Facile Domino Access to Chiral Bicyclo[3.2.1]octanes and Discovery of a New Catalytic Activation Mode

Hydrogen Bond Assisted Three-Component Tandem Reactions to Access N-Alkyl-4-Quinolones

Hydrogen-bonding catalytic reactions have gained great interest. Herein, a hydrogen-bond-assisted three-component tandem reaction for the efficient synthesis of N-alkyl-4-quinolones is described. This novel strategy features the first proof of polyphosphate ester (PPE) as a dual hydrogen-bonding catalyst and the use of readily available starting materials for the preparation of N-alkyl-4-quinolones. The method provides a diversity of N-alkyl-4-quinolones in moderate to good yields. The compound 4h demonstrated good neuroprotective activity against N-methyl-ᴅ-aspartate (NMDA)-induced excitotoxicity in PC12 cells.

Asymmetric Hydroaminoalkylation of Alkenylazaarenes via Cooperative Photoredox and Chiral Hydrogen-Bonding Catalysis

Chiral hydrogen-bonding (H-bonding) catalytic asymmetric conjugate addition to activated olefins has been widely used to access enantioenriched molecules containing stereocenters at the β-position of the olefin activating groups. Herein, we report the first highly enantioselective radical-based manifold. Under a dual organocatalyst system involving a chiral phosphoric acid and DPZ as the photoredox sensitizer, transformations of N-arylglycines, in which aryls with CF₃ substituents are introduced, with alkenyl azaarenes afforded valuable hydroaminoalkylation adducts with satisfactory results. In addition to the diversity of azaarenes, the method can be used to construct aryl-, alkyl- and silyl-substituted stereocenter. Control experiments and density functional theory calculations were performed to elucidate a plausible reaction mechanism and the origin of stereoselectivity, wherein nonclassical H-bonding interactions were found to assist chiral catalysts in offering sufficient enantiocontrol.

Stereoselective organocascades: from fundamentals to recent developments

Reaction sequences where more bonds are sequentially formed (cascade reactions) may be started either by a stoichiometric or by a catalytic reagent, and proceed in an enantio- diastereo- or non-stereo- selective manner. A wide variety of such strategies has been developed, including both stoichiometric and catalytic ones.

Within the widely developed cascade reactions field, this chapter is not meant to be omni-comprehensive, but to offer an as much as possible complete overview on organocatalytic stereoselective methods. We embrace the more general definitions by Tietze and Denmark, considering as cascade reactions all those one-pot processes that involve two or more bond formations, where each subsequent step is enabled by a structural change caused by the previous one. We will include both two- and multi-component reactions where one or more organocatalysts may be responsible either for all or just some of the occurring transformations. Organocascades will be reported according to the number of involved catalytic cycles.

In the following paragraphs, only cascade reactions that are stereoselective by means of a chiral catalyst will be considered. It will be shown that multiple possibilities, relying on different catalysis modes, are available to achieve the same reaction sequence.

N,N-Diacylaminals as Emerging Tools in Synthesis: From Peptidomimetics to Asymmetric Catalysis

N,N-Diacylaminals are flexible molecular scaffolds that have commonly been utilized as amide surrogates in peptidomimetics. The singularities of this motif as an N-acyl imine equivalent and as hydrogen-bond donor have recently opened new synthetic opportunities, especially in the field of asymmetric catalysis. This concept article highlights this diverse synthetic potential and provides the elements necessary for further developments.

Stereodivergent Synthesis of Camphor-Derived Diamines and Their Application as Thiourea Organocatalysts

A series of 18 regio- and stereo-chemically diverse chiral non-racemic 1,2-, 1,3-, and 1,4-diamines have been synthesized from commercial (1S)-(+)-ketopinic acid and (1S)-(+)-10-camphorsulfonic acid. The structures of the diamines are all based on the d-(+)-camphor scaffold and feature isomeric diversity in terms of regioisomeric attachment of the primary and the tertiary amine function and the exo/endo-isomerism. Diamines were transformed into the corresponding noncovalent bifunctional thiourea organocatalysts, which have been evaluated for catalytic activity in the conjugative addition of 1,3-dicarbonyl nucleophiles (dimethyl malonate, acetylacetone, and dibenzoylmethane) to trans-β-nitrostyrene. The highest enantioselectivity was achieved in the reaction with acetylacetone as nucleophile using endo-1,3-diamine derived catalyst 52 (91.5:8.5 er). All new organocatalysts 48-63 have been fully characterized. The structures and the absolute configurations of eight intermediates and thiourea derivative 52 were also determined by X-ray diffraction.

