Organocatalytic asymmetric robinson annulation of alpha,beta-unsaturated aldehydes: applications to the total synthesis of (+)-palitantin

Kinetic Rationalization of Nonlinear Effects in Asymmetric Catalytic Cascade Reactions under Curtin-Hammett Conditions

Observations of nonlinear effects of catalyst enantiopurity on product enantiomeric excess in asymmetric catalysis are often used to infer that more than one catalyst species is involved in one or more reaction steps. We demonstrate here, however, that in the case of asymmetric catalytic cascade reactions, a nonlinear effect may be observed in the absence of any higher order catalyst species or any reaction step involving two catalyst species. We illustrate this concept with an example from a recent report of an organocatalytic enantioselective [10 + 2] stepwise cyclization reaction. The disruption of pre-equilibria (Curtin-Hammett equilibrium) in reversible steps occurring prior to the final irreversible product formation step can result in an alteration of the final product ee from what would be expected based on a linear relationship with the enantiopure catalyst. The treatment accounts for either positive or negative nonlinear effects in systems over a wide range of conditions including "major-minor" kinetics or the more conventional "lock-and-key" kinetics. The mechanistic scenario proposed here may apply generally to other cascade reaction systems exhibiting similar kinetic features and should be considered as a viable alternative model whenever a nonlinear effect is observed in a cascade sequence of reactions.

Palitantin derivatives from the Antarctic fungus Geomyces sp. 3-1

Two new polyketides, palitantins B and C (1 and 2), along with one known related compound (+)-palitantin (3) were obtained from the culture of the Antarctic fungus Geomyces sp. 3-1. The structures of the new compounds were elucidated by detailed analysis of HRESIMS, NMR, CD, and ECD data. Compound 3 showed potent PTP1B inhibitory activity with an IC₅₀ value of 7.9 μM (ursolic acid as positive control, IC₅₀ = 8.3 μM).

Organocatalytic total synthesis of bioactive compounds based on one-pot methodologies

The combination of one-pot methodologies to asymmetric organocatalysis allow a green and direct access to many types of complex highly functionalized chiral products, including important key intermediates in total syntheses of important bioactive compounds. A series of chiral organocatalysts have already been successfully applied to such syntheses. This report collects major developments in the total synthesis of biologically active products based on the use of enantioselective organocatalytic domino/tandem reactions as key steps. It is divided into two parts dealing successively with reactions based on the use of proline-derived catalysts and other organocatalysts.

1,3-Cyclohexadien-1-Als: Synthesis, Reactivity and Bioactivities

In synthetic organic chemistry, there are very useful basic compounds known as building blocks. One of the main reactions wherein they are applied for the synthesis of complex molecules is the Diels-Alder cycloaddition. This reaction is between a diene and a dienophile. Among the most important dienes are the cyclic dienes, as they facilitate the reaction. This review considers the synthesis and reactivity of one of these dienes with special characteristics-it is cyclic and has an electron withdrawing group. This building block has been used for the synthesis of biologically active compounds and is present in natural compounds with interesting properties.

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.

Organocatalysis and Beyond: Activating Reactions with Two Catalytic Species

Since the beginning of the millennium, organocatalysis has been gaining a predominant role in asymmetric synthesis and it is, nowadays, a foundation of catalysis. Synergistic catalysis, combining two or more different catalytic cycles acting in concert, exploits the vast knowledge acquired in organocatalysis and other fields to perform reactions that would be otherwise impossible. Merging organocatalysis with photo-, metallo- and organocatalysis itself, researchers have ingeniously devised a range of activations. This feature review, focusing on selected synergistic catalytic approaches, aims to provide a flavor of the creativity and innovation in the area, showing ground-breaking examples of organocatalysts, such as proline derivatives, hydrogen bond-mediated, Cinchona alkaloids or phosphoric acids catalysts, which work cooperatively with different catalytic partners.

Novel binaphthyl and biphenyl α- and β-amino acids and esters: organocatalysis of asymmetric Diels-Alder reactions. A combined synthetic and computational study

Asymmetric catalysis of the Diels-Alder reaction between cyclopentadiene and cinnamaldehydes has been studied using as catalysts a range of novel α- and β-aminoacids and aminoesters with binaphthyl and biphenyl backbones, providing enantioselectivities of up to 62% ee. B3LYP/6-31G* calculations, including free energy corrections, have been carried out on a binaphthyl catalyst example to identify transition state structures and to aid in the identification of major enantiomers. The calculated product ratios agree well with the experimental data; the transition states identified involve preferential approach of cyclopentene along a trajectory adjacent to the acid/ester group. The four lowest energy transition states display a stabilizing dipolar interaction between the carbonyl group oxygen atom and a terminal proton of the diene unit.

