An expedient total synthesis of (-)-dactylolide and formal synthesis of (-)-zampanolide

Synthetic efforts on the road to marine natural products bearing 4-O-2,3,4,6-tetrasubstituted THPs: an update

Scientific literature is inundated with secondary metabolites from marine sources. In this ocean of natural products, the presence of recurring patterns has traditionally led scientists to unravel the biosynthetic mechanisms that naturally yield these products, as well as to imitate Nature to prepare them in the laboratory, especially when promising bioactivities and stimulating molecular architectures are involucrate. For instance, natural products containing multisubstituted oxygenated rings and macrocyclic lactones are recurrently selected as targets for developing total syntheses. Thus, in the last decades a noteworthy number of synthetic works regarding miyakolide, madeirolide A and representative compounds of polycavernosides, lasonolides and clavosolides have come to fruition. Up to now, these families of macrolides are the only marine natural products bearing a tetrasubstituted tetrahydropyran ring with carbon substituents at positions 2, 3 and 6, as well as an oxygen at position 4. Their splendid structures have received the attention of the synthetic community, up to the point of starring in dozens of articles, and even some reviews. This work covers all the synthetic studies towards miyakolide and madeirolide A, as well as the synthetic efforts performed after the previous specialised reviews about lasonolide A, polycavernoside A and clavosolides, published in 2006, 2007 and 2016, respectively. In total, this review summarises 22 articles in which these marine natural products with 4-O-2,3,4,6-tetrasubstituted tetrahydropyrans have the leading role.

Stereoselective Total Synthesis of Aspergillide A: A Visible Light-Mediated Photoredox Access to the Trisubstituted Tetrahydropyran Core

A stereoselective total synthesis of natural product aspergillide A is reported. The adopted strategy relies on the direct access to the key tetrahydropyran core through a visible light-mediated photoredox reaction from an allylic alcohol and iodoacetic acid. In a single manipulation, a γ-iodo-δ-valerolactone is obtained through an atom transfer radical addition followed by in situ acid-catalyzed lactonization. The obtained lactone possesses three functionalized sites, which were seized to link the required substituents in the final product and thus completing the total synthesis of aspergillide A.

Synthesis, conformational preferences, and biological activity of conformational analogues of the microtubule-stabilizing agents, (-)-zampanolide and (-)-dactylolide

Zampanolide and dactylolide are microtubule-stabilizing polyketides possessing potent cytotoxicity towards a variety of cancer cell lines. Using our understanding of the conformational preferences of the macrolide core in both natural products, we hypothesized that analogues lacking the C17-methyl group would maintain the necessary conformation for bioactivity while reducing the number of synthetic manipulations necessary for their synthesis. Analogues 3, 4 and 5 were prepared via total synthesis, and their conformational preferences were determined through computational and high-field NMR studies. While no observable activities were present in dactylolide analogues 3 and 4, zampanolide analogue 5 exhibited sub-micromolar cytotoxicity. Herein, we describe these efforts towards understanding the structure- and conformation-activity relationships of dactylolide and zampanolide.

Zampanolides B-E from the Marine Sponge Cacospongia mycofijiensis: Potent Cytotoxic Macrolides with Microtubule-Stabilizing Activity

Four new compounds (2-5) structurally related to the microtubule-stabilizing agent (-)-zampanolide (1) have been isolated from the Tongan marine sponge Cacospongia mycofijiensis. Three of these new structures, zampanolides B-D (2-4), exhibit nanomolar cytotoxicity toward the HL-60 cell line, are antimitotic, and induce in vitro tubulin polymerization at levels comparable to 1. Zampanolide E (5), saturated at C-8/C-9, was significantly less potent and does not stabilize purified tubulin, even at 10-fold higher concentrations. The structural differences across these compounds reveal a plasticity of the zampanolide pharmacophore. While unsaturation is required at Δ⁸, the configuration of this alkene and those of Δ⁴ and Δ^4' have little effect on tubulin polymerization. The first natural co-occurrence of 1 and (-)-dactylolide (6) from the same sponge extract is also noted.

Intermolecular Allylic C-H Etherification of Internal Olefins

Herein we report on the development of an oxidative allylic C-H etherification reaction, utilizing internal olefins and alcohols as simple precursors. Key advances include the use of RhCp* complexes to promote the allylic C-H functionalization of internal olefins and the compatibility of the oxidative conditions with oxidatively sensitive alcohols, enabling the direct etherification reaction. Preliminary mechanistic studies, consistent with C-H functionalization as the rate determining step, are presented.

