Kinetic resolution of β-ketoesters with quaternary stereocenters via a carbene-catalyzed benzoin reaction

Total Synthesis of (+)-Hyacinthacine A1 Using a Chemoselective Cross-Benzoin Reaction and a Furan Photooxygenation-Amine Cyclization Strategy

We report the shortest synthesis of glycosidase inhibitor (+)-hyacinthacine A₁ using a highly chemoselective N-heterocyclic carbene-catalyzed cross-benzoin reaction as well as a furan photooxygenation-amine cyclization strategy. This is the first such cyclization on a furylic alcohol, an unprecedented reaction due to the notorious instability of the formed intermediates. The photooxygenation strategy was eventually incorporated into a three-step one-pot process that formed the requisite pyrrolizidine framework of (+)-hyacinthacine A₁.

N-Heterocyclic carbene (NHC)-catalyzed oxidative [3+2] annulation of dioxindoles and enals: mechanism, role of NHC, role of a mixture of bases with different strength, and origin of stereoselectivity

Over recent years, in-depth understanding of the mechanism of oxidative N-heterocyclic carbene (NHC) catalyzed reactions in the presence of a mild oxidant and the structure of key radical intermediates have been considered as an important challenge in organic chemistry. Furthermore, the role of using a mixture of bases with different strengths is unclear in NHC-catalyzed reactions. In this paper, the detailed competing oxidative mechanisms, origin of stereoselectivity, and role of the NHC-organocatalyst in the NHC-catalyzed reactions of dioxindoles with enals were studied using the density functional theory method. In addition, the roles of newly produced Brønsted acids of the applied bases, i.e.DBU·H+ and DABCO·H+, are examined. The computational results indicated that the oxidation of the Breslow intermediate by nitrobenzene (NB) occurs first through a hydrogen atom transfer (HAT) pathway from the Breslow intermediate, and then it is oxidized into acyl azolium by single electron transfer (SET). We found that the energy barrier of the proton transfer processes is remarkably reduced by the conjugated Brønsted acid of the weaker base in the solution. Further, the calculated results revealed that the NHC catalyst has different behavior before and after the oxidation of the Breslow intermediate in these reactions. Before oxidation, the nucleophilicity of R1 increased by adding R1 to NHC, while, after the oxidation process, the electrophilicity of R1 increases, and as a result the product of oxidation, α, β unsaturated acyl azolium, acts as an electrophile. This mechanistic study paves the way for the rational design of oxidative NHC-catalyzed reactions.

Minimization of Amounts of Catalyst and Solvent in NHC-Catalyzed Benzoin Reactions of Solid Aldehydes: Mechanistic Consideration of Solid-to-Solid Conversion and Total Synthesis of Isodarparvinol B

Attempts were made to minimize the amounts of catalyst and solvent in the NHC-catalyzed benzoin reactions of solid aldehydes. In some case, solid-to-solid conversions proceeded in the solvent-free NHC-catalyzed benzoin reactions. Even if a mixture of the substrate, N-heterocyclic carbene (NHC) precursor, and inorganic base was initially a powdery solid, the reaction did proceed at reaction temperature lower than the melting points of each compound. The solid mixture partially melted or became a slurry or suspension in the meantime. We call this solid/liquid mixture a semisolid state. The reaction giving an optically active product was faster than that giving a racemic mixture of the same product. Melting-point depression was observed for a series of mixtures of the substrate and product in different substrate/product ratios. Solvent-free solid-to-solid conversions were accelerated by the formation of a semisolid state resulting from the melting-point depression of the solid substrate accompanied by the product formation. In the case of solid substrates with high melting points, melting-point depression was useless, and the addition of a small amount of solvent was needed. The first total synthesis of isodarparvinol B was achieved via the NHC-catalyzed intramolecular benzoin reaction using a small amount of solvent as an additive.

Construction of 1-Tetralols Bearing Two Contiguous Quaternary Chiral Centers through a Rhodium-Catalyzed Enantioselective Desymmetrization Cascade Reaction

A novel and efficient access to polyfunctionnalized chiral 1-tetralols, bearing two contiguous quaternary carbon stereocenters, has been developed from various and easily accessible alkynyl-1,3-diketones, through a cascade process including a regioselective alkyne insertion, a 1,4-Rh shift, and a nucleophilic addition step via the desymmetrization of the 1,3-diketone moiety thanks to an appropriate rhodium-chiral diene complex in the presence of arylboronic acids.

Asymmetric catalytic construction of fully substituted carbon stereocenters using acyclic α-branched β-ketocarbonyls: the “Methyl Rule” widely exists

This review illustrates the recent advances in catalytic asymmetric α-functionalization of acyclic β-ketocarbonyls. A thorough survey of all these reactions indicates the existance of a general principle which is called the “Methyl Rule”.

Synthesis of chromans and kinetic resolution of 2-aryl-3-nitro-2H-chromenes via the NHC-bound azolium homoenolate pathway

Herein, an effective kinetic resolution (KR) of racemate 2-aryl-3-nitro-2H-chromenes to afford synthetically valuable chromans and 2H-chromenes through an NHC-bound acyl azolium homoenolate pathway has been demonstrated.