Optically pure nitrogenous compounds, and especially nitrogen-containing heterocycles, have drawn intense research attention because of their frequent isolation as natural products. These compounds have wide-ranging biological and pharmaceutical activities, offering potential as new drug candidates. Among the various synthetic approaches to nitrogenous heterocycles, the use of asymmetric multicomponent reactions (MCRs) catalyzed by chiral phosphoric acids has recently emerged as a particularly robust tool. This method combines the prominent merits of MCRs with organocatalysis, thus affording enantio-enriched nitrogenous heterocyclic compounds with excellent enantioselectivity, atom economy, bond-forming efficiency, structural diversity, and complexity. In this Account, we discuss a variety of asymmetric MCRs catalyzed by chiral phosphoric acids that lead to the production of structurally diverse nitrogenous heterocycles. In MCRs, three or more reagents are combined simultaneously to produce a single product containing structural contributions from all the components. These one-pot processes are especially useful in the construction of heterocyclic cores: they can provide a high degree of both complexity and diversity for a targeted set of scaffolds while minimizing the number of synthetic operations. Unfortunately, enantioselective MCRs have thus far been relatively underdeveloped. Particularly lacking are reactions that proceed through imine intermediates, which are formed from the condensation of carbonyls and amines. The concomitant generation of water in the condensation reaction can deactivate some Lewis acid catalysts, resulting in premature termination of the reaction. Thus, chiral catalysts typically must be compatible with water for MCRs to generate nitrogenous compounds. Recently, organocatalytic MCRs have proven valuable in this respect. Brønsted acids, an important class of organocatalysts, are highly compatible with water and thereby offer great potential as chiral catalysts for multicomponent protocols that unavoidably release water molecules during the course of the reaction. We present a detailed investigation of several MCRs catalyzed by chiral phosphoric acids, including Biginelli and Biginelli-like reactions; 1,3-dipolar cycloadditions; aza Diels-Alder reactions; and some other cyclization reactions. These approaches have enabled the facile preparation of 3,4-dihydropyrimidinones, pyrrolidines, piperidines, and dihydropyridines with high optical purity. The synthetic applications of these new protocols are also discussed, together with theoretical studies of the reaction transition states that address the regio- and stereochemistry. In addition, we briefly illustrate the application of a recently developed strategy that involves relay catalysis by a binary system consisting of a chiral phosphoric acid and a metal complex. This technique has provided access to new reactions that generate structurally diverse and complex heterocycles. Enantioselective organocatalytic MCRs remain a challenge, b...

Novel organic molecules containing an l-proline amide moiety and a terminal hydroxyl for catalyzing direct asymmetric aldol reactions of aldehydes in neat acetone are designed and prepared. Catalyst 3d, prepared from l-proline and (1S,2S)-diphenyl-2-aminoethanol, exhibits high enantioselectivities of up to 93% ee for aromatic aldehydes and up to >99% ee for aliphatic aldehydes. A theoretical study of transition structures demonstrates the important role of the terminal hydroxyl group in the catalyst in the stereodiscrimination. Our results suggest a new strategy in the design of new organic catalysts for direct asymmetric aldol reactions and related transformations because plentiful chiral resources containing multi-hydrogen bond donors, for example, peptides, might be adopted in the design.

The privileged spiro[pyrrolidin-3,3'-oxindole] derivatives exhibit important biological activities. An enantioselective organocatalytic approach to the rapid synthesis of spiro[pyrrolidin-3,3'-oxindole] derivatives with high enantiopurity and structural diversity is described. The asymmetric catalytic three-component 1,3-dipolar cycloaddition of a broad range of methyleneindolinones with aldehydes and amino esters in the presence of chiral phosphoric acid provides spirooxindole derivatives in high yield with unusual regiochemistry and excellent stereoselectivities (up to 98% ee) under mild conditions. The straightforward construction of spirooxindole skeletons with high stereo- and regioselectivity suggests a new avenue to medicinal chemistry and diversity-oriented synthesis. Theoretical calculations disclosed that both the azomethine ylide and the methyleneindolinone are hydrogen-bonded with the phosphoric acid, which accounted for the high enantio- and regioselectivity and indicated that the unusual regioselectivity results from the stabilization stemming from the favorable pi-pi stacking interaction between the oxo-indole ring and the conjugated esters.

L-proline amides derived from various chiral beta-amino alcohols that bear substituents with various electron natures at their stereogenic centers are prepared and evaluated for catalyzing the direct Aldol reaction of 4-nitrobenzaldehyde with acetone. Catalysts with strong electron-withdrawing groups are found to exhibit higher catalytic activity and enantioselectivity than their analogues with electron-donating groups. The presence of 2 mol % catalyst 4g significantly catalyzes the direct Aldol reactions of a wide range of aldehydes with acetone and butanone, to give the beta-hydroxy ketones with very high enantioselectivities ranging from 96% to >99% ee. High diastereoselectivity of 95/5 was observed for the anti Aldol product from the reaction of cyclohexanone, and excellent enantioselectivity of 93% ee was provided for anti Aldol product from the reaction of cyclopentanone.

Consecutive hydroamination/asymmetric transfer hydrogenation under relay catalysis of an achiral gold complex/chiral Brønsted acid binary system has been described for the direct transformation of 2-(2-propynyl)aniline derivatives into tetrahydroquinolines with high enantiomeric purity.

