Total synthesis and stereochemical assignment of the salicylate antitumor macrolide lobatamide C(1)

Total Synthesis of Lobatamides A and C

The total synthesis of lobatamides A (1 a) and C (1 c) via a common bislactone intermediate is reported. The allylic aryl moiety including a trisubstituted Z-olefin was constructed by hydroboration of a 1,1-disubstituted allene and subsequent Migita-Kosugi-Stille coupling. Although the seco acid proved to be highly unstable even in the presence of weak bases, Zhao macrolactonization under acidic conditions via the α-acyloxyenamide successfully provided the common bislactone intermediate. Hydrozirconation-iodination of the terminal alkyne and subsequent copper-mediated coupling with primary amides proceeded successfully in the presence of the sensitive bislactone framework. The developed synthetic route enables the late-stage installation of enamide side chains, which are crucial structures for V-ATPase inhibition.

Marine Natural Products from Flora and Fauna of the Western Australian Coast: Taxonomy, Isolation and Biological Activity

Marine natural products occurring along the Western Australian coastline are the focus of this review. Western Australia covers one-third of the Australian coast, from tropical waters in the far north of the state to cooler temperate and Antarctic waters in the south. Over 40 years of research has resulted in the identification of a number of different types of secondary metabolites including terpenoids, alkaloids, polyketides, fatty acid derivatives, peptides and arsenic-containing natural products. Many of these compounds have been reported to display a variety of bioactivities. A description of the compound classes and their associated bioactivities from marine organisms found along the Western Australian coastline is presented.

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.

Pd-catalyzed C–N coupling of vinylbromides and sulfonimidamides: a facile synthesis of N′-vinylsulfonimidamides

N′-Vinyl sulfonimidamides were synthesized through Pd-catalyzed C–N coupling; the product's utility were further demonstrated by preparing N′-alkyl sulfonimidamides through hydrogenation.

Ester coupling reactions – an enduring challenge in the chemical synthesis of bioactive natural products

In this review we investigate the use of complex ester fragment couplings within natural product total syntheses. Using examples from the literature up to 2014 we illustrate the state-of-the-art as well as the challenges within this area of organic synthesis.

Cu-mediated enamide formation in the total synthesis of complex peptide natural products

Cu-mediated C(sp(2))-N bond formation has received intense interest recently, and has been applied to the total synthesis of a wide variety of structurally complex natural products. This review covers the synthetic assembly of peptide natural products in which Cu-mediated enamide formation is the key transformation. The total syntheses of cyclopeptide alkaloids, pacidamycin D, and yaku'amide A exemplify the versatility of the Cu-catalyzed cross-coupling reaction in comparison to other synthetic methods.

Importance and synthesis of benzannulated medium-sized and macrocyclic rings (BMRs)

Cyclic molecular frameworks, especially the benzannulated medium-sized and macrocyclic ring (BMR) systems, constitute an integral component of a large number of biologically significant natural or synthetic molecules.

Palladium-catalyzed 1,4-difunctionalization of butadiene to form skipped polyenes

A palladium-catalyzed 1,4-addition across the commodity chemical 1,3-butadiene to afford skipped polyene products is reported. Through a palladium σ → π → σ allyl isomerization, two new carbon-carbon bonds are formed with high regioselectivity and trans stereoselectivity of the newly formed alkene. The utility of this method is highlighted by the successful synthesis of the ripostatin A skipped triene core.

Direct C-H functionalization of enamides and enecarbamates by using visible-light photoredox catalysis

Direct C-H functionalization of various enamides and enecarbamates was realized through visible-light photoredox catalyzed reactions. Under the optimized conditions using [Ir(ppy)(2)(dtbbpy)PF(6)] as photocatalyst in combination with Na(2)HPO(4), enamides such as N-vinylpyrrolidinone could be easily functionalized by irradiation of the reaction mixture overnight in acetonitrile with visible light. The scope of the reaction with respect to enamide and enecarbamate substrates by using diethyl 2-bromomalonate for the alkylation reaction was explored, followed by an investigation of the scope of alkylating reagents used to react with the enamides and enecarbamates. The results indicated that reaction takes place with quite broad substrate scope, however, tertiary enamides with an internal C=C double bond in the E configuration could not be alkylated. Alkylation of N-vinyl tertiary enamides and enecarbamates gave monoalkylated products exclusively in the E configuration. Alkylation of N-vinyl secondary enamides gave doubly alkylated products. Double bond migration was observed in the reaction of electron-deficient bromides such as 3-bromoacetyl acetate with N-vinylpyrrolidinone. A mechanism is proposed for the reaction that is different from reported reactions of SOMOphiles with a nonfunctionalized C=C double bond. Further tests on the trifluoromethylation and arylation of enamides and enecarbamates under similar conditions showed that the reactions could serve as a mild, practical, and environmentally friendly approach to various functionalized enamides and enecarbamates.

