Palladium-catalyzed cascade cyclization-coupling reactions of 2-bromo-1,6-enynes with organoboronic acids

Cyclization of ortho-ethynylbiaryls as an emerging versatile tool for the construction of polycyclic arenes

Cyclization and cycloisomerization of ortho-aryl(ethynyl)arenes provide an easy direct access to fused polycyclic aromatic carbo- and heterocycles. This methodology has demonstrated an impressive progress in the recent years. The goal of this review is to give a comprehensive outlook on the synthetic potential, scope, limitations, and mechanistic aspects of the cyclization reactions. The material is arranged according to the activation method that can be used to induce cyclization: pyrolysis, metal catalysis, electrophilic activation, radical induction, base catalysis. Particular attention is paid to the specificity of ortho-ethynylbiaryls with a heterocyclic central core.

The bibliography includes 257 references.

Enantioselective Synthesis of All-Carbon Quaternary Centers Structurally Related to Amaryllidaceae Alkaloids

Enantioselective synthesis of all-carbon quaternary centers remains a considerable challenge for synthetic organic chemists. Here, we report a two-step protocol to synthesize such centers including tandem cyclization/Suzuki cross-coupling followed by halocarbocyclization. During this process, two rings, three new C-C bonds and a stereochemically defined all-carbon quaternary center are formed. The absolute configuration of this center is controlled by the stereochemistry of the adjacent stereocenter, which derives from an appropriate enantioenriched starting material. Using this method, we synthesized polycyclic compounds structurally similar to Amaryllidaceae alkaloids in high enantiomeric excesses. Because these products resemble naturally occurring compounds, our protocol can be used to synthesize various potentially bioactive compounds.

Ynamide carbopalladation: a flexible route to mono-, bi- and tricyclic azacycles

Bromoenynamides represent precursors to a diversity of azacycles by a cascade sequence of carbopalladation followed by cross-coupling/electrocyclization, or reduction processes. Full details of our investigations into intramolecular ynamide carbopalladation are disclosed, which include the first examples of carbopalladation/cross-coupling reactions using potassium organotrifluoroborate salts; and an understanding of factors influencing the success of these processes, including ring size, and the nature of the coupling partner. Additional mechanistic observations are reported, such as the isolation of triene intermediates for electrocyclization. A variety of hetero-Diels-Alder reactions using the product heterocycles are also described, which provide insight into Diels-Alder regioselectivity.

Ligand-controlled divergent formation of alkenyl- or allylboronates catalyzed by Pd, and synthetic applications

Pd-catalysed reactions of allyl carbonates containing alkynes with B2(pin)2 afford either alkenylboronates by cascade borylative cyclisation, or allylboronates. The reaction outcome is controlled by a suitable choice of ligands. Boronates have been applied to a number of synthetic applications.

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.

A palladium-catalyzed three-component cross-coupling of conjugated dienes or terminal alkenes with vinyl triflates and boronic acids

A three-component coupling of vinyl triflates and boronic acids to alkenes catalyzed by palladium is reported. Using 1,3-dienes, selective 1,2-alkene difunction-alization is observed, whereas the use of terminal alkenes results in 1,1-alkene difunctionalization. The reaction outcome is attributed to the formation of stabilized, cationic Pd-π-allyl intermediates to regulate β-hydride elimination.

Copper-catalysed bromoalkynylation of arynes

Arynes were found to be inserted into a C(sp)-Br sigma-bond of bromoalkynes in the presence of a copper catalyst, giving (alkynyl)bromoarenes in a straightforward manner.

Rhodium-catalyzed tandem cyclization-cycloaddition reactions of enynebenzaldehydes: construction of polycyclic ring systems

o-(1,6-Enynyl)benzaldehydes underwent a novel mode of cycloaddition using Rh(I)-precatalyst, via[3+2] cycloaddition of presumed dipolar carbonyl ylide intermediate generated by Rh-catalyst and the utility of this mechanistically intriguing enyne cyclization can be found in a number of polycyclic natural product skeletons.