Gold(I)-Catalyzed Cycloisomerization of 3-Alkoxyl-1,6-diynes: A Facile Access to Bicyclo[2.2.1]hept-5-en-2-ones

A novel gold‐catalyzed cycloisomerization of 1,6‐diynes was achieved, providing an atom‐economic approach to a diverse set of bicyclo[2.2.1]hept‐5‐en‐2‐ones in moderate to good yields. With unsymmetrical starting materials with two different internal alkynyl substituents, to some extent, the regioselectivity could be controlled by both electronic and steric factors. This unprecedented reactivity pattern may inspire new and unconventional strategies for the preparation of bridged ring systems.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Catalytic 1,2-Rearrangements: Organocatalyzed Michael/Semi-Pinacol-like Rearrangement Cascade of 1,3-Diones and Nitroolefins

New types of organocatalytic 1,2-rearrangements, which resemble the Smiles-like or semi-pinacol-like rearrangement, of Michael adducts of 1,3-dicarbonyl-2-alkyl compounds and nitroalkenes have been realized. Unlike the well-known conjugate addition, the reaction affords the 1-phenyl-1-nitroalkanes via unprecedented rearrangement and cascade reactions. Structures of the appropriate products were unambiguously characterized by X-ray crystallography.

Organo-catalyzed asymmetric cascade annulation reaction for the construction of bi-spirocyclic pyrazolone and oxindole derivatives

Organo-catalyzed tandem reaction between β,γ-unsaturated α-ketoesters and α-arylidene pyrazolinones was developed, and it provided chiral bi-spirocyclic pyrazolone and oxindole derivatives in high yields with good to excellent stereoselectivity.

Transition state analysis of an enantioselective Michael addition by a bifunctional thiourea organocatalyst

The mechanism of the enantioselective Michael addition of diethyl malonate to trans-β-nitrostyrene catalyzed by a tertiary amine thiourea organocatalyst is explored using experimental 13C kinetic isotope effects and density functional theory calculations. Large primary 13C KIEs on the bond-forming carbon atoms of both reactants suggest that carbon-carbon bond formation is the rate-determining step in the catalytic cycle. This work resolves conflicting mechanistic pictures that have emerged from prior experimental and computational studies.

2-Hydroxybenzophenone as a Chemical Auxiliary for the Activation of Ketiminoesters for Highly Enantioselective Addition to Nitroalkenes under Bifunctional Catalysis

An organocatalytic system is presented for the Michael addition of monoactivated glycine ketimine ylides with a bifunctional catalyst. The ketimine bears an ortho hydroxy group, which increases the acidity of the methylene hydrogen atoms and enhances the reactivity, thus allowing the synthesis of a large variety of α,γ-diamino acid derivatives with excellent stereoselectivity.

Organocatalyzed Synthesis of [3.2.1] Bicyclooctanes

Organocatalysis constitutes one of the main research areas in organic chemistry from the last two decades. This chemistry has been applied to the synthesis of many natural products and structures in a manner that reduces the residues and so the ecological impact. In this review, we consider the work that has been done for the synthesis of bicyclo[3.2.1]octane framework. This structure is present in many natural products with very important biological activities.

Intramolecular Hydrogen Bond Activation: Thiourea-Organocatalyzed Enantioselective 1,3-Dipolar Cycloaddition of Salicylaldehyde-Derived Azomethine Ylides with Nitroalkenes

An organocatalytic strategy for the synthesis of tetrasubstituted pyrrolidines with monoactivated azomethine ylides in high enantiomeric excess and excellent exo/endo selectivity is presented. The key to success is the intramolecular activation via hydrogen bonding through an o-hydroxy group, which allows the dipolar cycloaddition to take place in the presence of azomethine ylides bearing only one activating group. The intramolecular hydrogen bond in the azomethine ylide and the intermolecular hydrogen bond with the catalyst have been demonstrated by DFT calculations and mechanistic proofs to be crucial for the reaction to proceed.

Enantioselective Michael Addition of Cyclic β-Diones to α,β-Unsaturated Enones Catalyzed by Quinine-Based Organocatalysts

An enantioselective (52–98% ee) Michael addition between cyclic β-diones and α,β-unsaturated enones was established in the presence of quinine-based primary amine or squaramide. A variety of cinnamones were smoothly converted into the desired 3,4-dihydropyrans in moderate to high yields (63–99%). Chalcones were also suitable acceptors and gave rise to the expected adducts in satisfactory yields (31–99%). The resulting adducts readily underwent further modification to form fused 4H-pyran or 2,3-dihydrofuran.

Construction of tetrahydropyranoquinoline derivativesviaan asymmetric organocatalytic aza-Michael-IED/HAD cascade reaction

A highly efficient organocatalytic asymmetric aza-Michael-IED/HAD cascade reaction of (E)-ethyl 4-(2-(4-methylphenylsulfonamido)phenyl)-2-oxobut-3-enoate and enals is reported.