Asymmetric Bisvinylogous Aldolation of Aldehydes via 2-Oxonia-Cope Rearrangement Enabling Total Stereochemical Control

Highly stereoselective 2-oxonia-Cope rearrangement reactions between newly designed bisvinylogous aldolation synthons and aldehydes, which can provide ε-hydroxy-α,β,γ,δ-unsaturated esters with excellent enantioselectivities, as well as with unprecedented E- and Z-selectivities without regioselectivity issues, are described.

Recent advances in microtubule-stabilizing agents

Highly dynamic mitotic spindle microtubules are superb therapeutic targets for a group of chemically diverse and clinically successful anticancer drugs. Microtubule-targeted drugs disrupt microtubule dynamics in distinct ways, and they are primarily classified into two groups: microtubule destabilizing agents (MDAs), such as vinblastine, colchicine, and combretastatin-A4, and microtubule stabilizing agents (MSAs), such as paclitaxel and epothilones. Systematic discovery and development of new MSAs have been aided by extensive research on paclitaxel, yielding a large number of promising anticancer compounds. This review focuses on the natural sources, structural features, mechanisms of action, structure-activity relationship (SAR) and chemical synthesis of MSAs. These MSAs mainly include paclitaxel, taccalonolides, epothilones, FR182877 (cyclostreptin), dictyostatin, discodermolide, eleutherobin and sarcodictyins, zampanolide, dactylolide, laulimalides, peloruside and ceratamines from natural sources, as well as small molecular microtubule stabilizers obtained via chemical synthesis. Then we discuss the application prospect and development of these anticancer compounds.

Advances in the synthesis of glycosidic macrolides: clavosolides A-D and cyanolide A

Covering: 2005 to 2016Clavosolides A-D and cyanolide A are glycosidic macrolides and represent a new family of marine natural products. They possess a number of unusual structural features and have attracted considerable interest from the synthetic community. This review presents a comprehensive survey of all aspects of the clavosolides A-D and cyanolide A. Specific topics include isolation, structure determination, biological activity, and synthetic approaches.

Dictyoceratidan poisons: Defined mark on microtubule-tubulin dynamics

Tubulin/microtubule assembly and disassembly is characterized as one of the chief processes during cell growth and division. Hence drugs those perturb these process are considered to be effective in killing fast multiplying cancer cells. There is a collection of natural compounds which disturb microtubule/tubulin dis/assemblage and there have been a lot of efforts concerted in the marine realm too, to surveying such killer molecules. Close to half the natural compounds shooting out from marine invertebrates are generally with no traceable definite mechanisms of action though may be tough anti-cancerous hits at nanogram levels, hence fatefully those discoveries conclude therein without a capacity of translation from laboratory to pharmacy. Astoundingly at least 50% of natural compounds which have definite mechanisms of action causing disorders in tubulin/microtubule kinetics have an isolation history from sponges belonging to the Phylum: Porifera. Poriferans have always been a wonder worker to treat cancers with a choice of, yet precise targets on cancerous tissues. There is a specific order: Dictyoceratida within this Phylum which has contributed to yielding at least 50% of effective compounds possessing this unique mechanism of action mentioned above. However, not much notice is driven to Dictyoceratidans alongside the order: Demospongiae thus dictating the need to know its select microtubule/tubulin irritants since the unearthing of avarol in the year 1974 till date. Hence this review selectively pinpoints all the compounds, noteworthy derivatives and analogs stemming from order: Dictyoceratida focusing on the past, present and future.

Preparation of conjugated dienoates with Bestmann ylide: Towards the synthesis of zampanolide and dactylolide using a facile linchpin approach

Bestmann ylide [(triphenylphosphoranylidene)ketene] acts as a chemical linchpin that links nucleophilic entities, such as alcohols or amines, with carbonyl moieties to produce unsaturated esters and amides, respectively. In this work, the formation of α,β,γ,δ-unsaturated esters (dienoates) is achieved through the coupling of Bestmann ylide, an alcohol and an α,β-unsaturated aldehyde. Primary and secondary alcohols, including allylic alcohols, are suitable substrates; the newly formed alkene has an E-geometry. Strategically, this represents a highly efficient route to unsaturated polyketide derivatives. A linchpin approach to the synthesis of a major fragment of the natural products zampanolide and dactylolide is investigated using Bestmann ylide to link the C16-C20 alcohol with the C3-C8 aldehyde fragment.

Recent applications of the hetero Diels–Alder reaction in the total synthesis of natural products

The synthetic utility and potential power of the Diels–Alder (D–A) reaction in organic chemistry is evident.

