to show that, in contrast to expectation, synthetic 5 and ent-5 were only marginally active in these cell lines. This work clearly underscores the vital role of total synthesis in the establishment of the structure and biological activity of natural products.

Remote Asymmetric Induction Reactions using a E, E-Vinylketene Silyl N, O-Acetal and the Wide Range Stereocontrol Strategy for the Synthesis of Polypropionates

The construction of libraries of acyclic polyketides remains a challenging topic, mostly due to the difficulties associated with finding the right balance between diversity and brevity for the synthetic routes leading to polyketides. Recently, relatively short methods have been developed and applied to the synthesis of natural products. However, these short routes often suffer from limited diversity with respect to the arrangement of functional groups and stereochemistry, as these usually require reactions that direct multiple functional groups simultaneously in one step. Therefore, methods that combine a small number of reaction steps with structural diversity remain an attractive research target for the construction of acyclic polyketide libraries. In 2004, we reported a remote asymmetric induction reaction using chiral vinylketene silyl N, O-acetal 1, which is commensurate to an anti-selective vinylogous Mukaiyama aldol reaction. Ever since, this reaction has been applied to the synthesis of numerous natural products, as this synthetic route is short and efficient on account of the simultaneous introduction of both asymmetric centers and the multiply functionalized carbon chain. Recently, we have developed a variety of this remote asymmetric induction reaction based on the E, E-vinylketene N, O-acetal 1, which includes syn-selective vinylogous Mukaiyama aldol reactions, as well as alkylation, acylation, and bromination reaction. These reactions provide polypropionates in a highly stereoselective manner. The proposed transition states of these reactions are discussed in this Account. Additionally, we have developed a new short synthesis of polypropionates by combining reactions for the remote asymmetric induction and the functionalization of double bonds (wide-range stereocontrol, WRS). The remote asymmetric induction reaction simultaneously constructs the stereogenic centers at the central part of the products and introduces the α,β-unsaturated imide, while the new strategy is based on the initial construction of the central part of the molecule and a subsequent functionalization of the surroundings (WRS). This strategy successfully furnished stereoisomers in a few steps, and the stereodivergent synthesis of 2,4,6-trimethyloctanoic acid derivatives was accomplished. This strategy should also be feasible to construct an acyclic polyketide library. Moreover, we applied this method to the concise synthesis of natural products. In this Account, the development of remote asymmetric induction reactions and the new WRS strategy are described. Applications of the WRS strategy as well as reactions for the stereodivergent synthesis of polypropionates and natural products are also described. The aforementioned acyclic polyketide library should be constructed in the future with the help of the WRS strategy and become a powerful tool in drug discovery.

Secondary Metabolites from Polar Organisms

Polar organisms have been found to develop unique defences against the extreme environment environment, leading to the biosynthesis of novel molecules with diverse bioactivities. This review covers the 219 novel natural products described since 2001, from the Arctic and the Antarctic microoganisms, lichen, moss and marine faunas. The structures of the new compounds and details of the source organism, along with any relevant biological activities are presented. Where reported, synthetic and biosynthetic studies on the polar metabolites have also been included.

Cold-water marine natural products, 2006 to 2016

This is an update report on marine natural products isolated from cold-water organisms in the last decade, following the previous review that covered the literature up to 2005. Emphasis is on structural assignments and biological activity.

Direct asymmetric syntheses of chiral aldehydes and ketones via N-acyl chiral auxiliary derivatives including chiral Weinreb amide equivalents

This article covers N-acyl chiral auxiliary-based approaches to the asymmetric synthesis of enantiopure aldehydes and ketones. The use of diastereoisomerically pure N-acyl derivatives of chiral auxiliaries (including chiral Weinreb amide equivalents) and their conversion to the corresponding enantiopure aldehydes and ketones in a single synthetic operation by treatment with a hydride reducing agent or an organometallic reagent, respectively, are highlighted.

The vinylogous Mukaiyama aldol reaction (VMAR) in natural product synthesis

This review will provide an overview on the recent developments of polyketide synthesis using the vinylogous Mukaiyama aldol reaction for the construction of advanced intermediates. In general, four different motifs can be constructed efficiently using the recent developments of asymmetric variants of this strategy.

The impact of the Mukaiyama aldol reaction in total synthesis

Four decades since Mukaiyama's first reports on the successful application of silicon and boron enolates in directed aldol reactions, the ability of this highly controlled carbon-carbon bond-forming method to simultaneously define stereochemistry, introduce complexity, and construct the carbon skeleton with a characteristic 1,3-oxygenation pattern has made it a powerful tool for natural product synthesis. This Minireview highlights a number of representative total syntheses that demonstrate the impact of the Mukaiyama aldol reaction and discusses the underlying mechanistic rationale that determines the stereochemical outcomes.

Marine natural products

This review covers the literature published in 2011 for marine natural products, with 870 citations (558 for the period January to December 2011) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1152 for 2011), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Ivorenolide A, an unprecedented immunosuppressive macrolide from Khaya ivorensis: structural elucidation and bioinspired total synthesis

Ivorenolide A (1), a novel 18-membered macrolide featuring conjugated acetylenic bonds and five chiral centers, was isolated from Khaya ivorensis. The structure of 1 was fully determined by spectroscopic analysis, single-crystal X-ray diffraction, and bioinspired total synthesis. Both compound 1 and its synthetic enantiomer 2 showed potent and selective immunosuppressive activity.

Bioactive substances with anti-neoplastic efficacy from marine invertebrates: Bryozoa, Mollusca, Echinodermata and Urochordata

The marine environment provides a rich source of natural products with potential therapeutic application. This has resulted in an increased rate of pharmaceutical agents being discovered in marine animals, particularly invertebrates. Our objective is to summarize the most promising compounds which have the best potential and may lead to use in clinical practice, show their biological activities and highlight the compounds currently being tested in clinical trials. In this paper, we focused on Bryozoa, Mollusca, Echinodermata and Urochordata.