Cyclobutenedione-Based Method for the Synthesis of Substituted 2-Pyridinones and Dihydro-2-pyridinones

Construction of Seven-Membered Oxacycles Using a Rh(I)-Catalyzed Cascade C-C Formation/Cleavage of Cyclobutenol Derivatives

Herein, we describe the synthesis of substituted oxepane derivatives through the skeletal remodeling of 4-hydroxy-2-cyclobutenones, which are readily prepared from commercially available dialkyl squarates upon their reaction with acrylonitrile. Mechanistically, a Rh(I)-catalyzed C-C bond formation and cleavage cascade is proposed. Specifically, a fused [3.2.0] bicycle is proposed to form from dialkyl squarate-derived cyclobutenols via an unusual Rh(I)-catalyzed intermolecular oxa-Michael addition of a tertiary alcohol with acrylonitrile, followed by an intramolecular conjugate addition/migratory insertion. Subsequent C(sp3)-C(sp3) bond cleavage through a Rh-catalyzed β-carbon elimination is then theorized to furnish the oxepane scaffold. Computational studies support the formation of an intermediate [3.2.0] bicycle but also point to an alternative pathway for the formation of the oxepane products involving a Rh(III) intermediate. Additional studies have shown the overall process to be stereoretentive. The functional groups that are introduced in this process can be leveraged to form fused or bridged ring systems.

Isocyanide insertion across the Pd–C bond of allenyl and propargyl palladium complexes bearing phosphoquinoline as a spectator ligand. Synthesis of a palladium complex bearing a coordinated cyclobutenyl fragment

A palladium complex bearing a cyclobutenyl group directly coordinated to metal has been obtained as a result of the insertion of an isocyanide across the Pd–C bond.

Reactive iron carbonyl reagents via reaction of metal alkoxides with Fe(CO)5 or Fe2(CO)9: synthesis of cyclobutenediones via double carbonylation of alkynes

Alkoxy bases such as t-BuOK react with Fe(CO)(5) to give reactive iron carbonyl intermediates that in turn react with alkynes at 70 °C in THF to give 1,2-cyclobutenediones in 70-93% yields after CuCl(2)·2H(2)O oxidation. A novel 1,2-diacyloxyferrole derivative was isolated in the reaction of diphenylacetylene with Fe(CO)(5)/t-BuOK in the presence of acetyl chloride in contrast to the formation of a 1,4-diacyloxyferrole complex formed in the reaction using Fe(CO)(5)/Me(3)NO. The Fe(2)(CO)(9)/t-BuOK reagent system also converts the alkynes to corresponding cyclobutenediones in 63-90% yields under similar reaction conditions.

Pd-catalyzed, Cu(I)-mediated cross-couplings of bisarylthiocyclobutenediones with boronic acids and organostannanes

Bisarylthiocyclobutenedione 7 reacted smoothly with a variety of both organostannanes and (hetero)arylboronic acids in the presence of a catalytic amount of Pd and a stoichiometric amount of CuTC to produce symmetrical disubstituted cyclobutenediones in yields that range from 37 to 94% (18 examples).

Selective Cross-Couplings. Sequential Stille−Liebeskind/Srogl Reactions of 3-Chloro-4-arylthiocyclobutene-1,2-dione

The synthesis and initial reactivity studies of 2 are described. It was found that it participates in Stille couplings exclusively at the C-Cl site with a number of organostannanes (58-71% yield) in the absence of Cu(I). Then, these new derivatives were functionalized at the C-S site with boronic acids by switching to the Liebeskind-Srogl reaction conditions (in the presence of a Cu(I) carboxylate) to yield the bifunctionalized cyclobutenediones (44-90% yield).

Microwave-Assisted Synthesis of New Polysubstituted Dienaminoesters and Their Cyclization to 3-Bromo-2(1H)-Pyridinones

A microwave-assisted, telescoped synthesis, involving a Michael-type addition followed by intramolecular cyclization, provides an effective entry to the polysubstituted 3-bromo-2(1H)-pyridinone core.

New Synthesis of Semisquaric Acid Derivatives via Chlorinated N-(Cyclobutylidene)amines

The synthesis and reactivity of new chlorinated N-(cyclobutylidene)amines leading to new synthetic pathways toward various substituted cyclobutenediones is described.