Catalytic asymmetric synthesis of 1,2-diamines

The asymmetric catalytic synthesis of 1,2-diamines has received considerable interest, especially in the last ten years, due to their presence in biologically active compounds and their applications for the development of synthetic building blocks, chiral ligands and organocatalysts. Synthetic strategies based on C-N bond-forming reactions involve mainly (a) ring opening of aziridines and azabenzonorbornadienes, (b) hydroamination of allylic amines, (c) hydroamination of enamines and (d) diamination of olefins. In the case of C-C bond-forming reactions are included (a) the aza-Mannich reaction of imino esters, imino nitriles, azlactones, isocyano acetates, and isothiocyanates with imines, (b) the aza-Henry reaction of nitroalkanes with imines, (c) imine-imine coupling reactions, and (d) reductive coupling of enamines with imines, and (e) [3+2] cycloaddition with imines. C-H bond forming reactions include hydrogenation of CN bonds and C-H amination reactions. Other catalytic methods include desymmetrization reactions of meso-diamines.

Catalytic Kinetic Resolution and Desymmetrization of Amines

Optically active amines represent critically important subunits in bioactive natural products and pharmaceuticals, as well as key scaffolds in chiral catalysts and ligands. Kinetic resolution of racemic amines and enantioselective desymmetrization of prochiral amines have proved to be efficient methods to access enantioenriched amines, especially when the racemic or prochiral amines were easy to prepare while the chiral ones are difficult to be accessed directly. In this review, we systematically summarized the development of kinetic resolution and desymmetrization of amines through nonenzymatic asymmetric catalytic approaches in the last two decades.

New Strategy To Access Enantioenriched Cyclohexadienones: Kinetic Resolution of para-Quinols by Organocatalytic Thiol-Michael Addition Reactions

Existing stereoselective routes to 2,5-cyclohexadienones involve either desymmetrization of an achiral substrate or have attempted to perform an asymmetric dearomatization of a phenol. Herein, we report proof-of-principle experiments aimed at developing a kinetic resolution as an alternative method for accessing enantioenriched 2,5-cyclohexadienones. More specifically, chiral bifunctional thiourea catalysts were used to promote the addition of 2-thionapthalene into unsymmetric para-quinols. The selectivity of the kinetic resolution was found to be quite sensitive to substitution around the substrate.

Effective Guest Inclusion by a 6-O-Modified β-Cyclodextrin Dimer in Organic Solvents

A 6-O-tert-butyldimethylsilylated β-cyclodextrin (TBDMS-β-CD) dimer, in which two TBDMS-β-CD rings are connected in a head-to-head fashion by a m-xylylene linker, effectively forms inclusion complexes with pyrene and naphthalene in nonpolar organic solvents such as cyclohexane and benzene. This TBDMS-β-CD dimer shows a higher inclusion ability toward these guests than a TBDMS-β-CD dimer bearing a p-xylylene linker due to the greater cooperation of the two TBDMS-β-CD rings for the guest inclusion. Unlike the corresponding monomer, the TBDMS-β-CD dimer bearing a m-xylylene linker is also a good host even in polar organic solvents such as tetrahydrofuran. High chiral recognition of aromatic amines and alcohol is realized by utilizing inclusion within the cavity of the TBDMS-β-CD dimer in cyclohexane. In particular, an extremely high binding selectivity for (S)-1-(1-naphthyl)ethylamine and (S)-1-(1-naphthyl)ethanol over the corresponding (R)-isomers is achieved. Moreover, by utilizing the high chiral recognition with the TBDMS-β-CD dimer in cyclohexane, a non-enzymatic kinetic resolution of racemic 1-(1-naphthyl)ethylamine via enantioselective N-benzoylation is attained with an enantiomer excess of up to 87 % and an s-factor of 15.

Chiral Thioureas Promote Enantioselective Pictet-Spengler Cyclization by Stabilizing Every Intermediate and Transition State in the Carboxylic Acid-Catalyzed Reaction

An investigation of the mechanism of benzoic acid/thiourea co-catalysis in the asymmetric Pictet-Spengler reaction is reported. Kinetic, computational, and structure-activity relationship studies provide evidence that rearomatization via deprotonation of the pentahydro-β-carbolinium ion intermediate by a chiral thiourea·carboxylate complex is both rate- and enantioselectivity-determining. The thiourea catalyst induces rate acceleration over the background reaction mediated by benzoic acid alone by stabilizing every intermediate and transition state leading up to and including the final selectivity-determining step. Distortion-interaction analyses of the transition structures for deprotonation predicted using density functional theory indicate that differential π-π and C-H···π interactions within a scaffold organized by multiple hydrogen bonds dictate stereoselectivity. The principles underlying rate acceleration and enantiocontrol described herein are expected to have general implications for the design of selective transformations involving deprotonation of high-energy intermediates.

