Highly Enantioselective Synthesis of (R)-α-Alkylserines via Phase-Transfer Catalytic Alkylation of o-Biphenyl-2-oxazoline-4-carboxylic Acid tert-Butyl Ester Using Cinchona-Derived Catalysts

A catalytic highly enantioselective allene approach to oxazolines

A completely different approach for the asymmetric construction of oxazoline derivatives with an excellent enantioselectivity via the palladium-catalyzed coupling-cyclization of N-(buta-2,3-dienyl)amides with aryl iodides has been reported.

Oxazolines are a very important class of heterocyclic compounds. However, catalytic enantioselective syntheses are very limited. Here, a highly enantioselective palladium-catalyzed coupling-cyclization of readily available N-(buta-2,3-dienyl) amides with aryl or 1-alkenyl iodides has been developed for the asymmetric construction of oxazoline derivatives. Many synthetically useful functional groups are tolerated in this reaction. The absolute configuration of the chiral center in the products has been established by X-ray diffraction study. A model for prediction of the absolute configuration of the chiral center in the products from this cyclic enantioselective nucleophilic allylation has been proposed. The synthetic potentials based on the unique structure of the products formed have also been demonstrated.

Efficient synthesis of oxazoles by dirhodium(II)-catalyzed reactions of styryl diazoacetate with oximes

An efficient one-step synthesis of multi-functionalized oxazole derivatives is achieved in high yield by dirhodium(II)-catalyzed reactions of styryl diazoacetate with aryl oximes.

Cinchona-based phase-transfer catalysts for asymmetric synthesis

Phase-transfer catalysis is one of the most useful methodologies for practical syntheses given its operational simplicity and mild reaction conditions that enable its application in industrial processes. Cinchona alkaloids have been a popular, natural source of practical organocatalysts due largely to their excellent commercial availability and low cost. Since the first Cinchona alkaloid-derived phase-transfer catalysts was disclosed in 1981, diverse generations of Cinchona-derived phase-transfer catalysts have been developed and successfully applied to various asymmetric syntheses. In this feature article, we describe the generation of Cinchona-derived chiral phase-transfer catalysts according to the development stages and our efforts toward the design of polymeric Cinchona phase-transfer catalysts, the effects of the electronic functional group incorporated in the catalysts, and their application in asymmetric organic reactions.

Recent advances in asymmetric phase-transfer catalysis

The use of chiral nonracemic onium salts and crown ethers as effective phase-transfer catalysts have been studied intensively primarily for enantioselective carbon-carbon or carbon-heteroatom bond-forming reactions under mild biphasic conditions. An essential issue for optimal asymmetric catalysis is the rational design of catalysts for targeted reaction, which allows generation of a well-defined chiral ion pair that reacts with electrophiles in a highly efficient and stereoselective manner. This concept, together with the synthetic versatility of phase-transfer catalysis, provides a reliable and general strategy for the practical asymmetric synthesis of highly valuable organic compounds.

Enantioselective Synthesis of (R)- and (S)-α-Alkylcysteines via Phase-Transfer Catalytic Alkylation

We reported efficient enantioselective synthetic methodologies for (R)-alpha-alkylcysteines and (S)-alpha-alkylcysteines. The phase-transfer catalytic alkylation of 2-phenyl-2-thiazoline-4-carboxylic acid tert-butyl ester and 2-o-biphenyl-2-thiazoline-4-carboxylic acid tert-butyl ester, in the presence of chiral catalysts (1 or 2), gave the corresponding alkylated products, which could be hydrolyzed to provide (R)-alpha-alkylcysteines (67->99% ee) and (S)-alpha-alkylcysteines (66-88% ee), respectively.

Highly Enantioselective Synthesis of (2S)-α-(Hydroxymethyl)-glutamic Acid by the Catalytic Michael Addition of 2-Naphthalen-1-yl-2-oxazoline-4-carboxylic Acid tert-Butyl Ester

[reaction: see text]. Highly enantioselective synthesis of a potent metabotropic receptor ligand, (2S)-alpha-(hydroxymethyl)-glutamic acid (2, HMG) was accomplished by the catalytic Michael addition of 2-naphthalen-1-yl-2-oxazoline-4-carboxylic acid tert-butyl ester (3b), using the phosphazene base, BEMP, in CH(2)Cl(2) at -60 degrees C in the presence of (S)-binaphthyl quaternary ammonium salt 4.