Rhodium(II)-Catalyzed Stereocontrolled Synthesis of Dihydrofuran-3-imines from 1-Tosyl-1,2,3-triazoles

Advances Toward Amphidinolides C, F and U: Isolations, Synthetic Studies and Total Syntheses

Amphidinolides C, F, and U, including C2-C4 analogs, are highly cytotoxic marine macrolides, mainly isolated from dinoflagellates of the genus Amphidinium. All these polyketides share a 75 % or more similar structure, highlighted by a macrolactone ring, at least one trans-2,5-substituted-THF motif and a characteristic polyenic side chain. From their isolation and absolute configurational assignment, the total synthesis of these marine macrolides represented an intense challenge to the organic synthesis community over the last 15 years, with around 14 research groups engaged in this inspiring task. In the first part of this review, we present the different approaches to the isolation and characterization of these natural products, including the most recent analogs, which may cast doubt on the biogenetic origin of these compounds. The various synthetic approaches to the total synthesis of C, F, and U amphidinolides are presented in a second part, focusing on key reactions and/or innovative strategies. The review concludes in a third section summarizing the successful approaches leading to the total synthesis of one of the members of this amphidinolide subfamily.

Diastereoselective Synthesis of Highly Functionalized Proline Derivatives

An efficient way to access highly functionalized proline derivatives was developed based on a Cu(I)-catalyzed reaction between CF₃-substituted allenynes and tosylazide, which involved a cascade of [3 + 2]-cycloaddition/ketenimine and a rearrangement/Alder-ene cyclization to afford the new proline framework with a high diastereoselectivity.

Convergent Synthesis of the C1-C29 Framework of Amphidinolide F

The complete carbon framework of the macrocyclic marine natural product amphidinolide F has been prepared by a convergent synthetic route in which three fragments of similar size and complexity have been coupled. Key features of the syntheses of the fragments include the stereoselective construction of the tetrahydrofuran in the C1–C9 fragment by oxonium ylide (free or metal-bound) formation and rearrangement triggered by the direct generation of a rhodium carbenoid from 1-sulfonyl-1,2,3-triazole, the highly diastereoselective aldol reaction between a boron enolate and an aldehyde with 1,4-control to prepare the C10–C17 fragment, and the formation of the tetrahydrofuran in the C18–C29 fragment by intramolecular nucleophilic ring opening of an epoxide with a hydroxyl group under acidic conditions.

Access to (Z)-β-Substituted Enamides from N1-H-1,2,3-Triazoles

A direct ring-opening/nucleophilic substitution reaction of N1-H-1,2,3-triazoles has been described. Divergent (Z)-β-halogen- or sulfonyl-substituted enamides could be stereospecifically synthesized in a tunable manner. This strategy might not only enable a new ring-opening method of N1-H-1,2,3-triazoles under nonmetal catalysis and mild reaction conditions but also offer a good opportunity to reliably access versatile (Z)-β-substituted enamides that could be used as synthetic precursors for further synthetic transformations.

Grob-type fragmentation of an oxonium ylide generated from α-imino rhodium carbene

A Grob-type fragmentation of an oxonium ylide generated from α-imino rhodium carbene and tethered tetrahydrofuran was realized, leading to functionalized acroleins.

Total Syntheses of 11-Acetoxy-4-deoxyasbestinin D, 4-Deoxyasbestinin C, Asbestinin-10, -20, -21 and -23

Six members of the asbestinin family of marine diterpene natural products have been synthesized in an efficient and stereoselective manner from a single oxa‐bridged intermediate. Five of these natural products have not been synthesized previously and the structures of four of them have been confirmed as those proposed originally or following revisions to the original structures. The fifth natural product—asbestinin‐21—has been shown to be a diastereomer of the compound that had been proposed previously.

Rhodium-catalyzed Sommelet–Hauser type rearrangement of α-diazoimines: synthesis of functionalized enamides

An efficient rhodium catalyzed Sommelet–Hauser type rearrangement of sulfur ylides derived from α-thioesters and N-sulfonyl-1,2,3-triazoles has been successfully accomplished for the synthesis of various functionalized enamides.

Cascade Rh(ii) and Yb(iii) catalyzed synthesis of substituted naphthofurans via transannulation of N-sulfonyl-1,2,3-triazoles with β-naphthols

Synthesis of substituted naphthofurans has been achieved via an efficient cascade Rh(ii)/Yb(iii) catalyzed transannulation of N-sulfonyl-1,2,3-triazoles with β-naphthols.

Rhodium(ii)-catalyzed annulation of N-sulfonyl-1,2,3-triazoles with 1,3,5-triazinanes to produce octahydro-1H-purine derivatives: a combined experimental and computational study

The Rh(ii)-catalyzed annulation of N-sulfonyl-1,2,3-triazoles with 1,3,5-triazinanes leading to the octahydro-1H-purine derivatives with moderate to good yields is described in this communication.

Rhodium(ii)-catalyzed divergent intramolecular tandem cyclization of N- or O-tethered cyclohexa-2,5-dienones with 1-sulfonyl-1,2,3-triazole: synthesis of cyclopropa[cd]indole and benzofuran derivatives

A novel rhodium(ii)-catalyzed divergent intramolecular tandem cyclization of N- or O-tethered cyclohexa-2,5-dienones and 1-sulfonyl-1,2,3-triazole is disclosed, affording cyclopropa[cd]indole and benzofuran derivatives in moderate to good yields.

