Trapping of Oxonium Ylide with Isatins: Efficient and Stereoselective Construction of Adjacent Quaternary Carbon Centers

Tandem Insertion/[3,3]-Sigmatropic Rearrangement Involving the Formation of Silyl Ketene Acetals by Insertion of Rhodium Carbenes into S-Si Bonds

Allyl 2-diazo-2-phenylacetates are shown to react with trimethylsilyl thioethers in the presence of rhodium(II) catalysts to generate α-allyl-α-thio silyl esters. The transformation involves a tandem process involving formal rhodium-catalyzed insertion of the carbene group into the S-Si bond to generate a silyl ketene acetal, followed by a spontaneous Ireland-Claisen rearrangement. The silyl ester products were isolated as the corresponding carboxylic acids after aqueous workup. Intramolecular cyclopropanation of the allyl fragment generally competes with addition of the heteroatom to the carbene center. The reaction occurs under mild conditions and in high yield, allowing for rapid entry into rearrangement tetrasubstituted products. Propargyl esters were shown to generate the corresponding α-allenyl products.

Visible light mediated aerobic oxidative hydroxylation of 2-oxindole-3-carboxylate esters: an alternative approach to 3-hydroxy-2-oxindoles

A convenient aerobic oxidative hydroxylation of 3-substituted oxindoles under mild reaction conditions is described herein. This process was accomplished by the activation of molecular oxygen in the air in the presence of a photocatalyst under the irradiation of visible light. The desired product was delivered in up to 89% yield without the addition of base or stoichiometric oxidant.

A Rh(II)-catalyzed multicomponent reaction by trapping an α-amino enol intermediate in a traditional two-component reaction pathway

Multicomponent reactions (MCRs) represent an ideal organic synthesis tool for the rapid construction of complex molecules due to their step and atom economy. Compared to two-component reactions, the development of new MCRs has been greatly limited during the 170 years since the first MCR was reported. Theoretically, the trapping of an active intermediate generated from two components by a third component could change the traditional two-component reaction pathway, leading to the discovery of MCRs. We report an example of the trapping of α-imino enols generated in situ from 1-sulfonyl-1,2,3-triazoles via α-imino metal carbene species by vinylimine ions using C(2)-substituted indoles and paraformaldehyde as precursors in the presence of a rhodium(II) catalyst. The traditional enol-ketone transformation pathway was suspended by the trapping procedure and efficiently switched to an MCR pathway to produce α-amino-β-indole ketones in moderate to good yields. Unexpectedly, the resulting products and the theoretical density functional theory (DFT) calculation results indicated that the enolic carbon had a stronger nucleophilicity than the well-known traditional enamic carbon in the trapping process. The reaction mechanism was investigated using control experiments and detailed DFT calculations, and the synthetic application of the products was also illustrated. The developed strategy provides a mild and rapid access to α-amino-β-indole ketones and suggests a rationale for the discovery of MCRs by trapping an active intermediate with a third component in a traditional two-component reaction pathway.

Facile creation of 3-substituted-3-hydroxy-2-oxindoles by arginine-catalyzed aldol reactions of α,β-unsaturated ketones with isatins

An efficient approach for the synthesis of 3-substituted-3-hydroxy-2-oxindoles has been achieved via an aldol reaction of α,β-unsaturated ketones and isatins using arginine as an organocatalyst. A range of 3-substituted-3-hydroxy-2-oxindoles were obtained in moderate to high (up to 99%) yields. These 3-substituted-3-hydroxy-2-oxindoles with an additional enone moiety provide an opportunity for further elaboration of the products and for potentially interesting biological activities. In addition, the formation of 3-substituted-3-hydroxy-2-oxindole 3a was confirmed by X-ray crystallography. The possible reaction mechanism reveals that the reaction proceeds via a double action process.

Iron porphyrin-catalyzed three-component reaction of ethyl diazoacetate with aliphatic amines and β,γ-unsaturated α-keto esters

An iron porphyrin-catalyzed three-component reaction of ethyl diazoacetate with aliphatic amines and β,γ-unsaturated α-keto esters is reported. The use of iron porphyrin catalyst allows aliphatic amines to be used as the substrate without encountering catalyst poisoning issue and a series of β-hydroxy-α-amino esters are produced in high yields with excellent regioselectivities.

Novel multicomponent reactions via trapping of protic onium ylides with electrophiles

Multicomponent reactions (MCRs) are one-pot processes in which three or more starting materials form a product that incorporates the structural features of each reagent. These reactions date back to the mid-19th century, when Strecker first prepared α-aminonitriles through the condensation of aldehydes with ammonia and hydrogen cyanide. In addition to affording products with structural complexity and diversity, MCRs offer the advantages of simplicity, synthetic efficiency, synthetic convergence, and atom economy. Therefore, they have played an important role in modern synthetic organic chemistry and drug-discovery research. The irreversible trapping of an active intermediate generated from two components by a third one offers an effective way to discover novel MCRs. In cases where the intermediate from the first two components is reactive enough to generate a two-component byproduct, it becomes challenging to control of the chemoselectivity of these MCRs over the side reaction. For example, researchers had expected that ammonium/oxonium ylides, high energy intermediates that have acidic protons and basic carbanions attached to adjacent carbons, would be too reactive to be intercepted by external electrophiles. Instead, a very fast 1,2-proton transfer would neutralize the charge separation, resulting in a stable N-H/O-H insertion product. In this Account, we present our efforts toward the development of novel MCRs via trapping of the active ammonium/oxonium ylide intermediates with a number of electrophiles. In these reactions, a "delayed proton transfer" that occurs after the trapping process produces novel multicomponent coupling products. Thus, transition-metal-catalyzed MCRs of diazocarbonyl compounds, anilines/alcohols, and electrophiles efficiently afford polyfunctional molecules such as α-amino-β-hydroxy acids, α-hydroxy-β-amino acids, α,β-diamino acids, and α,β-dihydroxy acid derivatives. We have also applied a cooperative catalysis strategy to some of these MCRs leading to reactions with high chemo-, diastereo-, and enantioselectivity. These MCRs also provide solid experimental evidence for the existence of the active protic onium ylides.

Novel tetracyclic structures from the synthesis of thiolactone-isatin hybrids

A simple and straightforward synthetic approach to potential anti-infective thiolactone-isatin hybrids led to the discovery of novel tetracyclic compounds which bear a macrocylic motif containing an unusual bridged amide bond.

Asymmetric construction of quaternary stereocenters by direct organocatalytic amination of 3-substituted oxindoles

Quinidine derivative (QD)(2)PYR was found to catalyze the asymmetric direct amination of unprotected prochiral 3-oxindole with DIAD to construct quaternary stereocenters at the C3 position with excellent enantioselectivity.