Zampanolide and dactylolide: cytotoxic tubulin-assembly agents and promising anticancer leads

Zampanolide is a marine natural macrolide and a recent addition to the family of microtubule-stabilizing cytotoxic agents. Zampanolide exhibits unique effects on tubulin assembly and is more potent than paclitaxel against several multi-drug resistant cancer cell lines. A high-resolution crystal structure of αβ-tubulin in complex with zampanolide explains how taxane-site microtubule-stabilizing agents promote microtubule assemble and stability. This review provides an overview of current developments of zampanolide and its related but less potent analogue dactylolide, covering their natural sources and isolation, structure and conformation, cytotoxic potential, structure-activity studies, mechanism of action, and syntheses.

Macrocyclic drugs and synthetic methodologies toward macrocycles

Macrocyclic scaffolds are commonly found in bioactive natural products and pharmaceutical molecules. So far, a large number of macrocyclic natural products have been isolated and synthesized. The construction of macrocycles is generally considered as a crucial and challenging step in the synthesis of macrocyclic natural products. Over the last several decades, numerous efforts have been undertaken toward the synthesis of complex naturally occurring macrocycles and great progresses have been made to advance the field of total synthesis. The commonly used synthetic methodologies toward macrocyclization include macrolactonization, macrolactamization, transition metal-catalyzed cross coupling, ring-closing metathesis, and click reaction, among others. Selected recent examples of macrocyclic synthesis of natural products and druglike macrocycles with significant biological relevance are highlighted in each class. The primary goal of this review is to summarize currently used macrocyclic drugs, highlight the therapeutic potential of this underexplored drug class and outline the general synthetic methodologies for the synthesis of macrocycles.

Total Synthesis of Potent Antitumor Macrolide, (-)-Zampanolide: An Oxidative Intramolecular Cyclization-Based Strategy

A detailed account of the enantioselective total synthesis of (-)-zampanolide, a macrolide marine natural product with high anti-cancer activity is described. For synthesis of the 4-methylene tetrahydropyran unit of (-)-zampanolide, initially, we relied upon an oxidative C-H activation of an alkenyl ether and intramolecular cyclization to provide the substituted tetrahydropyran ring. However, this strategy was unsuccessful. Subsequently, we found that a cinnamyl ether is critical for the successful oxidative intramolecular cyclization reaction. The synthesis also features a cross metathesis reaction to construct a tri-substituted olefin, a ring-closing metathesis to form a highly functionalized macrolactone and a chiral phosphoric acid promoted N-acyl aminal formation to furnish (-)-zampanolide stereoselectively and in good yield. The synthetic (-)-zampanolide had effects on cultured cells and on tubulin assembly consistent with properties reported for the natural product.

Toward an enantioselective synthesis of (-)-zampanolide: preparation of the C9-C20 region

Progress toward the synthesis of the microtubule-stabilizing agent, (-)-zampanolide, is reported. Construction of the 2,6-cis-tetrahydropyran ring was accomplished utilizing ether transfer methodology in conjunction with an intramolecular radical cyclization reaction. Efficient installation of the C16-C20 side chain relied on a one-pot cross-metathesis/olefination sequence, Sharpless epoxidation, and selective reduction of a vinyl epoxide.

Synthesis of Functionalized 4-Methylenetetrahydropyrans by Oxidative Activation of Cinnamyl or Benzyl Ethers

Oxidative activation of benzyl or cinnamyl ether bearing allylsilane derivatives using a catalytic amount of DDQ and 2 equivalents of CAN in the presence of PPTS provided functionalized 4-methylenetetrahydropyrans in good yields and excellent diastereoselectivity. The reaction could be applied to the synthesis of a variety of substituted tetrahydropyran derivatives.

A formal synthesis of (-)-cyanolide A featuring a stereoselective Mukaiyama aldol reaction and oxocarbenium reduction

The formal synthesis of the marine natural product (-)-cyanolide A is presented. The synthetic strategy is centered on two acyclic diastereoselective reactions and a single cyclic reaction with modest to excellent dr based on an initial stereocenter. Most notable is a highly stereoselective oxocarbenium reduction based on a "mismatched" reactive conformer to afford the β-C-glycoside subunit leading to an efficient synthesis of the diolide aglycon in 12 overall steps.

Enantioselective total synthesis of (-)-zampanolide, a potent microtubule-stabilizing agent

An enantioselective total synthesis of zampanolide has been accomplished using a novel DDQ/Brønsted acid promoted cyclization as the key reaction. The synthesis features cross-metathesis to construct the trisubstituted olefin and a ring-closing metathesis to form the macrolactone. The final N-acyl aminal formation was stereoselectively accomplished by an organocatalytic reaction.

