Chiral Secondary Amine Research Articles

Enantiodivergence by minimal modification of an acyclic chiral secondary aminocatalyst

The development of enantiodivergent catalysis for the preparation of both enantiomers of a chiral compound is of importance in pharmaceutical and bioorganic chemistry. With the design of a class of reactive and stereoselective organocatalysts, acyclic chiral secondary amines, a method for achieving the enantiodivergence is developed simply by changing the secondary N-i-Bu- to N-Me-group within the catalyst architecture while maintaining the same absolute configuration of the catalysts, which modulates the catalyst conformation. This catalyst-controlled enantiodivergent method not only enables challenging asymmetric transformations to occur in an enantiodivergent manner but also features a high level of stereocontrol and broad scope that is demonstrated in eight different reactions (90 examples), all delivering both enantiomers of a range of structurally diverse products including hitherto less accessible, yet important, compounds in good yields with high stereoselectivities.

Evolved enzyme improves route to drug candidate

Chemists at the pharmaceutical company GlaxoSmithKline had some problems with the route originally devised to install a chiral secondary amine in the drug candidate GSK2879552. To make a key intermediate of the molecule, which is in Phase II clinical trials for the treatment of small-cell lung cancer and acute leukemia, they first had to separate enantiomers of the secondary amine. Then, following reductive amination, they had to use a complex extraction process that required several environmentally unfriendly solvents and generated boron waste. Looking to make their synthesis greener, simpler, and less expensive, the chemists turned to imine reductase enzymes. Such enzymes are known to perform reductive amination reactions, but until now they have been used only with simple amines and carbonyl compounds and in conditions that aren’t amenable to industrial syntheses. The GSK team, led by Gheorghe-Doru Roiban, put an imine reductase enzyme from the bacterium Saccharothrix espanaensis through three

Advances in the Asymmetric Total Synthesis of Natural Products Using Chiral Secondary Amine Catalyzed Reactions of α,β-Unsaturated Aldehydes.

Chirality is one of the most important attributes for its presence in a vast majority of bioactive natural products and pharmaceuticals. Asymmetric organocatalysis methods have emerged as a powerful methodology for the construction of highly enantioenriched structural skeletons of the target molecules. Due to their extensive application of organocatalysis in the total synthesis of bioactive molecules and some of them have been used in the industrial synthesis of drugs have attracted increasing interests from chemists. Among the chiral organocatalysts, chiral secondary amines (MacMillan’s catalyst and Jorgensen’s catalyst) have been especially considered attractive strategies because of their impressive efficiency. Herein, we outline advances in the asymmetric total synthesis of natural products and relevant drugs by using the strategy of chiral secondary amine catalyzed reactions of α,β-unsaturated aldehydes in the last eighteen years.

Synthesis of Phenylcyclopropane-Based Secondary Amine Catalysts and Their Applications in Enamine Catalysis.

A novel chiral motif based on a phenylcyclopropane scaffold has been designed, and a facile synthetic route to the key intermediate for the synthesis of phenylcyclopropane-based chiral secondary amines has been developed. Newly synthesized chiral amines function as effective catalysts for several asymmetric reactions through enamine intermediates.

Organocatalytic Asymmetric Inverse‐Electron‐Demand Diels‐Alder Reaction of Pyrrolidone‐Dienes with Enals

AbstractA new pyrrolidone‐diene synthon was devised, and the inverse‐electron‐demand Diels‐Alder reaction of pyrrolidone‐dienes with enals catalyzed by chiral secondary amines has been disclosed. A variety of enantioenriched, multifunctionalized hexahydro‐isoindol‐1‐one derivatives were obtained in good yields (up to 75%) with excellent stereoselectivities (up to >20:1 dr and >99% ee).magnified image

Synergistic Catalysis for Asymmetric [3 + 2] Cycloadditions of 2-Indolylmethanols with α,β-Unsaturated Aldehydes.

A catalytic asymmetric [3 + 2] cycloaddition of 2-indolylmethanols with α,β-unsaturated aldehydes was developed for the first time. This transformation was achieved by a synergistic catalytic system consisting of a palladium complex, a Brønsted acid, and a chiral secondary amine to synthesize biologically active cyclopenta[b]indole derivatives with excellent diastereo- and enantioselectivities (up to >20:1 dr, up to 99% ee).

Effective asymmetric Michael addition of anthrone to nitroalkenes using chiral tetraoxacalix[2]arene[2]triazines as organocatalysts.

Novel chiral secondary amines bearing a tetraoxacalix[2]arene[2]triazine scaffold were created and used for catalytic asymmetric Michael reaction of anthrone with nitroalkenes. The relevant adducts were obtained in good to excellent yields (82%-98%) and enantioselectivities (75%-98%).

Asymmetric Direct Remote Michael Addition Reactions of Allyl Furfurals via Dearomative Trienamine and Tetraenamine Catalysis

Abstract2‐Allyl‐3‐furfural could generate dearomative trienamine species in the presence of a chiral secondary amine, and underwent an asymmetric remote ϵ‐regioselective Michael addition reaction with isatin‐derived α,α‐dicyanoalkenes. Meanwhile, the more challenging dearomative tetraenamine intermediate from 5‐allyl‐2‐furfural could be similarly utilized in very remote η‐regioselective Michael additions. In general, good yields with moderate to high enantioselectivity were obtained by employing a concerted activation strategy of bifunctional secondary amine/thiourea catalysts.