Catalytic Asymmetric Synthesis of 3,4-Disubstituted Cyclohexadiene Carbaldehydes: Formal Total Synthesis of Cyclobakuchiols A and C

The first catalytic approach for the asymmetric synthesis of 3,4-disubstituted cyclohexadiene carbaldehydes through an inverse-electron-demand Diels-Alder reaction is described. A variety of arylacetaldehydes and α,β,γ,δ-unsaturated aldehydes are tested under the mild reaction conditions catalyzed by l-proline to obtain the trans diastereomeric products with good yields and high enantioselectivities. The scope of this methodology is further extended to the asymmetric synthesis 3,4-disubstituted cyclohexane carbaldehydes and their derivatives. The practicality of this method is demonstrated by the gram-scale synthesis. This methodology is successfully applied for the formal total synthesis of cyclobakuchiol A, an antipyretic and anti-inflammatory agent, and cyclobakuchiol C.

Organocatalytic Transannular Approach to Stereodefined Bicyclo[3.1.0]hexanes

A diastereodivergent approach to highly substituted bicyclo[3.1.0]hexanes has been developed through a transannular alkylation reaction that builds up the bicyclic core employing asymmetric organocatalysis as the tool for the installation of all stereocenters. On one hand, a Michael/Michael cascade process between enals and 4-alkenyl sulfamidate imines under the iminium/enamine activation manifold provides an oxathiazole-2,2-dioxide-fused cyclohexane adduct that, after isolation, is subsequently engaged in a transannular alkylation/hydrolysis through enamine activation by the use of a primary amine. On the other hand, the corresponding C-2 epimers are directly obtained from the same starting materials in a single operation through a cascade Michael/Michael/transannular alkylation/hydrolysis sequence through sequential iminium/enamine/enamine combination of aminocatalytic activation manifolds.

Diastereoselective synthesis of chiral 1,3-cyclohexadienals

A novel approach to the production of chiral 1,3-cyclohexadienals has been developed. The organocatalysed asymmetric reaction of different β-disubstituted-α,β-unsaturated aldehydes with a chiral α,β-unsaturated aldehyde in the presence of a Jørgensen-Hayashi organocatalyst provides easy and stereocontrolled access to the cyclohexadienal backbone. This method allows for the synthesis of potential photoprotective chiral 1,3-cyclohexadienals and extra extended conjugation compounds in a simple manner.

NMR and experimental reinvestigation of the condensation reaction between γ-methylene-α,β-unsaturated aldehydes and propargyl aldehydes

An experimental and computational study of the reaction between γ-methylene enals and propargyl aldehydes uncovered a deconjugative aldol condensation.

Asymmetric cycloaddition reactions catalysed by diarylprolinol silyl ethers

Organocatalysis as a paint palette for asymmetric cycloaddition reactions.

Dynamic resolution of 2-cyclohexylidene acetaldehydes through organocatalytic dienamine [4+2] cycloaddition

Organocatalytic formed dienamines are shown to be involved in dynamic resolution of 2-cyclohexylidene acetaldehydes.

The stereoselective formation of highly substituted CF3-dihydropyrans as versatile building blocks

The highly enantioselective dienamine-mediated formation of 5-bromo-6-(trifluoromethyl)-3,4-dihydro-2H-pyrans from α,β-unsaturated aldehydes and α-bromo-(trifluoromethyl)-enones employing a C2-symmetric aminocatalyst is described. The products are demonstrated to be applicable in coupling reactions directly onto the ring, thereby granting access to a broad scope of highly substituted 6-(trifluoromethyl)-dihydropyran compounds.

Organocatalytic asymmetric strategies to carbocyclic structures by γ-alkylation-annulation sequences

Attractive carbocyclic structures are accessed via a highly regio- and enantioselective aminocatalytic γ-addition of cyclic enals to vinyl phosphonates followed by a one-pot intramolecular Horner-Wadsworth-Emmons reaction. It is also demonstrated that nitro olefins can act as electrophiles in a similar reaction concept, providing carbocycles in equally high stereoselectivity.

