A convenient synthetic procedure for the preparation of chiral thiohydantoins derived from natural α‐amino acids is reported. Salient features of this method include ready availability of the starting materials, mild reaction conditions, and high yields of the desired products. The prepared thiohydantoins were tested as organocatalysts in asymmetric Michael addition reactions, under solvent‐free reaction conditions. Thiohydantoin 10 d turned out to be the most efficient catalyst, and its catalytic activity can be significantly enhanced by the use of an equimolar amount of salicylic acid as additive. Furthermore, highest diastereo‐ and enantioselectivity is achieved when the reaction is carried out at −5.5 °C, without detriment of the reaction rate. Regarding the reaction’s scope, the asymmetric Michael reaction catalyzed by 10 d was found to maintain its efficiency with various Michael acceptors.
Novel organocatalytic systems based on the recently developed (S)-proline derivative (2S)-[5-(benzylthio)-4-phenyl-(1,2,4-triazol)-3-yl]-pyrrolidine supported on mesoporous silica were prepared and their efficiency was assessed in the asymmetric aldol reaction. These materials were fully characterized by FT-IR, MS, XRD, and SEM microscopy, gathering relevant information regarding composition, morphology, and organocatalyst distribution in the doped silica. Careful optimization of the reaction conditions required for their application as catalysts in asymmetric aldol reactions between ketones and aldehydes afforded the anticipated aldol products with excellent yields and moderate diastereo- and enantioselectivities. The recommended experimental protocol is simple, fast, and efficient providing the enantioenriched aldol product, usually without the need of a special work-up or purification protocol. This approach constitutes a remarkable improvement in the field of heterogeneous (S)-proline-based organocatalysis; in particular, the solid-phase silica-bonded catalytic systems described herein allow for a substantial reduction in solvent usage. Furthermore, the supported system described here can be recovered, reactivated, and reused several times with limited loss in catalytic efficiency relative to freshly synthesized organocatalysts.
The synthesis of phenyl-γ-hydroxyketones through Heck coupling and subsequent opening of the tetrahydrofuran ring by the nucleophilic attack of a water molecule catalyzed by PdCl2·Gly2 under microwave irradiation.
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