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.

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”.

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

Chiral β-ketoesters bearing fully substituted carbon centers are important building blocks in organic synthesis. Mono-substituted ketoesters have been widely used to synthesize the above compounds through asymmetric additions or substitutions. The limitations of these protocols mainly exist in the substrate scopes, and α-methyl or α-fluoro-substituted β-ketoesters or acetyl acetates are frequently used owing to their relatively higher reactivity. To break through this limitation, we employed N-heterocyclic carbene-catalyzed kinetic resolution to achieve the access to enantioenriched β-ketoesters with quaternary stereocenters. Sterically more bulky groups such as benzyl, allyl, phenyl and cyclopropyl groups are all tolerated using this method.

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.