Enantioselective alkylation using a new C3 symmetric amine-based chiral phase-transfer catalyst

Ion Pair Catalyst - Pentanidinium

In this account, we further describe our already developed N-sp² hybrid guanidinium as an efficient phase-transfer catalyst and ion pair catalysis based on N-sp² hybrid pentanidinium and its application in some new reactions. The sp³ hybrid quaternary ammonium salt has a tetrahedral structure, which means that three sides of it can be effectively steric, allowing the remaining side to be close to the substrate. However, the sp² hybrid ammonium salt allows the substrate to form ion pairs from both directions respectively, so it is a greater challenge to control the stereoselectivity of the reaction. Van der Waals forces, such as hydrogen bonds and π - π ${\pi -\pi }$ interactions, have been used to make electrophiles approach from a certain direction, leading to a higher enantioselectivity. Based on the above idea, we designed an N-sp² hybrid phase-transfer catalyst, pentanidinium. Pentanidinium has five conjugated nitrogen atoms, one of which has a formal positive charge, which is necessary for it to become an ion pair catalyst. We have confirmed that pentanidinium can catalyze α-hydroxylation of 3-substituted-2-oxindoles, Michael addition of 3-alkyloxindoles with vinyl sulfone, and alkylation reactions of sulfenate anions and dihydrocoumarins, desymmetrization of pro-chiral sulfinate to afford enantioenriched sulfinate esters. Pentanidinium with side chain structure changes can also be catalyzed efficiently with enantioconvergent halogenophilic nucleophilic substitution, including azidation and thioesterification. In the reaction catalyzed by pentanidinium, it always attracts us with the advantages of low catalytic load and good enantioselectivity.

A systematic investigation of quaternary ammonium ions as asymmetric phase-transfer catalysts. Synthesis of catalyst libraries and evaluation of catalyst activity

Despite over three decades of research into asymmetric phase-transfer catalysis (APTC), a fundamental understanding of the factors that affect the rate and stereoselectivity of this important process are still obscure. This paper describes the initial stages of a long-term program aimed at elucidating the physical organic foundations of APTC employing a chemoinformatic analysis of the alkylation of a protected glycine imine with libraries of enantiomerically enriched quaternary ammonium ions. The synthesis of the quaternary ammonium ions follows a diversity-oriented approach wherein the tandem inter[4 + 2]/intra[3 + 2] cycloaddition of nitroalkenes serves as the key transformation. A two-part synthetic strategy comprised of (1) preparation of enantioenriched scaffolds and (2) development of parallel synthesis procedures is described. The strategy allows for the facile introduction of four variable groups in the vicinity of a stereogenic quaternary ammonium ion. The quaternary ammonium ions exhibited a wide range of activity and to a lesser degree enantioselectivity. Catalyst activity and selectivity are rationalized in a qualitative way on the basis of the effective positive potential of the ammonium ion.

Design of Binaphthyl-Modified Symmetrical Chiral Phase-Transfer Catalysts: Substituent Effect of 4,4′,6,6′-Positions of Binaphthyl Rings in the Asymmetric Alkylation of a Glycine Derivative

A series of symmetrical chiral phase-transfer catalysts with 4,4',6,6'-tetrasubstituted binaphthyl units have been designed, and these aryl- and trialkylsilyl-substituted phase-transfer catalysts, which included a highly fluorinated catalyst, were prepared. The chiral efficiency of these chiral phase-transfer catalysts was investigated in the asymmetric alkylation of tert-butylglycinate-benzophenone Schiff base under mild phase-transfer conditions, and the eminent substituent effect of the 4,4',6,6'-positions of the binaphthyl units on enantioselection was observed. In particular, the OctMe2Si-substituted catalyst was found to be highly efficient for the phase-transfer alkylation of tert-butylglycinate-benzophenone Schiff base with various alkyl halides, including sec-alkyl halides. The highly fluorinated catalyst was also utilized as a recyclable chiral phase-transfer catalyst by simple extraction with fluorous solvents.

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.

Catalytic asymmetric phase-transfer Michael reaction and Mannich-type reaction of glycine Schiff bases with tartrate-derived diammonium salts

Catalytic asymmetric Michael and Mannich-type reactions of glycine Schiff bases with chiral two-center organocatalysts, tartrate-derived diammonium salts (TaDiASs), are described. On the basis of conformational studies, optimized TaDiASs with a 2,6-disubstituted cyclohexane spiroacetal were newly designed. These TaDiASs catalyzed the asymmetric Michael and Mannich-type reactions of glycine Schiff bases with higher enantioselectivity than previous catalysts. In the Mannich-type reaction, aromatic N-Boc-protected imines (Boc = tert-butoxycarbonyl) as well as enolizable alkyl imines were applicable. As a synthetic application of the catalytic asymmetric Mannich-type reaction with the optimized TaDiASs, we developed a catalytic asymmetric total synthesis of (+)-nemonapride, which is an antipsychotic agent.

