A Decade of Advancements in Pauson–Khand‐Type Reactions

Mechanism of the Palladium-Catalyzed Synthesis of Münchnones: The Role of Ligands in N-Acyl Iminium Salt Carbonylation

The palladium-catalyzed carbonylative coupling of imines, acid chlorides, and dipolarophiles can provide efficient routes to prepare nitrogen-containing heterocycles. One challenge in developing this reaction, and in the creation of more active catalyst systems, is the lack of data on how this complex transformation proceeds. To address this, we report here the results of our mechanistic studies on this system, and in particular the formation of mesoionic münchnones. This includes the synthesis of key catalytic intermediates, model reactions, and kinetic studies that support the role of these compounds in catalysis. Together, these studies provide a clear picture of the impact of catalyst structure, ligands, and palladium nanoparticles on facilitating the carbonylation of in situ generated iminium salts, and suggest an avenue for the creation of more active catalyst systems.

Total Syntheses of (+)-Marrubiin and (-)-Marrulibacetal

A stereoselective total synthesis of (+)-marrubiin has been accomplished starting from a chiral building block via the CyNH2-promoted Pauson-Khand reaction (PKR) followed by oxidative cleavage of the resultant cyclopentenone ring. Two successive oxidations and internal transacetalization culminated in the total synthesis of the antispasmodic labdane diterpenoid (-)-marrulibacetal.

Homogeneous rhodium(i)-catalysis in de novo heterocycle syntheses

Rh(i)-catalysed reactions often employ mild reaction conditions and offer excellent functional group tolerance, making them ideal transformations for the preparation of complex molecules. This review surveys examples of these synthetically useful transformations as applied to the synthesis of various heterocycles.

Synthesis of tricyclic indolizidines from ethyl isocyanoacetate

Tricyclic indolizidines were synthesized in good yields from commercially available ethyl isocyanoacetate by a novel sequential alkylation, thiol-mediated radical cyclization, N-alkylation, and microwave-assisted Pauson-Khand reaction.

Lewis acid catalyzed intramolecular [3 + 2] cross cycloadditions of cobalt-alkynylcyclopropane 1,1-diesters with carbonyls for construction of medium-sized and polycyclic skeletons

A Lewis acid catalyzed intramolecular [3 + 2] cross cycloaddition of cobalt-alkynylcyclopropane 1,1-diesters with carbonyls has been successfully developed. Together with simple and efficient postcycloadditions of the cobalt-alkyne moiety, a general and efficient strategy for construction of structurally complex and diverse medium-sized skeletons and related polycycles was supplied successfully.

Nickel(0)-catalyzed enantioselective annulations of alkynes and arylenoates enabled by a chiral NHC ligand: efficient access to cyclopentenones

Cyclopentenones are versatile structural motifs of natural products as well as reactive synthetic intermediates. The nickel-catalyzed reductive [3+2] cycloaddition of α,β-unsaturated aromatic esters and alkynes constitutes an efficient method for their synthesis. Here, nickel(0) catalysts comprising a chiral bulky C1-symmetric N-heterocyclic carbene ligand were shown to enable an efficient asymmetric synthesis of cyclopentenones from mesityl enoates and internal alkynes under mild conditions. The bulky NHC ligand provided the cyclopentenone products in very high enantioselectivity and led to a regioselective incorporation of unsymmetrically substituted alkynes.

A "catch-and-release" protocol for alkyne-tagged molecules based on a resin-bound cobalt complex for peptide enrichment in aqueous media

The development of new and mild protocols for the specific enrichment of biomolecules is of significant interest from the perspective of chemical biology. A cobalt-phosphine complex immobilised on a solid-phase resin has been found to selectively bind to a propargyl carbamate tag, that is, "catch", under dilute aqueous conditions (pH 7) at 4 °C. Upon acidic treatment of the resulting resin-bound alkyne-cobalt complex, the Nicholas reaction was induced to "release" the alkyne-tagged molecule from the resin as a free amine. Model studies revealed that selective enrichment of the alkyne-tagged molecule could be achieved with high efficiency at 4 °C. The proof-of-concept was applied to an alkyne-tagged amino acid and dipeptide. Studies using an alkyne-tagged dipeptide proved that this protocol is compatible with various amino acids bearing a range of functionalities in the side-chain. In addition, selective enrichment and detection of an amine derived from the "catch and release" of an alkyne-tagged dipeptide in the presence of various peptides has been accomplished under highly dilute conditions, as determined by mass spectrometry.

Asymmetric allylation/Pauson-Khand reaction: a simple entry to polycyclic amines. Application to the synthesis of aminosteroid analogues

Asymmetric allylation of o-iodoarylsulfinylimines has been achieved in high diastereoselectivities. The thus-obtained o-iodoarylhomoallylic sulfinamides participate in a subsequent Sonogashira coupling followed by a diastereoselective intramolecular Pauson-Khand reaction. In this way, tricyclic amines showing a unique benzo-fused indenyl backbone were obtained. The methodology has been applied to the synthesis of amino steroid analogues.

Chiral metal nanoparticle-catalyzed asymmetric C-C bond formation reactions

Chiral ligand-modified metal nanoparticles possess an attractive potential for application in asymmetric synthesis. This article focuses on chiral-nanoparticle-catalyzed asymmetric C-C bond formation reactions and discusses the nature of the active species.

Nucleoside analogues with a 1,3-diene-Fe(CO)3 substructure: stereoselective synthesis, configurational assignment, and apoptosis-inducing activity

The synthesis and stereochemical assignment of two classes of iron-containing nucleoside analogues, both of which contain a butadiene-Fe(CO)3 substructure, is described. The first type of compounds are Fe(CO)3-complexed 3'-alkenyl-2',3'-dideoxy-2',3'-dehydro nucleosides (2,5-dihydrofuran derivatives), from which the second class of compounds is derived by formal replacement of the ring oxygen atom by a CH2 group (carbocyclic nucleoside analogues). These compounds were prepared in a stereoselective manner through the metal-assisted introduction of the nucleobase. Whilst the furanoid intermediates were prepared from carbohydrates (such as methyl-glucopyranoside), the carbocyclic compounds were obtained by using an intramolecular Pauson-Khand reaction. Stereochemical assignments based on NMR and CD spectroscopy were confirmed by X-ray structural analysis. Biological investigations revealed that several of the complexes exhibited pronounced apoptosis-inducing properties (through an unusual caspase 3-independent but ROS-dependent pathway). Furthermore, some structure-activity relationships were identified, also as a precondition for the design and synthesis of fluorescent and biotin-labeled conjugates.