Chiral Brønsted Acid-Catalyzed Metal-Free Asymmetric Direct Reductive Amination Using 1-Hydrosilatrane.

The asymmetric direct reductive amination of prochiral ketones with aryl amines using 1-hydrosilatrane with a chiral Brønsted acid catalyst is reported. This is the first known example of chiral Brønsted acid-catalyzed asymmetric reductive amination using a silane as the hydride source. The reaction features a highly practical reducing reagent and proceeds efficiently at room temperature without a specialized reaction setup or equipment to exclude air or moisture. This method provides high conversion and enantiomeric excess up to 84% of the desired chiral secondary amines with minimal side products.

Formal [3+2] cycloaddition of vinylcyclopropane azlactones to enals using synergistic catalysis

The present study reports the asymmetric cyclization of enals with vinylcyclopropane azlactones efficiently catalyzed by the combination of achiral Pd(0) complexes and chiral secondary amines. Corresponding spirocyclic azlactones were produced in high yields with moderate diastereoselectivities and excellent enantioselectivities. This protocol provides an efficient and easily-performed route to spirocyclic scaffolds, and densely functionalized cyclopentanes containing quaternary carbon centers.

Organocatalytic Enantioselective Addition of α-Aminoalkyl Radicals to Isoquinolines.

With a dual organocatalytic system involving a chiral phosphoric acid and a dicyanopyrazine-derived chromophore (DPZ) photosensitizer and under the irradiation with visible light, an enantioselective Minisci-type addition of α-amino acid-derived redox-active esters (RAEs) to isoquinolines has been developed. A variety of prochiral α-aminoalkyl radicals generated from RAEs were successfully introduced on isoquinolines, providing a range of valuable α-isoquinoline-substituted chiral secondary amines in high yields with good to excellent enantioselectivities.

Carboxylates as Nucleophiles in the Enantioselective Ring-Opening of Formylcyclopropanes under Iminium Ion Catalysis.

In this work, carboxylic acids, which are typically regarded as poor nucleophiles, are demonstrated to be competent reagents to promote the ring-opening of formylcyclopropanes after activation of the latter through iminium ion formation. Under optimized reaction conditions, a variety of γ-acyloxy-substituted aldehydes can be obtained in high yields and enantioselectivities through the desymmetrization of substituted meso-formylcyclopropanes in the presence of a chiral secondary amine as catalyst.

A Chiral Secondary Amine–Amidophosphane Precatalyst for Silver-Catalyzed Asymmetric 1,3-Dipolar Cycloaddition Reactions

A class of multifunctional amidophosphanes derived from chiral 1,2-diphenylethylenediamines and natural α-amino acids has been developed. Among these, in combination with silver(I) salts, a chiral secondary amine–amidophosphane precatalyst has been demonstrated as being a highly efficient multifunctional precatalyst in the asymmetric 1,3-dipolar cycloaddition of azomethine ylides, including a series of heterocyclic, aliphatic, and 2-substituted azomethine ylides, and aromatic α,β-unsaturated aldehyde derived imino esters with different electron-deficient alkenes, as well as the three-component reaction of α-imino esters generated in situ by using N,N′-diisopropylcarbodiimide as dehydrating agent. Under optimal conditions, highly functionalized endo-adducts were obtained in high to excellent yields (up to 99% yield) and enantioselectivities (up to >99.9% ee).

Asymmetric Benzylic Allylic Alkylation Reaction of 3-Furfural Derivatives by Dearomatizative Dienamine Activation.

The dearomatizative dienamine-type ortho-quinodimethane species are smoothly generated between 2-alkyl-3-furfurals and chiral secondary amine catalysts, which undergo asymmetric benzylic allylic alkylation reactions with 2-nitroallylic acetates efficiently. A spectrum of densely functionalized 3-furfural derivatives are delivered in moderate to high yields with good to excellent diastereo- and enantioselectivity (up to 98 % yield, >19:1 d.r., >99 % ee). The latent transformations allow the facile production of some enantioenriched architectures, such as 1,1,2,2-tetraarylethanes and triarylmethanes, which are not easily available from other protocols.

Organocatalytic Asymmetric Synthesis of Trifluoromethylated Tetrahydrocarbazoles by a Vinylogous Michael/Aldol Formal [4+2] Annulation

A new protocol for the organocatalytic asymmetric synthesis of highly functionalized tetrahydrocarbazoles has been developed based on a vinylogous Michael/aldol reaction sequence between 3‐(trifluoroacetyl)‐2‐methyl‐indoles and enals. This formal [4+2] annulation occurs by cooperative catalysis between an achiral Brønsted base and a chiral secondary amine organocatalyst to afford tetrahydrocarbazole products bearing three vicinal stereocenters, including a trifluoromethylated tetrasubstituted stereocenter, in excellent yields, diastereo‐ and enantioselectivities.

