Mechanism of the gold(I)-catalyzed Rautenstrauch rearrangement: a center-to-helix-to-center chirality transfer

The mechanism of the stereospecific gold(I)-catalyzed Rautenstrauch rearrangement of (E)-1-ethynyl-2-methyl-but-2-en-yl acetate to 3,4-dimethyl-cyclopent-2-enone has been computationally addressed using DFT (B3LYP/6-31G, SDD for Au). Our results indicate that the bond formation event follows the Au(I)-induced acetyl transfer to the vicinal alkyne and that it is the helicity of the pentadienyl cation intermediate which keeps memory of the chiral information. The fidelity of the center-to-helix-to-center chirality transfer requires that the rates of helix interconversion and pivaloyl rotation are slower than the cyclization, as calculations predict.     

Synthesis of functionalized pyrroles via gold(I)-catalyzed aza-Claisen-type rearrangement

Gold(I)-catalyzed cyclization of pentenynyl allyl tosylamides allows the rapid construction of functionalized pyrroles. The concerted aza-Claisen-type mechanism induces a complete selectivity of the process and allows the easy formation of quaternary centers.     

Synthesis of functionalized furans via gold(I)-catalyzed Claisen-type rearrangement

Gold(I)-catalyzed cyclization of pentenynyl allyl ethers allows the rapid construction of functionalized furans. The concerted oxy-Claisen-type mechanism induces a complete selectivity of the process and allows the easy formation of quaternary centers.     

Stereoselective synthesis of vinylsilanes by a gold(I)-catalyzed acetylenic sila-cope rearrangement

Cationic tri-tert-butylphosphinegold(I) serves as a catalyst in the sila-Cope rearrangement of acetylenic allylsilanes. When phenol is employed as a nucleophile, the reaction allows for the stereoselective synthesis of vinylsilanes. Alternatively, use of methanol as a nucleophile leads to cyclic vinylsilanes, which can be viewed as latent vinylsilanes that are revealed on treatment with a mild Lewis acid. Thus, both of these reagents serve as useful reagents for stereoselective synthesis of trisubstituted olefins through transition-metal-catalyzed cross-coupling reactions.     

Synthesis of indenyl ethers by gold(I)-catalyzed intramolecular carboalkoxylation of alkynes

The gold(I)-catalyzed carboalkoxylation of alkynes to form indanone derivatives from readily available ortho-acetylenic benzylic ethers is described. Importantly, the gold(I)-catalyzed rearrangement of enantioenriched benzylic ethers proceeds with chirality transfer, thus providing a practical method for the enantioselective synthesis of indenyl ethers.     

Gold(I)-catalyzed propargyl Claisen rearrangement

Homoallenic alcohols are prepared from propargyl vinyl ethers using a trinuclear gold(I)-oxo complex, [(Ph3PAu)3O]BF4, as a catalyst for propargyl Claisen rearrangement at room temperature. The gold(I)-catalyzed reaction is effective for a diverse collection of propargyl vinyl ethers, including substrates containing aryl and alkyl groups at the propargylic position, and hydrogen, aryl, and alkyl substituents at the alkyne terminus. Tertiary propargyl vinyl ethers can be employed in the reaction, at slightly elevated temperatures, to afford tetrasubstituted allenes. Importantly, the rearrangement of 1,2-disubstituted vinyl ethers proceeds with excellent diastereoselectivity, and the rearrangement of chiral nonracemic propargyl vinyl ethers proceeds with excellent chirality transfer to furnish enantioenriched allenes.     

Gold(I)-catalyzed intramolecular rearrangement of vinylidenecyclopropanes

Vinylidenecyclopropanes 1 can undergo intramolecular rearrangement to give the corresponding functionalized 1,2-dihydronaphthalenes in the presence of gold catalyst under mild conditions. A plausible rearrangement mechanism has been proposed on the basis of control experiments.     

Synthesis of benzonorcaradienes by gold(I)-catalyzed [4+3] annulation

A formal [4 + 3]-annulation of vinyl arenes and diynyl propargyl esters is described. A mechanism involving cationic phosphinegold(I)-catalyzed tandem cyclopropanation/hydroarylation to produce the benzonorcaradiene products is proposed. In accord with this mechanism, the alkynyl cyclopropane can also be prepared with excellent regio- and diastereocontrol.     

Synthesis of 2-cyclohexenone derivatives via gold(I)-catalyzed hydrative cyclization of 1,6-diynes

The gold(I) complex (MeAuPPh3) was found to be a highly effective catalyst for the hydrative cyclization of 1,6-diynes to form the corresponding 3-methyl hex-2-enone derivatives with good to excellent yield. The proposed mechanism is described.     