Mechanistic studies on the asymmetric alkylation of amino ester enolates using a copper(ii)salen catalyst

Hammett data indicate that the asymmetric alkylation of enolates catalysed by copper(II)salen complex 1, proceeds by an asynchronous S(N)2 reaction and that the role of the catalyst is to enhance the nucleophilicity of the enolate.

Enantioselective syntheses and biological studies of aeruginosin 298-A and its analogs: application of catalytic asymmetric phase-transfer reaction

Aeruginosin 298-A was isolated from the freshwater cyanobacterium Microcystis aeruginosa (NIES-298) and is an equipotent thrombin and trypsin inhibitor. A variety of analogs were synthesized to gain insight into the structure-activity relations. We developed a versatile synthetic process for aeruginosin 298-A as well as several attractive analogs, in which all stereocenters were controlled by catalytic asymmetric phase-transfer reaction promoted by two-center asymmetric catalysts and catalytic asymmetric epoxidation promoted by a lanthanide-BINOL complex. Furthermore, serine protease inhibitory activities of aeruginosin 298-A and its analogs were examined.

Enantioselective Total Syntheses of Several Bioactive Natural Products Based on the Development of Practical Asymmetric Catalysis

I present herewith enantioselective total syntheses of several bioactive natural products, such as (-)-strychnine, (+)-decursin, (-)-cryptocaryolone diacetate, (-)-fluoxetine, and aeruginosin 298-A, based on practical asymmetric catalyses (Michael reaction, epoxidation, and phase-transfer reaction) that I developed with co-workers in Prof. Shibasaki's group over the past 5 years. In the first part of this review, I discuss the great improvement of catalyst efficiency in an ALB-catalyzed asymmetric Michael reaction of malonate and application to the pre-manufacturing scale (greater than kilogram scale) and enantioselective total synthesis of (-)-strychnine with the development of novel domino cyclization. To broaden the substrate generality of the Michael reaction, we developed a highly stable, storable, and reusable La-O-linked-BINOL complex. Further extension of the reaction using beta-keto ester as a Michael donor was achieved with the development of a La-NR-linked-BINOL complex, thereby improving indole alkaloid syntheses. In the second section, I discuss enantioselective total synthesis of (+)-decursin using catalytic asymmetric epoxidation. To achieve the synthesis, we developed a new La-BINOL-Ph(3)As = O (1:1:1) complex catalyst system, which has much higher reactivity and broader substrate generality than the previously developed catalyst systems. This allowed us to achieve catalytic asymmetric epoxidation of alpha,beta-unsaturated carboxylic acid derivatives with high enantioselectivity and broad substrate generality for the first time by changing the lanthanide metal and reaction conditions. Among them, catalytic asymmetric epoxidation of alpha,beta-unsaturated morpholinyl amides is quite useful in terms of synthetic utility of the corresponding alpha,beta-epoxy morpholinyl amides. Highly catalyst-controlled enantio- or diastereoselective epoxidation of the alpha,beta-unsaturated morpholinyl amides, coupled with diastereoselective reduction of beta-hydroxy ketones, enabled the synthesis of all possible stereoisomers of 1,3-polyol arrays with successful enantioselective total synthesis of several 1,3-polyol natural products, such as (-)-cryptocaryolone diacetate. In addition, the development of a new regioselective epoxide-opening reaction of alpha,beta-epoxy amides to the corresponding alpha- and beta-hydroxy amides enhanced the usefulness of the present epoxidation and was applied to the enantioselective total synthesis of (-)-fluoxetine. In the final section, I report the development of a new asymmetric two-center organocatalyst (TaDiAS) and its application to the enantioselective synthesis of aeruginosin 298-A and its analogues. Because of the remarkable structural diversity of TaDiAS, a practical asymmetric phase-transfer reaction with broad substrate generality was achieved. As a result, we succeeded in developing a highly versatile synthetic method for aeruginosin 298-A and its analogues. Inhibitory activity studies of the compounds against the serine protease trypsin provided preliminary information about their structure-activity relations.