Ring strain-promoted allylic transposition of cyclic silyl ethers

Relief of the ring strain of medium-sized rings promotes a regioselective allylic transposition of a C-O bond when catalyzed by rhenium oxide. Through the allylic transposition, eight-membered cyclic silyl ethers undergo ring contraction to the corresponding six-membered siloxacycles.

The medicinal potential of promising marine macrolides with anticancer activity

Marine natural products have become a major source of new chemical entities in the discovery of potential anticancer agents that potently suppress various molecular targets. In particular, the marine macrolides, which include an array of novel biomolecules endowed with outstanding cytotoxic and/or antiproliferative activities, are a prominent class of marine natural products that offer continued promise for breakthroughs in anticancer research. Herein we highlight some recent studies of promising marine macrolides, paying particular attention to their discovery, anticancer activities, mechanisms of action, chemical synthesis, and representative analogues.

Microtubule-stabilizing drugs from marine sponges: focus on peloruside A and zampanolide

Marine sponges are an excellent source of bioactive secondary metabolites with potential therapeutic value in the treatment of diseases. One group of compounds of particular interest is the microtubule-stabilizing agents, the most well-known compound of this group being paclitaxel (Taxol), an anti-cancer compound isolated from the bark and leaves of the Pacific yew tree. This review focuses on two of the more recent additions to this important class of drugs, peloruside A and zampanolide, both isolated from marine sponges. Peloruside A was isolated from Mycale hentscheli collected in New Zealand coastal waters, and it already shows promising anti-cancer activity. Two other potent bioactive compounds with different modes of action but isolated from the same sponge, mycalamide A and pateamine, will also be discussed. The fourth compound, zampanolide, most recently isolated from the Tongan sponge Cacospongia mycofijiensis, has only recently been added to the microtubule-stabilizing group of compounds, and further work is in progress to determine its activity profile relative to peloruside A and other drugs of this class.

Total synthesis of (-)-zampanolide and questionable existence of (-)-dactylolide as the elusive biosynthetic precursor of (-)-zampanolide in an Okinawan sponge

A new and concise total synthesis of (-)-zampanolide, (-)-1, and (-)-dactylolide, (-)-2, is described. Synthetic highlights include (i) a mild Horner-Wadsworth-Emmons reaction providing the seco acid, (ii) an unusual stepwise cross-coupling reaction of a 1,1-dibromodiene with inversion of olefin geometry, and (iii) specific O-Michael reaction conditions using catalytic LHMDS with TMEDA for the synthesis of functionalized 2,6-cis-tetrahydropyran. A marine sponge extract was analyzed for the presence of (-)-2 as the biosynthetic precursor of (-)-zampanolide.

An efficient formal synthesis of (-)-clavosolide a featuring a "mismatched" stereoselective oxocarbenium reduction

The enantioselective formal synthesis of the polyketide marine natural product (-)-clavosolide A is presented. The construction of the beta-C-glycoside subunit is highlighted by a one-pot oxocarbenium cation formation/reduction sequence. Yamaguchi dimerization afforded the diolide aglycon in 18 steps (longest linear sequence).

Studies Toward the Synthesis of (-)-Zampanolide: Preparation of the Macrocyclic Core

Studies towards the synthesis of the macrocyclic core of (-)-zampanolide are reported. The synthetic approach features a one-pot reduction/vinylogous aldol reaction for construction of the C15-C20 fragment, an intramolecular silyl-modified Sakurai (ISMS) reaction for construction of the 2,6-cis-disubstituted exo-methylene pyran subunit, and use of an sp2-sp3 Stille reaction for macrocyclization.

Total synthesis of (-)-dactylolide and formal synthesis of (-)-zampanolide via target oriented beta-C-glycoside formation

The total synthesis of (-)-dactylolide and formal synthesis of (-)-zampanolide via target oriented beta-C-glycoside formation is described. The two key reactions involved a stereoselective reduction of the appropriate oxocarbenium cation and a highly chemo- and diastereoselective ring-closing metathesis protocol for the formation of the macrocyclic core. In addition to the described chemistry, in vitro screening of the antipode of natural dactylolide against the NCI's 60 cancer cell line helped to illuminate the critical importance of the N-acyl hemiaminal side chain of natural zampanolide for its reported potent nanomolar cytotoxicities. Furthermore, by means of the in vitro screen of (-)-dactylolide, a promising cancer therapeutic lead has now emerged for a variety of carcinomas. More specifically, (-)-dactylolide exhibited GI50 values in the nanomolar (25-99 ng/mL) range against the four cell lines HL-60, K-562, HCC-2998, and SF-539, while displaying modest LC50 values.