Enantioselective Synthesis of Spirocyclic Benzopyranones by Rhodium‐Catalyzed Intermolecular [4+2] Annulation

Transition Metal-Catalyzed Enantioselective Synthesis of Chiral Five- and Six-Membered Benzo O-heterocycles

Enantiomerically enriched five- and six-membered benzo oxygen heterocycles are privileged architectures in functional organic molecules. Over the last several years, many effective methods have been established to access these compounds. However, comprehensive documents cover updated methodologies still in highly demand. In this review, recent transition metal catalyzed transformations lead to chiral five- and six-membered benzo oxygen heterocycles are presented. The mechanism and chirality transfer or control processes are also discussed in details.

Rhodium-Catalyzed Asymmetric C-H Functionalization Reactions

This review summarizes the advancements in rhodium-catalyzed asymmetric C-H functionalization reactions during the last two decades. Parallel to the rapidly developed palladium catalysis, rhodium catalysis has attracted extensive attention because of its unique reactivity and selectivity in asymmetric C-H functionalization reactions. In recent years, Rh-catalyzed asymmetric C-H functionalization reactions have been significantly developed in many respects, including catalyst design, reaction development, mechanistic investigation, and application in the synthesis of complex functional molecules. This review presents an explicit outline of catalysts and ligands, mechanism, the scope of coupling reagents, and applications.

Photoinduced Synthesis of Functionalized Oxacyclic Spirooxindoles Via Ring Expansion

A versatile photochemical ring-expansion protocol for the synthesis of oxacyclic spirooxindoles under catalyst-free conditions is described. The reaction is enabled by the use of unstrained O-containing heterocycles with 3-diazoindolin-2-ones under visible-light irradiation. Several synthetic advantages for this method are exhibited, including mild conditions, good functional group tolerance, operational simplicity, and scalability. Mechanistic studies indicate that the transformation may proceed through the formation of oxonium ylide intermediate followed by an ionic cyclization.

A Unified Catalytic Asymmetric (4+1) and (5+1) Annulation Strategy to Access Chiral Spirooxindole-Fused Oxacycles

A unified catalytic asymmetric (N+1) (N=4, 5) annulation reaction of oxindoles with bifunctional peroxides has been achieved in the presence of a chiral phase-transfer catalyst (PTC). This general strategy utilizes peroxides as unique bielectrophilic four- or five-atom synthons to participate in the C-C and the subsequent umpolung C-O bond-forming reactions with one-carbon unit nucleophiles, thus providing a distinct method to access the valuable chiral spirooxindole-tetrahydrofurans and -tetrahydropyrans with good yields and high enantioselectivities under mild conditions. DFT calculations were performed to rationalize the origin of high enantioselectivity. The gram-scale syntheses and synthetic utility of the resultant products were also demonstrated.

Box-copper catalyzed asymmetric inverse-electron-demand oxa-hetero-Diels–Alder reaction for efficient synthesis of spiro pyranyl-oxindole derivatives

A chiral Box/Cu catalyzed asymmetric IEDDA reaction between isatin-derived β,γ-unsaturated α-ketoesters and electron-rich olefins was developed, which provided chiral spiro oxindole-pyrans in excellent yields with excellent stereoselectivities.

Enantioselective synthesis of indanone spiro-isochromanone derivatives via a dinuclear zinc-catalyzed Michael/transesterification tandem reaction

An enantioselective Michael/transesterification tandem reaction of α-hydroxy indanones with ortho-ester chalcones was realized using dinuclear zinc catalysts. A series of enantiomerically pure spiro[indanone-2,3'-isochromane-1-one] derivatives were obtained in good yields with excellent stereoselectivities (up to >20 : 1 dr, up to >99% ee). This protocol could be conducted on a gram scale without affecting its stereoselectivities. In addition, the absolute stereochemistry of the products was determined by X-ray crystallographic analysis of 3ac, and a positive nonlinear effect was observed. Finally, a possible catalytic cycle was proposed to explain the origin of the enantioselectivity.

Ruthenium(0)-Catalyzed [4+2] Cycloaddition of Acetylenic Aldehydes with α-Ketols: Convergent Construction of Angucycline Ring Systems

Ruthenium(0) complexes modified by CyJohnPhos or RuPhos catalyze the successive C-C coupling of acetylenic aldehydes with α-ketols to form [4+2] cycloadducts as single diastereomers. This method enables convergent construction of type II polyketide ring systems of the angucycline class.

N-Heterocyclic carbene-catalyzed [4 + 2] cyclization of α,β-unsaturated carboxylic acids bearing γ-H with isatins: an enantioselective synthesis of spirocyclic oxindole–dihydropyranones

An NHC-catalyzed asymmetric [4 + 2] annulation of isatins and α,β-unsaturated carboxylic acids bearing γ-H gave spirocyclic oxindole–dihydropyranones successfully via an in situ activation strategy.