A series of binol- and H8-binol-based phosphoric acids have for the first time been evaluated for their ability to catalyze Biginelli reactions of aldehydes, thiourea or urea, and beta-keto esters. A new chiral phosphoric acid, derived from 3,3'-diphenyl-H8-binol, exhibited superior catalytic activity and enantioselectivity compared to its structural analogues, affording high enantioselectivities ranging from 85 to 97% ee with a wide scope of substrates. A metal-free preparation of optically active monastrol was achieved on the basis of the current process.

Asymmetric catalysis has been considered to be the most intriguing means for building collections of functionalized optically active compounds. In particular, metal and organocatalysis have been well established to allow many fundamentally different reactions. Metal catalysis has enabled the participation of a much broader scope of chemical bonds in organic transformations than are allowed by organocatalysis, while organocatalysis permits a broader scope of functional groups to undergo a diverse range of enantioselective transformations, individually, simultaneously, or sequentially. Theoretically, the combination of organocatalysts and metal complexes could probably render new transformations through the simultaneous or sequential activation and reorganization of multiple chemical bonds if the superior features of both the catalysts are adopted. In 2001, both our research group and Takemoto's group separately described an asymmetric allylation of glycine imino esters with allyl acetate catalyzed by palladium complexes and chiral ammonium salts. In these cases, the oxidative addition of palladium complexes to allyl acetate formed the π-allylic fragments, while the chiral ammonium salts were actually responsible for controlling the stereoselectivity. These reactions in fact marked the beginning of asymmetric organo/metal combined catalysis. Since then, asymmetric organocatalysis combined with metal catalysis, including cooperative catalysis, relay catalysis, and sequential catalysis, has been a versatile concept for the creation of unknown organic transformations. Sequential catalysis describes a one-pot reaction involving two or more incompatible catalytic cycles. Alternatively, cooperative and relay catalyses require high compatibility of principally distinct catalysts and will be the focus of this Account. The catalysts in cooperative catalytic reactions must be able to simultaneously and individually activate both substrates to drive a bond-forming reaction, while relay catalysis is basically defined as a cascade process in which two or more sequential bond-forming transformations are independently catalyzed by distinct catalysts. In the past decade, we have discovered a variety of binary catalytic systems consisting of metals, including Rh(II), Pd(0), Au(I), and Mg(II), and chiral organocatalysts, including chiral phosphoric acids and quinine-based bifunctional molecules, for cooperative catalysis and relay catalysis, allowing the accomplishment of many unprecedented asymmetric transformations. In this Account, these achievements will be summarized, particularly focusing on the description of the concept and proof of the concept, to demonstrate the robustness of combined organo/metal catalysis in the creation of efficient enantioselective transformations. In addition, elegant studies from other laboratories using chiral phosphoric acid/Au(I) for the establishment of asymmetric cascade reactions involving the carbon-carbon triple bond functionality and typical combined organo/metal catalytic systems, very rece...

L-Prolinamides 2, prepared from L-proline and simple aliphatic and aromatic amines, have been found to be active catalysts for the direct aldol reaction of 4-nitrobenzaldehyde with neat acetone at room temperature. They give moderate enantioselectivities of up to 46% enantiomeric excess (ee). The enantioselectivity increases as the amide NOH becomes a better hydrogen bond donor. L-Prolinamides 3, derived from the reaction of L-proline with ␣,␤-hydroxyamines such that there is a terminal hydroxyl group, show more efficient catalysis and higher enantioselectivities. In particular, catalyst 3h, prepared from L-proline and (1S,2S)-diphenyl-2-aminoethanol, exhibits high enantioselectivities of up to 93% ee for aromatic aldehydes and up to >99% ee for aliphatic aldehydes under ؊25°C. Model reactions of benzaldehyde with three enamines derived from the condensation of prolinamides with acetone have been studied by quantum mechanics calculations. The calculations reveal that the amide NOH and the terminal hydroxyl groups form hydrogen bonds with the benzaldehyde substrate. These hydrogen bonds reduce the activation energy and cause high enantioselectivity. Our results suggest a new strategy in the design of new organic catalysts for direct asymmetric aldol reactions and related transformations. The asymmetric aldol reaction is one of the most powerful methods for the construction of complex chiral polyol architectures (1). The great synthetic usefulness of the aldol reaction in organic synthesis has powered a rapid evolution of numerous highly enantioselective chiral catalysts (2). In general, asymmetric catalytic aldol reactions are classified into two main categories. One type of aldol reaction requires the preconversion of ketone or ester to a more active aldol donor, such as an enol ether or a ketene acetal by the use of a chiral Lewis acid (2) or Lewis base (3, 4) as the catalyst. The other type is called a direct aldol reaction, which is highly atomically economic (5). The development of efficient catalysts for asymmetric catalytic direct aldol reactions is a worthwhile endeavor. The first example of the asymmetric direct aldol reaction catalyzed by heterobimetallic complexes was reported by Shibasaki and coworkers (6, 7). Trost and coworkers (8, 9) have designed a zinc complex for the direct catalytic asymmetric aldol reaction with high enantioselectivities. Since the pioneering finding by List, Barbas, and their coworkers (10, 11) that L-proline could act as a catalyst in intermolecular direct aldol reaction, the concept of small organic molecules as catalysts has received increasing attention (12-32). However, efficient organic catalysts other than chiral amino acids for asymmetric direct aldol reactions are scarce (33,34). We seek to design small organic molecules with structural diversity for catalyzing organic transformations with high stereoselectivity and broad substrates. Here, we report on a class of organic catalysts, (S)-pyrrolidine-2-carboxamides (L-prolinamides), that efficiently catalyze the direct ...