Enantioselective acyl transfer catalysis by a combination of common catalytic motifs and electrostatic interactions

Catalysts that can promote acyl transfer processes are important to enantioselective synthesis and their development has received significant attention in recent years. Despite noteworthy advances, discovery of small-molecule catalysts that are robust, efficient, recyclable and promote reactions with high enantioselectivity can be easily and cost-effectively prepared in significant quantities (that is, >10 g) has remained elusive. Here, we demonstrate that by attaching a binaphthyl moiety, appropriately modified to establish H-bonding interactions within the key intermediates in the catalytic cycle, and a 4-aminopyridyl unit, exceptionally efficient organic molecules can be prepared that facilitate enantioselective acyl transfer reactions. As little as 0.5 mol% of a member of the new catalyst class is sufficient to generate acyl-substituted all-carbon quaternary stereogenic centres in quantitative yield and in up to 98:2 enantiomeric ratio (er) in 5 h. Kinetic resolution or desymmetrization of 1,2-diol can be performed with high efficiency and enantioselectivity as well.

Nucleophilic catalysts are widely used for acyl transfer reactions, but chiral variants can be difficult to design or synthesise. Here, the authors report catalysts with chirality imparted from a binaphtyl moiety with tert-alcohol unit that show both high activity and enantioselectivity for a range of acyl transfer reactions.

Organocatalytic, diastereo- and enantioselective synthesis of nonsymmetric cis-stilbene diamines: a platform for the preparation of single-enantiomer cis-imidazolines for protein-protein inhibition

The finding by scientists at Hoffmann-La Roche that cis-imidazolines could disrupt the protein–protein interaction between p53 and MDM2, thereby inducing apoptosis in cancer cells, raised considerable interest in this scaffold over the past decade. Initial routes to these small molecules (i.e., Nutlin-3) provided only the racemic form, with enantiomers being enriched by chromatographic separation using high-pressure liquid chromatography (HPLC) and a chiral stationary phase. Reported here is the first application of an enantioselective aza-Henry approach to nonsymmetric cis-stilbene diamines and cis-imidazolines. Two novel mono(amidine) organocatalysts (MAM) were discovered to provide high levels of enantioselection (>95% ee) across a broad range of substrate combinations. Furthermore, the versatility of the aza-Henry strategy for preparing nonsymmetric cis-imidazolines is illustrated by a comparison of the roles of aryl nitromethane and aryl aldimine in the key step, which revealed unique substrate electronic effects providing direction for aza-Henry substrate–catalyst matching. This method was used to prepare highly substituted cis-4,5-diaryl imidazolines that project unique aromatic rings, and these were evaluated for MDM2-p53 inhibition in a fluorescence polarization assay. The diversification of access to cis-stilbene diamine-derived imidazolines provided by this platform should streamline their further development as chemical tools for disrupting protein–protein interactions.

Conjugate-base-stabilized Brønsted acids: catalytic enantioselective Pictet-Spengler reactions with unmodified tryptamine

A conjugate-base-stabilized Brønsted acid facilitates catalytic enantioselective Pictet-Spengler reactions with unmodified tryptamine. The chiral carboxylic acid catalyst is readily assembled in just two steps and enables the formation of β-carbolines with up to 92% ee. Achiral acid additives or in situ Boc-protection facilitate catalyst turnover.

Preparation of (-)-Nutlin-3 using enantioselective organocatalysis at decagram scale

Chiral nonracemic cis-4,5-bis(aryl)imidazolines have emerged as a powerful platform for the development of cancer chemotherapeutics, stimulated by the Hoffmann-La Roche discovery that Nutlin-3 can restore apoptosis in cells with wild-type p53. The lack of efficient methods for the enantioselective synthesis of cis-imidazolines, however, has limited their more general use. Our disclosure of the first enantioselective synthesis of (-)-Nutlin-3 provided a basis to prepare larger amounts of this tool used widely in cancer biology. Key to the decagram-scale synthesis described here was the discovery of a novel bis(amidine) organocatalyst that provides high enantioselectivity at warmer reaction temperature (-20 °C) and low catalyst loadings. Further refinements to the procedure led to the synthesis of (-)-Nutlin-3 in a 17 g batch and elimination of all but three chromatographic purifications.

Chiral recognition and kinetic resolution of aromatic amines via supramolecular chiral nanocapsules in nonpolar solvents

Herein we report the first example of chiral recognition and kinetic resolution of aromatic amine guests using supramolecular nanocapsules assembled from cyclodextrin derivatives in nonpolar media. With these nanocapsules, an extremely high chiral recognition of 1-(1-naphthyl)ethylamine (1) in cyclohexane was achieved, with a binding selectivity of up to 41 for (S)-1 over (R)-1. In addition, kinetic resolution of 1 through enantioselective N-acylation was accomplished with an enantiomeric excess of up to 91%.