Evolution of a protecting-group-free total synthesis: studies en route to the neuroactive marine macrolide (-)-palmyrolide A

A full account of our synthetic work toward the first total synthesis of the neuroactive marine macrolide (-)-palmyrolide A is described. Our first-generation approach aimed to unlock the unknown C(5)-C(7) stereochemical relationship via the synthesis of four diastereomers of palmyrolide A aldehyde, a known degradation product. When these efforts provided inconclusive results, recourse to synthesizing all possible stereocombinations of the 15-membered macrolide was undertaken. These studies were critical in confirming the absolute stereochemistry, yielding the first total synthesis of (+)-ent-palmyrolide A. Subsequent to this work, the first protecting-group-free total synthesis of natural (-)-palmyrolide A is also reported.

Total syntheses of chelidonine and norchelidonine via an enamide-benzyne-[2 + 2] cycloaddition cascade

Total syntheses of chelidonine and norchelidonine featuring an enamide-benzyne-[2 + 2] cycloaddition initiated cascade is described. The cascade includes a pericyclic ring-opening and intramolecular Diels-Alder reaction.

Asymmetric total synthesis and absolute stereochemistry of the neuroactive marine macrolide palmyrolide A

The first asymmetric total synthesis and determination of the absolute configuration for the neuroactive marine macrolide palmyrolide A is described. The highlight of the synthesis is macrocyclization via trans-enamide formation catalyzed by copper(I) iodide and cesium carbonate. Comparison with the authentic spectral data confirms the synthesis of (+)-ent-palmyrolide A.

Studies on the Synthesis of Reidispongiolide A: Stereoselective Synthesis of the C(22)-C(36) Fragment

A highly stereoselective synthesis of the C(22)-C(36) fragment 2 of reidispongiolide A is described. This synthesis features the highly stereoselective mismatched double asymmetric crotylboration reaction of the aldehyde derived from 5 and the new chiral reagent (S)-(E)-7 that provides 12 with >15:1 d.r. Subsequent coupling of the derived vinyl iodide 3 with aldehyde 16 provided allylic alcohol 17, that was elaborated by three steps into the targeted reidispongiolide fragment 2.

Total synthesis of neopeltolide and analogs

Neopeltolide, a potent cytotoxin from a Carribean sponge, was synthesized through a brief sequence that highlights the use of ethers as oxocarbenium ion precursors. Other key steps include an acid-mediated etherification and sequence that features a Sonogashira reaction, an intramolecular alkyne hydrosilylation reaction, and a Tamao oxidation. The alkene that is required for the oxidative cyclization can be hydrogenated to provide access to the natural product or an epimer, or can be epoxidized or dihydroxylated to form polar analogs.

Enamide-benzyne-[2 + 2] cycloaddition: stereoselective tandem [2 + 2]-pericyclic ring-opening-intramolecular N-tethered [4 + 2] cycloadditions

Benzyne-[2 + 2] cycloadditions with enamides are described. This effort led to the development of a highly stereoselective tandem [2 + 2] cycloaddition-pericyclic ring-opening-intramolecular-N-tethered-[4 + 2] cycloaddition for rapid assembly of nitrogen heterocycles.

Oxidative carbocation formation in macrocycles: synthesis of the neopeltolide macrocycle

Macrocyclic oxocarbenium ions can be formed from macrolactones that contain benzylic or allylic ether groups through oxidative carbon-hydrogen-bond activation mediated by 2,3-dichloro-4,5-dicyanoquinone (DDQ). The applicability of this efficient reaction to complex-molecule synthesis was demonstrated by its use in a brief formal synthesis of neopeltolide (see retrosynthetic scheme) to form the tetrahydropyrone ring.

A sequential metal-catalyzed C-N bond formation in the synthesis of 2-amido-indoles

A sequential metal-catalyzed C-N bond formation employing ortho-haloaryl acetylenic bromides is described. The initial amidation is highly selective for C (sp)-N bond formation, leading to o-haloaryl-substituted ynamides that can be useful building blocks, while the overall sequence provides a facile construction of 2-amido-indoles.

Deep-sea natural products

This review covers the 390 novel marine natural products described to date from deep-water (>50 m) marine fauna, with details on the source organism, its depth and country of origin, along with any reported biological activity of the metabolites. Relevant synthetic studies on the deep-sea natural products have also been included.

A stereoselective intramolecular halo-etherification of chiral enamides in the synthesis of halogenated cyclic ethers

A stereoselective halo-etherification of chiral enamides is described here. This work provides an approach to halogen containing cyclic ethers and reveals further mechanistic insights to the chemistry of chiral enamides.