A Dehydrogenative Diels-Alder Reaction of Prenyl Derivatives with 2,3-Dichloro-5,6-dicyanobenzoquinone

Pd(II)-Catalyzed tandem selective dehydrogenative [4+2] annulation of 2-methyl-1,3-cycloalkanediones with olefins

A practical and effective palladium-catalyzed selective dehydrogenative [4+2] annulation of 2-methyl-1,3-cycloalkanediones with olefins was reported. The active 2-methylene-1,3-cycloalkanedione was in situ generated via Pd-catalyzed enolate oxidation processes, and it subsequently reacted with a wide variety of olefins to afford various polysubstituted dihydropyran derivatives in good to excellent yields.

Asymmetric Double Oxidative [3 + 2] Cycloaddition for the Synthesis of CF3-Containing Spiro[pyrrolidin-3,2'-oxindole]

An asymmetric double oxidative [3 + 2] cycloaddition is reported. Oxidation of 3-((2,2,2-trifluoroethyl)amino)indolin-2-ones and β-aryl-substituted aldehydes simultaneously and subsequent asymmetric cycloaddition in the presence of the chiral amino catalyst generated highly functionalized chiral CF₃-containing spiro[pyrrolidin-3,2'-oxindole] with four contiguous stereocenters stereoselectively, which is characterized by directly constructing two C-C bonds from four C(sp³)-H bonds. This new method features mild conditions, broad substrate scope, and excellent functional group compatibility.

A Dehydrogenative Inverse Electron Demand Diels-Alder Reaction for the Synthesis of Functionalized Pyranones

An efficient dehydrogenative inverse electron demand Diels-Alder reaction of isopropyl and prenyl benzene derivatives with electron-deficient dienes followed by decarboxylation has been reported for the first time. The much broader substrate scope of dienophiles and electron-deficient dienes led to biologically valuable pyranones in good to excellent yields.

Synthetic applications of hydride abstraction reactions by organic oxidants

Carbon-hydrogen bond functionalizations provide an attractive method for streamlining organic synthesis, and many strategies have been developed for conducting these transformations. Hydride-abstracting reactions have emerged as extremely effective methods for oxidative bond-forming processes due to their mild reaction conditions and high chemoselectivity. This review will predominantly focus on the mechanism, reaction development, natural product synthesis applications, approaches to catalysis, and use in enantioselective processes for hydride abstractions by quinone, oxoammonium ion, and carbocation oxidants. These are the most commonly employed hydride-abstracting agents, but recent efforts illustrate the potential for weaker ketone and triaryl borane oxidants, which will be covered at the end of the review.

Synthesis of Enantioenriched 3,4-Disubstituted Chromans through Lewis Base Catalyzed Carbosulfenylation

A method for the catalytic, enantioselective, carbosulfenylation of alkenes to construct 3,4-disubstituted chromans is described. Alkene activation proceeds through the intermediacy of enantioenriched, configurationally stable thiiranium ions generated from catalytic, Lewis base activation of an electrophilic sulfenylating agent. The transformation affords difficult-to-generate, enantioenriched, 3,4-disubstituted chromans in moderate to high yields and excellent enantioselectivities. A variety of substituents are compatible including electronically diverse functional groups as well as several functional handles such as aryl halides, esters, anilines, and phenols. The resulting thioether moiety is amenable to a number of functional group manipulations and transformations. Notably, the pendant sulfide was successfully cleaved to furnish a free thiol which readily provides access to most sulfur-containing functional groups which are present in natural products and pharmaceuticals.

Copper(I)/DDQ-Mediated Double-Dehydrogenative Diels-Alder Reaction of Aryl Butenes with 1,4-Diketones and Indolones

A copper(I)/DDQ-mediated double-dehydrogenative Diels-Alder (DDDA) reaction of simple butenes with 1,4-diketones and indolones has been established for the first time. This strategy is based on a tandem double-dehydrogenation/Diels-Alder reaction from nonprefunctionalized starting materials, in which both a diene and dienophile were in situ generated via activation of fourfold inert C(sp³)-H bonds in one catalytic system.

