Gold-catalyzed [3+2] cycloaddition/hydrolytic Michael addition/retro-aldol reactions of propargylic esters tethered to cyclohexadienones

Furan Synthesis via a Tandem 1,2-Acyloxy Migration/[3 + 2] Cycloaddition/Aromatization of Enol Ether-Tethered Propargylic Esters

An effective method for the synthesis of furans is developed via a tandem 1,2-acyloxy migration/intramolecular [3 + 2] cycloaddition/aromatization of enol ether-tethered propargylic esters. The reaction exhibits excellent functional group tolerance, broad substrate scope, and excellent chemoselectivity. The isolation of dihydrofuran intermediates in some cases gives more insight into the [3 + 2] cycloisomerization process.

Transition-Metal Catalyzed Stereoselective Desymmetrization of Prochiral Cyclohexadienones

The development of transition-metal catalyzed enantioselective and diastereoselective transformations has contributed many advances in the field of synthetic organic chemistry. Particularly, stereoselective desymmetrization of prochiral cyclohexadienones represents a powerful strategy for accessing highly functionalized and stereochemically enriched scaffolds, which are often found in biologically active compounds and natural products. In recent years, several research groups including our group have made a significant progress on transition-metal catalyzed stereoselective desymmetrizations of 2,5-cyclohexadienones. In this account, we will provide an overview of the recent developments in this area employing Pd, Cu, Rh, Au, Ag, Ni, Co, and Mn-catalysts.

Lewis Acid-Driven Meyer-Schuster-Type Rearrangement of Yne-Dienone

Developed herein is a Cu(II)-catalyzed Meyer-Schuster-type rearrangement of alkyne-tethered cyclohexadienone for the construction of m-enone-substituted phenols. The reaction involves an uncommon 5-exo-trig 1,6-enyne cyclization of alkyne-tethered-cyclohexadienone, aromatization-triggered C-O bond cleavage, and an electrocyclic 4π-ring-opening of oxetene intermediate. This atom-efficient transformation provides access to a wide range of synthetically important α-(m-substituted phenol)-α,β-unsaturated ketones, featuring a broad scope with labile functional group tolerance. The gram-scale demonstration makes this transformation synthetically viable. The synthetic application of α,β-unsaturated ketones is also showcased.

Rh-Catalyzed diastereoselective desymmetrization of enone tethered-cyclohexadienones via tandem arylative cyclization

The rhodium-catalyzed arylative cyclization of enone tethered-cyclohexadienones has been developed with high efficiency, thus providing cis-fused bicyclic enones in good yields and with excellent diastereoselectivities.

Retro-metal-ene versus retro-Aldol: mechanistic insight into Rh-catalysed formal [3+2] cycloaddition

Theoretical calculations have been performed to investigate the mechanism and stereoselectivity of rhodium-catalysed intramolecular [3+2] cycloaddition for construction of a substituted hexahydropentalene complex. A new C-C bond cleavage mechanism, retro-Aldol-type, is proposed and verified for this Rh-catalysed [3+2] cycloaddition reaction.

Dearomatization of Indole via Intramolecular [3 + 2] Cycloaddition: Access to the Pentacyclic Skeleton of Strychons Alkaloids

An efficient method to build various multisubstituted polycyclic indoline-annulated normal to medium-size rings through dearomatization of indole via a tandem 1,2-acyloxy migration/intramolecular [3 + 2] cycloaddition process is described. The pentacyclic skeleton of strychnine could be synthesized via this tandem cycloaddition and a further Mannich reaction. This approach would provide a novel strategy to the synthesis of strychons alkaloids.

Mechanism, chemoselectivity and enantioselectivity for the rhodium-catalyzed desymmetric synthesis of hydrobenzofurans: a theoretical study

A theoretical study of Rhodium-catalyzed desymmetrization of cyclohexadienones.

Gold(i)-catalyzed highly stereoselective synthesis of polycyclic indolines: the construction of four contiguous stereocenters

A convenient and efficient synthetic method has been developed to construct highly functionalized polycyclic indoline skeletons with four contiguous stereocenters.

