Total Synthesis of Njaoamine C by Concurrent Macrocycle Formation

Total Synthesis of the Marine Diterpenoids Caucanolide E and F by Alkyne gem-Hydrogenation: Rigorous Reassignment of Two Almost Indistinguishable Diastereomers

As the spectra of the diastereomeric marine seco-cembranoids caucanolide E and F are almost identical, the isolation team had not been able to firmly assign their stereostructure. The puzzle has now been solved by a synthetic approach predicated on biogenetic considerations, which suggested that both natural products are 10S configured but differ in the configuration of their C8 stereocenter. Under this proviso, they can be distinguished by comparison with synthetic samples that are deliberately 8S,10S and 8S,10R configured: only one natural product can match sign and magnitude of the [α]D of one of these reference compounds; this particular caucanolide must be the 8S,10S isomer. Both reference compounds needed for this benchmarking exercise were accessible from a single precursor via ruthenium catalyzed gem-hydrogenation of an enyne to furnish an alkoxyfuran in the first place, followed by hydrolysis; this key transformation leverages the electrophilic character of the pianostool ruthenium carbene intermediates passed through. The recorded data showed that the assignment proposed in the literature is incorrect and needs to be reversed; this conclusion was independently confirmed by a combined NMR/DFT approach using a CP3 probability analysis.

Marine natural products

Covering: January to the end of December 2023This review covers the literature published in 2023 for marine natural products (MNPs), with 582 citations (541 for the period January to December 2023) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, the submerged parts of mangroves and other intertidal plants. The emphasis is on new compounds (1220 in 340 papers for 2023), together with the relevant biological activities, source organisms and country of origin. Pertinent reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. An analysis of the progress in the study of prokaryote involvement in macro-invertebrate MNP production is discussed.

Oxoammonium-Catalyzed Oxidation of N-Substituted Amines

We report the development of oxoammonium-catalyzed oxidation of N-substituted amines via a hydride transfer mechanism. Steric and electronic tuning of catalyst led to complementary sets of conditions that can oxidize a broad scope of carbamates, sulfonamides, ureas, and amides into the corresponding imides. The reaction was further demonstrated on a 100-g scale using a continuous flow setup.

Exploring efficient and air-stable d2 Re(v) alkylidyne catalysts: toward room temperature alkyne metathesis

Transition metal-catalyzed alkyne metathesis has become a useful tool in synthetic chemistry. Well-defined alkyne metathesis catalysts comprise alkylidyne complexes of tungsten, molybdenum and rhenium. Non-d0 Re(v) alkylidyne catalysts exhibit advantages such as remarkable tolerance to air and moisture as well as excellent functional group compatibility. However, the known Re(v) alkylidynes with a pyridine leaving ligand require harsh conditions for activation, resulting in lower catalytic efficiency compared to d0 Mo(vi) and W(vi) alkylidynes. Herein, we report the first non-d0 alkylidyne complex capable of mediating alkyne metathesis at room temperature, namely, the Re(v) aqua alkylidyne complex Re([triple bond, length as m-dash]CCH2Ph)( Ph PO)2(H2O) (14). The aqua complex readily dissociates a water ligand in solution, confirmed by ligand substitution reactions with other σ-donor ligands. The aqua complex can be readily prepared on a large scale, and is stable to air and moisture in the solid state and compatible with a variety of functional groups. The versatile ability of the catalyst has been demonstrated through examples of alkyne cross-metathesis (ACM), ring-closing alkyne metathesis (RCAM), and acyclic diyne metathesis macrocyclization (ADIMAC) reactions. All in all, this work presents a solution for an efficient and air-stable alkyne metathesis catalytic system based on d2 Re(v)-alkylidynes.

Total Synthesis of the Norcembranoid Scabrolide B and Its Transformation into Sinuscalide C, Ineleganolide, and Horiolide

It was recognized only recently that the sister norcembranoids scabrolides A and B have notably different carbotricyclic scaffolds. Therefore, our synthesis route leading to scabrolide A could not be extended to its sibling. Rather, a conceptually new approach had to be devised that relied on a challenging intramolecular alkenylation of a ketone to forge the congested central cycloheptene ring at the bridgehead enone site; the required cyclization precursor was attained by a lanthanide-catalyzed Mukaiyama-Michael addition. The dissonant 1,4-oxygenation pattern was then installed by allylic rearrangement/oxidation of the enone, followed by suprafacial 1,3-transposition. Synthetic scabrolide B was transformed into sinuscalide C by dehydration and into ineleganolide by base-mediated isomerization/oxa-Michael addition, which has potential biosynthetic implications; under basic conditions, the latter compound converts into horiolide by an intricate biomimetic cascade.

Transition-Metal-Catalyzed Transformations for the Synthesis of Marine Drugs

Transition metal catalysis has contributed to the discovery of novel methodologies and the preparation of natural products, as well as new chances to increase the chemical space in drug discovery programs. In the case of marine drugs, this strategy has been used to achieve selective, sustainable and efficient transformations, which cannot be obtained otherwise. In this perspective, we aim to showcase how a variety of transition metals have provided fruitful couplings in a wide variety of marine drug-like scaffolds over the past few years, by accelerating the production of these valuable molecules.