(R)- and (S)-Proline-Derived Chiral Phosphoramides as Organo­catalysts for the Enantiodivergent Aldol Reaction of Isatins with Cyclohexanone in the Presence of Water

Novel organocatalysts derived from (R)- and (S)-proline and incorporating a chiral phosphoramide fragment were rationally designed and subsequently synthesized. These chiral compounds catalyze the enantioselective aldol addition reaction of cyclohexanone to prochiral isatins in the presence of water. These observations are particularly relevant since reports of asymmetric aldol reactions between cyclohexanone­ and isatins catalyzed by chiral secondary amines remain scarce, with primary amines being the most studied and successful catalysts. The present report includes a thorough evaluation of the new bifunctional catalysts that actually give rise to either enantiomer of the chiral product by proper selection of the configuration of the proline moiety.

Evidence for Triplet Sensitization in the Visible-Light-Induced [2+2] Photocycloaddition of Eniminium Ions.

Εniminium ions were prepared from the corresponding α,β‐unsaturated carbonyl compounds (enones and enals), and were found to be promoted to their respective triplet states by energy transfer. The photoexcited intermediates underwent intra‐ or intermolecular [2+2] photocycloaddition in good yields (50–78 %) upon irradiation at λ=433 nm or λ=457 nm. Iridium or ruthenium complexes with a sufficiently high triplet energy were identified as efficient catalysts (2.5 mol % catalyst loading) for the reaction. The intermolecular [2+2] photocycloaddition of an eniminium ion derived from a chiral secondary amine proceeded with high enantioselectivity (88 % ee).

Achieving Molecular Complexity via Stereoselective Multiple Domino Reactions Promoted by a Secondary Amine Organocatalyst.

In the last two decades, organocatalysis has emerged as an intensively investigated and rapidly growing area of research facilitating many known and many new transformations to provide efficient novel entries to complex molecules of high stereochemical purity. The organocatalysts have not only shown their efficiency for catalyzing the reactions in which one bond is formed, but they have also been effectively exploited in various versions of one-pot reactions. Domino reactions are one of the most important classes of one-pot reactions, where the target structure can be obtained in one pot without changing any reaction conditions while each reaction occurs as a consequence of the intermediates generated in previous steps. Owing to the synthetic importance and operational advantages associated with the use of organocatalysts and the development of domino reactions, various asymmetric transformations leading to a complex structure of choice have been explored. The early era of organocatalysis exhibits a limited growth in the development of asymmetric domino reactions with special emphasis on two reactions occurring one after the other. In 2006, our group made a step forward to develop more complex domino reactions catalyzed by a secondary amine organocatalyst, wherein three reactions take place in one pot to provide cyclohexene carbaldehydes bearing four stereogenic centers with excellent stereocontrol. This triggered our interest to develop new organocatalytic domino sequences, especially for multiple domino reactions. After our seminal contribution, domino reactions catalyzed by secondary amine organocatalysts not only became more popular, but they also could be catalyzed by other classes of organocatalysts, such as bifunctional hydrogen bonding catalysts, chiral Brønsted acids, and N-heterocyclic carbenes. The mode of activation in this triple domino reaction relied on the sequential generation of enamine and iminium intermediates using a proline-based chiral secondary amine organocatalyst. By employing this strategy, we have developed several triple domino reactions leading to the formation of carbo- and heterocyclic structures bearing multiple stereogenic centers with excellent levels of stereoselectivities. The applications of the secondary amine organocatalysts have been further extended to more complex quadruple domino sequences. Moreover, these multiple domino sequences have been combined successfully with other transformations in one pot to create densely functionalized polycyclic compounds. This Account gives an overview of our research in the area of organocatalytic asymmetric multiple domino reactions with special emphasis on the secondary amine catalyzed triple and quadruple domino reactions via a sequential generation of enamine and iminium intermediates. The multiple cascade reactions assisted by di- and tri-iminium and -enamine species as well as other types of organocatalysts have been excluded from the scope of this Account.

Chemo- and Regioselective Asymmetric Friedel–Crafts Reaction of Furans and Thiophenes with α,β-Unsaturated Aldehydes through Dual Activation

A highly chemo- and regioselective Friedel–Crafts alkylation reaction of furans and thiophenes has been developed, which relies on the activation from the remote conjugated Mukaiyama silyl enol ether motif. Excellent enantioslectivity is generally obtained in reactions with α,β-unsaturated aldehydes under the well-established iminium ion catalysis of a chiral secondary amine.

Enantioselective Multicomponent Reaction for Rapid Construction of 1,2,5-Triol Derivatives with Vicinal Chiral Centers.

1,2,5-Triol derivatives with vicinal chiral centers have been synthesized from simple starting materials by one-pot method in good yields and with an excellent enantioselectivity. This process was promoted by a chiral secondary amine and iridium(I) cocatalyzed three-component reaction of aryldiazoacetates and alcohols with enals as electrophiles followed by a reduction with NaBH4. Iridium(I)-associated oxonium ylide intermediates were efficiently generated and successfully trapped by the amine-activated enals via a selective 1,4-addition manner, generating enantioselective three-component coupling products.