Olefination of ketones using a gold(III)-catalyzed Meyer-Schuster rearrangement

An atom-economical and efficient olefination strategy for ketones is described. Ethoxyacetylide addition followed by a gold-catalyzed Meyer-Schuster rearrangement affords alpha,beta-unsaturated esters, generally in excellent overall yield from the starting ketones. The alkynophilicity of Au3+ promotes an interaction with the electron-rich acetylenes that catalyzes the Meyer-Schuster rearrangement selectively over other conceivable pathways.     

Assembly of digitoxin by gold(I)-catalyzed glycosidation of glycosyl o-alkynylbenzoates

Digitoxin, a clinically important cardiac trisaccharide, was assembled efficiently from digitoxigenin and 3,4-di-O-tert-butyldiphenylsilyl-d-digitoxosyl o-cyclopropylethynylbenzoate in 9 steps and 52% overall yield via alternate glycosylation and protecting group manipulation. The present synthesis showcases the advantage of the gold(I)-catalyzed glycosylation protocol in the synthesis of glycoconjugates containing acid-labile 2-deoxysugar linkages.     

Synthesis of fused carbocycles via a selective 6-endo dig gold(I)-catalyzed carbocyclization

A gold-catalyzed synthesis of fused carbocycles via a regioselective 6-endo dig process is reported. The selectivity can be modulated by the steric and electronic properties of gold(I) complexes. The ligands can influence the pathway selectivity for the first bond formation rather than through a common intermediate generated after an initial bond formation. This gold(I)-catalyzed transformation provides access to synthetically useful carbocyclic motifs that are found in numerous diterpenoid natural products.     

Rh(II)-catalyzed Sommelet-Hauser rearrangement

Catalytic Sommelet-Hauser rearrangement is reported. The Rh(II)-catalyzed reaction of aryldiazoacetates with ethyl benzylthioacetate affords Sommelet-Hauser rearrangement products in good to excellent yields. This reaction provides a reliable and efficient way to introduce a substituent to the ortho position of arylacetates.     

Synthesis of N-aminopyrazoles by Fe(II)-catalyzed rearrangement of 4-hydrazonomethyl-substituted isoxazoles

A novel effective method is reported for the preparation of 1-amino-1H-pyrazole-4-carboxylic acid derivatives by Fe(II)-catalyzed rearrangement of isoxazoles having (2,4-dinitrophenylhydrazono)methyl substituent at C4. The reaction proceeds smoothly for both E and Z isomers of 4-(hydrazonomethyl)isoxazoles, and this means it is not necessary to separate mixtures of E/Z-isomers of the hydrazones prepared by reaction of 5-methoxy/pirrolidino-4-carbonylisoxazoles and 2,4-dinitrophenylhydrazine. The rearrangement proceeds via the formation of an aziridine intermediate which can be isolated in certain cases. The 2-nitro group in the synthesized 1-[(2,4-dinitrophenyl)amino]-1H-pyrazole-4-carboxylic esters can be selectively reduced in two steps via acylation of the amino group followed by hydrogenation-deacylation using H₂-Pd/C.     

Synthesis of polysubstituted pyrroles via a gold(I)-catalyzed tandem three-component reaction at room temperature

A gold(I)-catalyzed three-component reaction of β-nitrostyrenes with 1,3-dicarbonyl compounds and primary amines to form polysubstituted pyrroles has been developed at room temperature in ethanol. The key advantages of the three-component reaction are the mild reaction conditions and environm entally safer solvent     

Regioselectivity switch: gold(i)-catalyzed oxidative rearrangement of propargyl alcohols to 1,3-diketones

The gold(I)-catalyzed oxidative rearrangement of propargyl alcohols provides an efficient and selective route to 1,3-diketones under mild conditions. Pyridine-N-oxides were used as external oxidants with, different from related substrates, no alkylidenecycloalkanones or oxetan-3-ones formed as side-products.     

Palladium(II)-catalyzed isomerization-Claisen rearrangement of 2-alkoxy diallyl ethers

Diallyl ethers bearing an enol ether react in the presence of PdCl₂ as catalyst to give α-allyl α-alkoxy ketones by selective isomerization via formal 1,2-H migration at the more substituted allyl group, followed by Claisen rearrangement. This rearrangement is also promoted by AuCl₃ and IrCl₃, although the yields are lower with these catalysts.     

Synthesis of cycloalkanone-fused cyclopropanes by Au(I)-catalyzed oxidative ene-yne cyclizations

Au(I)-catalyzed oxidative cyclizations that successfully convert 1,5-ene-ynes and a 1,6-ene-yne to the corresponding cycloalkanone-fused cyclopropanes are described. This Au(I)-catalyzed oxidative cyclization can be effectively applied to various substrates and is an alternative to the previously used intramolecular cyclopropanation that required potentially explosive diazo compound.