Development and Analysis of a Pd(0)-Catalyzed Enantioselective 1,1-Diarylation of Acrylates Enabled by Chiral Anion Phase Transfer

Enantioselective 1,1-diarylation of terminal alkenes enabled by the combination of Pd catalysis with a chiral anion phase transfer (CAPT) strategy is reported herein. The reaction of substituted benzyl acrylates with aryldiazonium salts and arylboronic acids gave the corresponding 3,3-diarylpropanoates in moderate to good yields with high enantioselectivies (up to 98:2 er). Substituents on the benzyl acrylate and CAPT catalyst significantly affect the enantioselectivity, and multidimensional parametrization identified correlations suggesting structural origins for the high stereocontrol.

Enantioselective Organocatalyzed Transformations of β-Ketoesters

The β-ketoester structural motif continues to intrigue chemists with its electrophilic and nucleophilic sites. Proven to be a valuable tool within organic synthesis, natural product, and medicinal chemistry, reports on chiral β-ketoester molecular skeletons display a steady increase. With the reignition of organocatalysis in the past decade, asymmetric methods available for the synthesis of this structural unit has significantly expanded, making it one of the most exploited substrates for organocatalytic transformations. This review provides comprehensive information on the plethora of organocatalysts used in stereoselective organocatalyzed construction of β-ketoester-containing compounds.

An asymmetric approach to bicyclo[2.2.1]heptane-1-carboxylates via a formal [4 + 2] cycloaddition reaction enabled by organocatalysis

An organocatalytic formal [4 + 2] cycloaddition reaction has been realized that permits rapid access to a wide range of bicyclo[2.2.1]heptane-1-carboxylates in a highly enantioselective manner from simple starting materials under mild and operationally simple conditions.

1H-Imidazol-4(5H)-ones and thiazol-4(5H)-ones as emerging pronucleophiles in asymmetric catalysis

Asymmetric catalysis represents a very powerful tool for the synthesis of enantiopure compounds. In this context the main focus has been directed not only to the search for new efficient chiral catalysts, but also to the development of efficient pronucleophiles. This review highlights the utility and first examples of 1H-imidazol-4(5H)-ones and thiazol-4(5H)-ones as pronucleophiles in catalytic asymmetric reactions.

(Thio)urea-mediated synthesis of functionalized six-membered rings with multiple chiral centers

Organocatalysis, now running its second decade of life, is being considered one of the main tools a synthetic chemist has to perform asymmetric catalysis. In this review the synthesis of six-membered rings, that contain multiple chiral centers, either by a ring closing process or by a functionalization reaction on an already existing six-membered ring, utilizing bifunctional (thio)ureas will be summarized. Initially, the use of primary amine-thioureas as organocatalysts for the above transformation is being discussed, followed by the examples employing secondary amine-thioureas. Finally, the use of tertiary amine-thioureas and miscellaneous examples are presented.

Enantioselective synthesis of 2,3-disubstituted trans-2,3-dihydrobenzofurans using a Brønsted base/thiourea bifunctional catalyst

The enantioselective synthesis of 2,3-disubstituted trans-2,3-dihydrobenzofuran derivatives via intramolecular Michael addition has been developed using a bifunctional tertiary amine–thiourea catalyst.

Highly stereoselective [4+2] and [3+2] spiroannulations of 2-(2-oxoindolin-3-ylidene)acetic esters catalyzed by bifunctional thioureas

A new Michael-Michael cascade reaction between 2-(2-oxoindolin-3-ylidene)acetic esters 1 and nitroenoates 2, catalyzed by bifunctional thioureas, is investigated. The combination of the two Michael reactions results in a novel and facile [4+2] or [3+2] spiroannulation process, which is characterized by the following features: 1) two carbon-carbon bonds and four stereocenters, including a quaternary spiro carbon, are formed under mild conditions; 2) an unprecedented and stereochemically defined substitution pattern on the spirocarbocyclic unit is obtained; 3) the double-bond configuration of the donor-acceptor nitroenoate 2 determines the absolute configuration of the spiro center, whereas the remaining stereocenters are formed under control of the catalyst. The effect on the final stereochemical outcome of structural variations of each starting material, catalyst, and experimental conditions is analyzed in detail. In particular, the use of specifically designed chiral nitroenoates enables diverse polyfunctional spirocyclohexane derivatives containing six consecutive stereogenic centers to be constructed. To our knowledge, this is the first asymmetric organocatalytic strategy enabling both five- and six-membered β-nitro spirocarbocyclic oxindoles.

Catalytic Enantioselective Synthesis of N,C(α),C(α)-Trisubstituted α-Amino Acid Derivatives Using 1H-Imidazol-4(5H)-ones as Key Templates

1H-Imidazol-4(5H)-ones are introduced as novel nucleophilic α-amino acid equivalents in asymmetric synthesis. These compounds not only allow highly efficient construction of tetrasubstituted stereogenic centers, but unlike hitherto known templates, provide direct access to N-substituted (alkyl, allyl, aryl) α-amino acid derivatives.