Selectfluor-Mediated Stereoselective [1 + 1 + 4 + 4] Dimerization of Styrylnaphthols

Stereoselective [1 + 1 + 4 + 4] dimerization of 1-styrylnaphthols has been developed by using Selectfluor as the oxidant for the first time. The reaction was compatible with various functional groups, giving a class of ethanodinaphtho[b,f][1,5]dioxocines with novel 3D skeletons. DFT calculations indicate that this method merges an intriguing stereoselective intermolecular 1 + 1 radical coupling to construct a bridged C-C bond and then an intramolecular [4 + 4] formal cycloaddition of the in situ generated o-quinone methide intermediate.

Gold-catalyzed generation of azafulvenium from an enyne sulfonamide: rapid access to fully substituted pyrroles

A one-pot gold-catalyzed protocol for the synthesis of fully substituted pyrroles from enyne sulfonamides has been developed.

Metal-Free Dehydrogenative Diels-Alder Reactions of Prenyl Derivatives with Dienophiles via a Thermal Reversible Process

An efficient dehydrogenative Diels-Alder reaction of prenyl derivatives with dienophiles has been developed. The reaction exhibits broad substrate scope and provides efficient access to cyclohexene derivatives with good to excellent yields. A reasonable mechanism involving a metal-free thermal reversible process is proposed.

Copper-Catalyzed Dehydrogenative Diels-Alder Reaction

A practical and effective copper-catalyzed dehydrogenative Diels-Alder reaction of gem-diesters and ketone with dienes has been established. The active dienophiles were generated in situ via a radical-based dehydrogenation process, which reacted with a wide variety of dienes to afford various polysubstituted cyclohexene derivatives in good to excellent yields.

Biomimetic Total Synthesis of (±)-Griffipavixanthone via a Cationic Cycloaddition-Cyclization Cascade

We report the concise, biomimetic total synthesis of the dimeric, Diels-Alder natural product griffipavixanthone from a readily accessible prenylated xanthone monomer. The key step utilizes a novel intermolecular [4+2] cycloaddition-cyclization cascade between a vinyl p-quinone methide and an in situ generated isomeric diene promoted by either Lewis or Brønsted acids. Experimental and computational studies of the reaction pathway suggest that a stepwise, cationic Diels-Alder cycloaddition is operative.

Quinone-related hexacyclic by-products in the production process of exemestane

Exemestane, a 3rd-generation aromatase inhibitor, is clinically used in the treatment of breast cancer in postmenopausal women. The key step of the industrial synthetic process, i.e., a dehydrogenation to introduce the Δ¹-unsaturation, is normally performed with quinones such as p-chloranil or DDQ. We observed the formation of two different hexacyclic by-products, depending on the quinone used in the oxidation step. These compounds arise from an initial [4+2] cycloaddition between the precursor 6-methylenandrost-4-ene-3,17-dione and the quinone reagent, followed by a twofold dehydrohalogenation (with p-chloranil) or dehydrogenation (with DDQ). The structures of these unprecedented hexacyclic adducts were determined by a combination of mass spectrometry, NMR techniques and crystallographic analysis.

Divergent Gold(I)-Catalyzed Skeletal Rearrangements of 1,7-Enynes

The gold(I) complex catalyzed cycloisomerization and skeletal rearrangement of 1,n-enynes (n=5-7) is a powerful methodology for the efficient synthesis of complex molecular architectures. In contrast to 1,6-enynes, readily accessible homologous 1,7-enynes are largely unexplored in such transformations. Here, the divergent skeletal rearrangement of all-carbon 1,7-enynes by catalysis with a cationic gold(I) complex is reported. Depending on electronic and steric factors, differently substituted 1,7-enynes react via different carbocations formed from a common gold carbene intermediate to yield on the one hand novel exocyclic allenes and on the other hand tricyclic hexahydro-anthracenes through a novel dehydrogenative Diels-Alder reaction.