Gold-Catalyzed Intramolecular Cyclizations of Cyclopropenes with Propargylic Esters

Homogeneous gold catalysts are interesting as they can act as potent carbophilic Lewis acids to activate the π bonds of alkynes, allenes, and alkenes. Many impressive applications for the formation of C−C or C−heteroatom bonds have been found due to the excellent functional group compatibility of these catalysts and the air and moisture tolerance of their reactions. Here, we have developed gold‐catalyzed novel intramolecular cycloisomerizations of nitrogen or oxygen‐tethered cyclopropenes with propargylic esters. The reaction proceeded through different pathways according to different substituent styles, affording 5‐azaspiro[2.5]oct‐7‐enes and bicyclo[4.1.0]heptanes.

Synthesis of α-Acyloxyketone Derivatives via the Platinum-Catalyzed Migration of Propargylic Esters

The synthesis of α-acyloxyketones via the migration of a propargylic ester followed by the intramolecular nucleophilic addition of the resulting allene was achieved using a cationic platinum catalyst. The optimized conditions for this transformation were determined to be 3 mol% of Pt(cod)Cl2, 3 mol% of AgNTf2, and 3 eq of water in toluene at 100 °C, and these conditions were successfully applied to the synthesis of a wide variety of α-aryl-α-acyloxyketones. The mechanism of this reaction was evaluated in detail based on the results of isotope labeling experiments using H2(18)O.

Mechanistic insight into water-modulated cycloisomerization of enynyl esters using an Au(i) catalyst

The Au(i)-catalyzed cycloisomerization reactions of 1,6-enylnyl ester are theoretically investigated to rationalize the observed product divergences by modulating H₂O participation.

Gold-catalyzed 1,2-acyloxy migration/intramolecular [3+2] 1,3-dipolar cycloaddtion cascade reaction: an efficient strategy for syntheses of medium-sized-ring ethers and amines

A highly efficient strategy for the formation of medium-sized-ring ethers and amines based on a gold-catalyzed cascade reaction, involving enynyl ester isomerization and intramolecular [3+2] cyclization, has been developed. Various multisubstituted medium-sized-ring unsaturated ethers and amines were obtained through this transformation. This method represents one of the relatively few transition metal catalyzed intramolecular cycloaddition reactions for medium-sized ring synthesis.

Asymmetric organocatalytic desymmetrization of 4,4-disubstituted cyclohexadienones at high pressure: a new powerful strategy for the synthesis of highly congested chiral cyclohexenones

A highly diastereoselective and enantioselective method for the asymmetric desymmetrization of 4,4-disubstituted cyclohexadienones at high pressure was developed.

Tandem dienone photorearrangement-cycloaddition for the rapid generation of molecular complexity

A tandem dienone photorearrangement-cycloaddition (DPC) reaction of novel cyclohexadienone substrates tethered with various 2π and 4π reaction partners resulted in the formation of polycyclic, bridged frameworks. In particular, use of alkynyl ether-tethered substrates led to (3 + 2) cycloaddition to afford strained alkenes which could be further elaborated by intra- and intermolecular cycloaddition chemistry to produce complex, polycyclic chemotypes.

Rhodium-catalyzed acyloxy migration of propargylic esters in cycloadditions, inspiration from the recent "gold rush"

Transition metal-catalyzed acyloxy migration of propargylic esters offers versatile entries to allene and vinyl carbene intermediates for various fascinating subsequent transformations. Most π-acidic metals (e.g. gold and platinum) are capable of facilitating these acyloxy migration events. However, very few of these processes involve redox chemistry, which are well-known for most other transition metals such as rhodium. The coupling of acyloxy migration of propargylic esters with oxidative addition, migratory insertion, and reductive elimination may lead to ample new opportunities for the design of new reactions. This tutorial review summarizes recent developments in Rh-catalyzed 1,3- and 1,2-acyloxy migration of propargylic esters in a number of cycloaddition reactions. Related Au- and Pt-catalyzed cycloadditions involving acyloxy migration are also discussed.