Modular synthesis of cyclic cis- and trans-1,2-diamine derivatives

Structurally diverse carbocycles with two vicinal nitrogen-substituents were prepared in expedient three-component reactions from simple amines, aldehydes, and nitroalkenes. trans,trans-6-Nitrocyclohex-2-enyl amines were obtained in a one-pot domino reaction involving condensation, tautomerisation, conjugate addition, and nitro-Mannich cyclisation. Upon employment of less nucleophilic carboxamides, a concerted Diels-Alder cycloaddition mechanism operated to give the corresponding cis,trans-nitrocyclohexenyl amides. Both types of substituted carbocycles offer ample opportunities for chemical manipulations at the core and periphery. Ring oxidation with MnO2 affords substituted nitroarenes. Reduction with Zn/HCl provides access to various trans- and cis-diaminocyclohexenes, respectively, in a straight-forward manner. With enantiopure secondary amines, a two-step synthesis of chiral nitrocyclohexadienes was developed (82-94% ee).

Organocatalytic, Asymmetric Eliminative [4+2] Cycloaddition of Allylidene Malononitriles with Enals: Rapid Entry to Cyclohexadiene-Embedding Linear and Angular Polycycles

A direct aminocatalytic synthesis has been developed for the chemo-, regio-, diastereo-, and enantioselective construction of densely substituted polycyclic carbaldehydes containing fused cyclohexadiene rings. The chemistry utilizes, for the first time, remotely enolizable π-extended allylidenemalononitriles as electron-rich 1,3-diene precursors in a direct eliminative [4+2] cycloaddition with both aromatic and aliphatic α,β-unsaturated aldehydes. The generality of the process is demonstrated by approaching 6,6-, 5,6-, 7,6-, 6,6,6-, and 6,5,6-fused ring systems, as well as biorelevant steroid-like 6,6,6,6,5- and 6,6,6,5,6-rings. A stepwise reaction mechanism for the key [4+2] addition is proposed as a domino bis-vinylogous Michael/Michael/retro-Michael reaction cascade. The utility of the malononitrile moiety as traceless activating group of the dicyano nucleophilic substrates is demonstrated.

Regioselectivity switch in chiral amine-catalysed asymmetric addition of aldehydes to reactive enals

A regioselectivity switch was realized in the chiral amine-catalysed asymmetric addition of aldehydes to reactive enals, and either aldol adducts or conjugate adducts were obtained in a stereoselective fashion.

Asymmetric vinylogous Michael reaction of cyclic enones with silyloxy furans

A chiral, primary diamine catalyzed enantioselective vinylogous Michael reaction of cyclohexenone/medium and large cyclic enones with various 2-silyloxyfuran has been explored.

Asymmetric formation of bridged benzoxazocines through an organocatalytic multicomponent dienamine-mediated one-pot cascade

An organocatalytic one-pot cascade leading to the stereoselective formation of novel bridged benzoxazocines is presented. The developed methodology is based on the first example of a γ-selective-Mannich-initiated cascade reaction and allows for direct annulation of the bridged benzoxazocines by incorporation of various α,β-unsaturated aldehydes, electron-rich anilines, and electron-deficient salicylaldehydes. The synthetic applicability of the products is demonstrated by relevant transformations.

Asymmetric synthesis of tetrahydroquinolines through a [3+2] cycloaddition controlled by dienamine catalysis

A dienamine-mediated enantioselective 1,3-dipolar cycloaddition catalyzed by a chiral prolinol silyl ether catalyst has been developed. Removal of the benzamide group of the intermediates could furnish chiral C-1 substituted tetrahydroisoquinolines (see scheme) in high yields and excellent stereoselectivities.

Organocatalyzed asymmetric synthesis of dihydrodibenzofurans based on a dienamine process

The first organocatalyzed asymmetric method for the synthesis of dihydrodibenzofurans based on a dienamine process has been developed. This two-step protocol works with a broad range of substrates and delivers only the cis-diastereomer in good yield with up to 91% ee. The enantioenriched products have been transformed to highly functionalized and partially hydrogenated dibenzofurans in excellent diastereoselectivities.

Oxidative γ-addition of enals to trifluoromethyl ketones: enantioselectivity control via Lewis acid/N-heterocyclic carbene cooperative catalysis

An oxidative γ-functionalization of enals under N-heterocyclic carbene (NHC) catalysis to give unsaturated δ-lactones is disclosed. Enantioselectivity control involving the relatively remote enal γ-carbon was achieved via Lewis acid [Sc(OTf)(3) or combined Sc(OTf)(3)/Mg(OTf)(2)] and NHC cooperative catalysis.