Intermolecular Diels-Alder Cycloaddition/Cross-Coupling Sequences of 2-Bromo-1,3-butadienes

Stereoselective synthesis of the spirocyclic core of 13-desmethyl spirolide C using an aza-Claisen rearrangement and an exo-selective Diels-Alder cycloaddition

13-Desmethyl spirolide C is a marine natural product of the cyclic imine class that demonstrates remarkable bioactivity against several biomarkers of Alzheimer's Disease, which renders its [7,6]-spirocyclic imine pharmacophore of significant synthetic interest. This work describes a facile and efficient synthesis of the [7,6]-spirocyclic core of 13-desmethyl spirolide C from inexpensive starting materials, featuring an aza-Claisen rearrangement to simultaneously set both stereocentres of the dimethyl moiety with complete atom economy, and a highly exo-selective Diels-Alder cycloaddition to construct the challenging contiguous tertiary and quaternary stereocentres of the spirocyclic core of 13-desmethyl spirolide C. A comprehensive study of the key Diels-Alder reaction was also performed to evaluate the stereoselectivity and reactivity of various functionalised dienes and protected lactam dienophiles, wherein the first successful exo-selective Diels-Alder cycloaddition to construct spirocyclic structures using a bromodiene and α-exo-methylene dienophiles is reported. This strategy not only establishes a more efficient stereoselective access to the spirocyclic core that can be used for the total synthesis of 13-desmethyl spirolide C, but also serves as a sound platform for convenient preparations of a range of spirocyclic analogues required for a comprehensive biological evaluation of this desirable pharmacophore.

Leveraging Metal and Ligand Reactive Sites for One Pot Reactions: Ligand-Centered Borenium Ions for Tandem Catalysis with Palladium

Tandem catalysts that perform two different organic transformations in a single pot are highly desirable because they enable rapid and efficient assembly of simple organic building blocks into more complex molecules. Many examples of tandem catalysis rely on metal-catalyzed reactions involving one or more metal complexes. Remarkably, despite surging interest in the development of chemically reactive (i. e., non-innocent) ligands, there are few examples of metal complexes that leverage ligand-centered reactivity to perform catalytic reactions in tandem with separate catalytic reactions at the metal. Here we report how multifunctional Pd complexes with triaminoborane-derived diphosphorus ligands, called TBDPhos, appear to facilitate borenium-catalyzed cycloaddition reactions at the ligand, and Pd-catalyzed Stille and Suzuki cross-coupling reactions at the metal. Both transformations can be accessed in one pot to afford rare examples of tandem catalysis using separate metal and ligand catalysis sites in a single complex.

Intramolecular Diels-Alder Reactions of α-Bromostyrene-Functionalized Unsaturated Carboxamides

Intramolecular cycloaddition reactions of α-bromostyrene-functionalized amides of monomethyl fumarate were investigated. The reaction of the amides with Et₃N in toluene at 110 °C gave 1,4-dihydronaphthalenes. The 1,4-dihydronaphthalenes may be produced via the intramolecular Diels-Alder reaction, proton transfer, and dehydrobromination by a base, along with C═C bond isomerization by proton transfer. The reaction of amide derivatives with halogen on a benzene ring and alkali metal carbonates in toluene at 110 °C gave naphthalene derivatives directly. Dehydrogenation of various 1,4-dihydronaphthalenes with cesium or rubidium carbonate in toluene at 110 °C gave naphthalene derivatives. The retardation by TEMPO, acceleration by air for some substrates, and density functional theory calculations suggest a radical mechanism caused by intervention of molecular oxygen.

Azomethine ylide cycloaddition of 1,3-dienyl esters: highly regio- and diastereoselective synthesis of functionalized pyrrolidinochromenes

An efficient protocol for the synthesis of tricyclic pyrrolidinochromenes has been developed via an intramolecular 1,3-dipolar cycloaddition of azomethine ylides generated in situ from 1,3-dienyl ester tethered O-hydroxyarylaldehyde and glycine esters. The reaction is highly regio- and diastereoselective in nature and provided the potentially bioactive pyrrolidine fused tricyclic cycloadducts in excellent yields with wide substrate scope. Interestingly this reaction constructs two rings and four contiguous stereogenic centers in which one of them is an all carbon quaternary center in a unique fashion.