Molybdenum(VI) Nitrido Complexes with Tripodal Silanolate Ligands. Structure and Electronic Character of an Unsymmetrical Dimolybdenum μ-Nitrido Complex Formed by Incomplete Nitrogen Atom Transfer

In contrast to a tungsten nitrido complex endowed with a tripodal silanolate ligand framework, which was reported in the literature to be a dimeric species with a metallacyclic core, the corresponding molybdenum nitrides 3 are monomeric entities comprising a regular terminal nitride unit, as proven by single-crystal X-ray diffraction (SC-XRD). Their electronic character is largely determined by the constraints imposed on the metal center by the podand ligand architecture. 95Mo nuclear magnetic resonance (NMR) and, to a lesser extent, 14N NMR spectroscopy allow these effects to be studied, which become particularly apparent upon comparison with the spectral data of related molybdenum nitrides comprising unrestrained silanolate, alkoxide, or amide ligands. Attempted nitrogen atom transfer from these novel terminal nitrides to [(tBuArN)3Mo] (Ar = 3,5-dimethylphenyl) as the potential acceptor stopped at the stage of unsymmetric dimolybdenum μ-nitrido complex 13a as the first intermediate along the reaction pathway. SC-XRD, NMR, electron paramagnetic resonance, and ultraviolet-visible spectroscopy as well as magnetometry in combination with density functional theory allowed a clear picture of the geometric and electronic structure of this mixed-valent species to be drawn. 13a is formally best described as an adduct of the type [(Mo[O])+III-(μN)-III-(Mo[N])+VI], S = 1/2 complex with (Mo[O])+III in the low-spin configuration, whereas related complexes such as [(AdS)3Mo-(μN)-Mo(NtBuAr)3] (19; Ad = 1-adamantyl) have previously been regarded in the literature as mixed-valent Mo+IV/Mo+V species. The spin population at the two Mo centers is uneven and notably larger at the more reduced Mo[O] atom, whereas the only spin present at the (μN) bridge is derived from spin polarization.

Total Synthesis of Keramaphidin B and Ingenamine by Base-Catalyzed Diels-Alder Reaction Using Dynamic Regioselective Crystallization

The control of the selectivity is a central issue in the total synthesis of complex natural products. In this paper, we report the total synthesis of (±)-keramaphidin B and (±)-ingenamine. The key reaction is a DMAP-catalyzed Diels-Alder reaction in which the regioselectivity is completely controlled by dynamic crystallization. Our synthesis successfully demonstrates that dynamic crystallization can be an alternative when the selectivity is not controlled by either kinetic or thermodynamic approaches in solution.

Total Synthesis of the Guangnanmycin A Alcohol

Guangnanmycin A is a recently discovered congener of the well-known antitumor drug lead leinamycin; its macrolactam ring, however, is even more strained than that of the parent compound. The first synthetic foray towards this challenging target is reported, which relies on molybdenum-catalyzed macrocyclization by ring closing alkyne metathesis (RCAM) followed by ruthenium-catalyzed redox isomerization of the propargyl alcohol thus formed; the resulting enone enabled the introduction of the yet missing exo-methylene group by a modified Peterson olefination. The signature disulfide moiety of guangnanmycin A was installed by strain-driven thia-Michael addition followed by conversion of the thioether thus formed into an unsymmetric disulfide with the aid of (methylthio)dimethylsulfonium tetrafluoroborate and MeSSMe. While this sequence furnished racemic guangnanmycin A alcohol in good overall yield, the final oxidation to the corresponding acid failed, most likely because of the exceptional sensitivity of the strained scaffold.

Total Synthesis of the Allenic Macrolide (+)-Archangiumide

Archangiumide is the first known macrolide natural product comprising an endocyclic allene. For the ring strain that this linear substructure might entail, it was planned to unveil the allene at a very late stage of the projected total synthesis; in actual fact, this was achieved as the last step of the longest linear sequence by using an otherwise globally deprotected substrate. This unconventional timing was made possible by a gold catalyzed rearrangement of a macrocyclic propargyl benzyl ether derivative that uses a -PMB group as latent hydride source to unveil the signature cycloallene; the protecting group therefore gains a strategic role beyond its mere safeguarding function. Although the gold catalyzed reaction per se is stereoablative, the macrocyclic frame of the target was found to impose high selectivity and a stereoconvergent character on the transformation. The required substrate was formed by ring closing alkyne metathesis (RCAM) with the aid of a new air-stable molybdenum alkylidyne catalyst.

From the Glovebox to the Benchtop: Air-Stable High Performance Molybdenum Alkylidyne Catalysts for Alkyne Metathesis

Molybdenum alkylidynes endowed with tripodal silanolate ligands belong to the most active and selective catalysts for alkyne metathesis known to date. This paper describes a new generation that is distinguished by an unprecedented level of stability and practicality without sacrificing the chemical virtues of their predecessors. Specifically, pyridine adducts of type 16 are easy to make on gram scale, can be routinely weighed and handled in air, and stay intact for many months outside the glovebox. When dissolved in toluene, however, spontaneous dissociation of the stabilizing pyridine ligand releases an active species of excellent performance and functional group tolerance. Specifically, a host of polar and apolar groups, various protic sites, and numerous basic functionalities proved compatible. The catalysts are characterized by crystallographic and spectroscopic means, including 95Mo NMR; their activity and stability are benchmarked in detail, and the enabling properties are illustrated by advanced applications to natural product synthesis. For the favorable overall application profile and ease of handling, complexes of this new series are expected to replace earlier catalyst generations and help encourage a more regular use of alkyne metathesis in general.