Dinuclear zinc-catalyzed desymmetric intramolecular aldolization: an enantioselective construction of spiro[cyclohexanone-oxindole] derivatives

A bis-ProPhenol zinc complex-catalyzed asymmetric desymmetrizing intramolecular aldol or aldol condensation reaction is reported, which furnishes spiro[cyclohexanone-oxindole] derivatives in good yields with moderate to high enantioselectivities.

Practical Pd(II)-Catalyzed C-H Alkylation with Epoxides: One-Step Syntheses of 3,4-Dihydroisocoumarins

Pd(II)-catalyzed ortho-alkylation of benzoic acids with both terminal and internal epoxides affords 3,4-dihydroisocoumarins in one step. The presence of potassium countercations is crucial for this reaction. Monoprotected amino acid ligands significantly promote this reaction, enabling the development of a practical C-H alkylation reaction using 0.5 mol % Pd catalyst. The inversion of stereochemistry in the C-H alkylation step is consistent with a redox-neutral SN2 nucleophilic ring-opening process as opposed to a Pd(II)/Pd(IV) pathway.

An Enantioselective Synthesis of Spirobilactams through Copper-Catalyzed Intramolecular Double N-Arylation and Phase Separation

Spirobicyclic structures are versatile building blocks for functional chiral molecules. An enantioselective synthesis of chiral spirobilactams via a copper-catalyzed double N-arylation was developed. Amplification of solution ee by in situ precipitation of the racemate was observed with this method and enantioenriched spirobilactams were obtained with excellent ee values through simple solid-solution phase separation.

Phosphine-catalyzed [4 + 2] cycloaddition of unsaturated pyrazolones with allenoates: a concise approach toward spiropyrazolones

Phosphine-catalyzed [4 + 2] cycloaddition of unsaturated pyrazolones with allenoates providing concise access to spiropyrazolones.

Trienamine catalysis with linear deconjugated 3,5-dienones

Deconjugated linear 3,5-dienones with substantial substitutions were used in β,ε-regioselective Diels–Alder cycloadditions with 3-olefinic oxindoles via trienamine catalysis of cinchona-based primary amines.

The psymberin story--biological properties and approaches towards total and analogue syntheses

Psymberin is a marine natural product which has attracted a great deal of interest since its isolation: While the highly cytotoxic compound was detected early on as an ingredient in a marine sponge, it took over a decade and 600 additional samples for the structure to eventually be assigned. In the last eight years fascinating synthetic and biosynthetic investigations have led to a more detailed understanding as well as a new starting point for structure-activity studies towards new antitumor compounds. The Review gives an in-depth insight into the progress in the field of the marine polyketide psymberin and demonstrates how organic synthesis is influencing neighboring scientific subjects.

Enantioselective Synthesis of Spiro[cyclohexane-1,3'-indolin]-2'-ones Containing Multiple Stereocenters via Organocatalytic Michael/Aldol Cascade Reactions

Organocatalytic reactions of 3-olefinic oxindoles and pentane-1,5-dial were investigated to provide access to substituted spirocyclohexane oxindoles via Michael/Aldol cascade reactions. Of particular interest, we have examined the stereochemical outcome of electron withdrawing and electron-donating groups on the oxindole ring nitrogen. Interestingly, we have observed that the N-protecting group on the oxindole has critical effect on aldol ring closure leading to ultimate stereochemical outcome of the hydroxyl center. The overall process is quite efficient and afforded products with multiple stereocenters in high yields and excellent enantioselectivities (>99% ee).

Biferrocene-Based Diphosphine Ligands: Synthesis and Application of Walphos Analogues in Asymmetric Hydrogenations

A total of four biferrocene-based Walphos-type ligands have been synthesized, structurally characterized, and tested in the rhodium-, ruthenium- and iridium-catalyzed hydrogenation of alkenes and ketones. Negishi coupling conditions allowed the biferrocene backbone of these diphosphine ligands to be built up diastereoselectively from the two nonidentical and nonracemic ferrocene fragments (R)-1-(N,N-dimethylamino)ethylferrocene and (S_Fc)-2-bromoiodoferrocene. The molecular structures of (S_Fc)-2-bromoiodoferrocene, the coupling product, two ligands, and the two complexes ([PdCl₂(L)] and [RuCl(p-cymene)(L)]PF₆) were determined by X-ray diffraction. The structural features of complexes and the catalysis results obtained with the newly synthesized biferrocene-based ligands were compared with those of the corresponding Walphos ligands.