The Expanding Utility of Rhodium-Iminocarbenes: Recent Advances in the Synthesis of Natural Products and Related Scaffolds

Rhodium-iminocarbenes that are derived from N-sulfonyl-1,2,3-triazoles have become an important class of reactive species and useful intermediates in organic synthesis. Over the last several years, many practical and versatile approaches involving rhodium-iminocarbene intermediates to synthetically challenging molecules (scaffolds) have been developed. This Minireview mainly summarizes the recent advance of rhodium-iminocarbene involved reactions in the synthesis of natural products and their related scaffolds by the end of 2017. Several applications in important pharmaceuticals are documented as well.

Metal-free synthesis of 2-aminonaphthalenes by intramolecular transannulation of 1-sulfonyl-4-(2-alkenylphenyl)-1,2,3-triazoles

A facile metal-free synthesis of naphthalenes by intramolecular transannulation of 1-sulfonyl-4-(2-alkenylphenyl)-1,2,3-triazoles was realized. The in situ formed ketenimine was proposed as the key intermediate, and the desired 2-aminonaphthalenes were generated in up to 87% yield in refluxing 1,2-dichloroethane without any catalyst or additive.

Understanding the chemoselectivities between carbonyl and hydroxyl groups in the Rh(ii)–azavinyl carbene involved reactions

Computational studies were carried out to understand the chemoselectivities between carbonyl and hydroxyl groups in the Rh(ii)–AVC involved reactions.

Synthesis of 6-Substituted Piperidin-3-ones via Rh(II)-Catalyzed Transannulation of N-Sulfonyl-1,2,3-triazoles with Electron-Rich Aromatic Nucleophiles

A highly diastereoselective rhodium(II)-catalyzed transannulation of aldehyde-tethered N-sulfonyl triazoles with electron-rich aromatic nucleophiles is reported for the first time to afford functionalized 6-substituted piperidin-3-ones. The reaction has a broad substrate scope including both aliphatic and aromatic N-sulfonyl-1,2,3-triazoles together with various aromatic nucleophiles. The addition of a catalytic amount of Lewis acid has proven to be crucial for the yield improvement. By employing this methodology, hardly accessible piperidin-3-ones bearing quaternary carbons could be obtained.

Mechanistic studies for dirhodium-catalyzed ring expansion reactions

Theoretical studies reveal the role of a dirhodium catalyst in the expansion reactions of azide tethered methylenecyclopropanes and account for the origin of product selectivity.

Rhodium(ii)-catalyzed intramolecular annulation of 1-sulfonyl-1,2,3-triazoles with indoles: facile synthesis of functionalized tetrahydro-β-carbolines

A novel rhodium(ii)-catalyzed intramolecular annulation of 1-sulfonyl-1,2,3-triazoles with indoles has been developed, which enables the facile synthesis of various 4-functionalized tetrahydro-β-carbolines and relevant scaffolds from readily available starting materials.

Straightforward regioselective construction of 3,4-dihydro-2H-1,4-thiazine by rhodium catalysed [3 + 3] cycloaddition of thiirane with 1-sulfonyl-1,2,3-triazole: a pronounced acid additive effect

A rhodium catalyzed regioselective coupling of thiirane with 1-sulfonyl-1,2,3-triazole has been realized to give a formal [3 + 3] cycloaddition product 3,4-dihydro-2H-1,4-thiazine via the 1,3-insertion model of azavinyl carbene.

Base-induced synthesis of N-dialkylaminomethyl-2H-1,2,3-triazoles from N-sulfonyl-1,2,3-triazoles

A facile synthetic method to access N-dialkylaminomethyl-2H-1,2,3-triazoles has been developed from N-sulfonyl-1,2,3-triazoles by a base induced reaction. Moreover, 4-phenyl-1H-1,2,3-triazoles can also give the same product even in the absence of a base at high temperature.

Tandem 1,2-sulfur migration and (aza)-Diels-Alder reaction of β-thio-α-diazoimines: rhodium catalyzed synthesis of (fused)-polyhydropyridines, and cyclohexenes

Rhodium catalyzed synthesis of substituted tetrahydropyridines was accomplished from readily accessible thio-tethered N-sulfonyl-1,2,3-triazoles. The reaction involves tandem rhodium catalyzed 1,2-sulfur migration in β-thio-α-diazoimines, generated from thio-tethered N-sulfonyl-1,2,3-triazoles, to thio-substituted 1-azadiene and subsequent self aza-Diels-Alder reaction. Interestingly, the methodology was effectively extended to the synthesis of fused tetrahydropyridines, dihydropyridines and cyclohexenes through the in situ trapping of the intermediate, 1-azadiene, with various dienophiles such as enol ether, enamine, ketene S,S-acetal, alkyne, alkene and diene. Furthermore, the direct conversion of propargyl sulfides to (fused)-tetrahydropyridines was also achieved through the successful integration of copper and rhodium catalysts in one-pot.