Dehydroabietane-type bifunctional organocatalysts in asymmetric synthesis: recent progress

Dehydroabietane-type bifunctional organocatalysts derived from rosane-type diterpenes of dehydroabietic acid (DHAA) and dehydroabietylamine (DA) have been utilized in a wide variety of highly enantioselective reactions. Since one well-documented review exclusively reported on the development of terpene-derived bifunctional thioureas in asymmetric organocatalysis in 2013, fragmentary progress on the dehydroabietane-type bifunctional thioureas and squaramides has been mentioned in other reviews. In this mini-review, we systematically analyze and reorganize the published literature on dehydroabietane-type bifunctional organocatalysts in the recent decade according to the type of catalysts. Our aim is for this review to provide helpful research information and serve as a foundation for further design and application of rosin-based organocatalysts.

Catalytic asymmetric oxa-Diels-Alder reaction of acroleins with simple alkenes

The catalytic asymmetric inverse-electron-demand oxa-Diels-Alder (IODA) reaction is a highly effective synthetic method for creating enantioenriched six-membered oxygen-containing heterocycles. Despite significant effort in this area, simple α,β-unsaturated aldehydes/ketones and nonpolarized alkenes are seldom utilized as substrates due to their low reactivity and difficulties in achieving enantiocontrol. This report describes an intermolecular asymmetric IODA reaction between α-bromoacroleins and neutral alkenes that is catalyzed by oxazaborolidinium cation 1f. The resulting dihydropyrans are produced in high yields and excellent enantioselectivities over a broad range of substrates. The use of acrolein in the IODA reaction produces 3,4-dihydropyran with an unoccupied C6 position in the ring structure. This unique feature is utilized in the efficient synthesis of (+)-Centrolobine, demonstrating the practical synthetic utility of this reaction. Additionally, the study found that 2,6-trans-tetrahydropyran can undergo efficient epimerization into 2,6-cis-tetrahydropyran under Lewis acidic conditions. This structural core is widespread in natural products.

Organocatalytic Strategies for the Development of the Enantioselective Inverse-electron-demand Hetero-Diels-Alder Reaction

Cycloaddition reactions, in particular Diels-Alder reactions, have attracted a lot of attention from organic chemists since they represent one of the most powerful methodologies for the construction of carbon-carbon bonds. In particular, inverse-electron-demand hetero-Diels-Alder reactions have been an important breakthrough for the synthesis of heterocyclic compounds. Among all their variants, the organocatalytic enantioselective version has been widely explored since the asymmetric construction of diversely functionalized scaffolds under reaction conditions encompassed within the green chemistry field is of great interest. In this review, a profound revision on the latest advances on the organocatalytic asymmetric inverse-electron demand hetero-Diels-Alder reaction is shown.

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.

Chiral Thioureas-Preparation and Significance in Asymmetric Synthesis and Medicinal Chemistry

For almost 20 years, thioureas have been experiencing a renaissance of interest with the emerged development of asymmetric organocatalysts. Due to their relatively high acidity and strong hydrogen bond donor capability, they differ significantly from ureas and offer, appropriately modified, great potential as organocatalysts, chelators, drug candidates, etc. The review focuses on the family of chiral thioureas, presenting an overview of the current state of knowledge on their synthesis and selected applications in stereoselective synthesis and drug development.

Organocatalytic asymmetric synthesis of both cis- and trans-configured pyrano[2,3-b]chromenes via different dehydration pathways

The enamine-catalyzed [3 + 3]-cycloaddition between chroman-2-ols and β,γ-unsaturated α-ketoesters is developed to access both enantiomers of cis- and trans-fused pyrano[2,3-b]chromene derivatives.

Catalytic Asymmetric Inverse-Electron-Demand Hetero-Diels-Alder Reactions

In this review, the recent developments in catalytic asymmetric inverse-electron-demand hetero-Diels-Alder reaction, which is recognized as one of the most powerful routes to construct highly functionalized and enantioenriched six-membered heterocycles, are described. The article is organized on the basis of different kinds of electron-deficient heterodienes, including α,β-unsaturated ketones/aldehydes, o-benzoquinones, α,β-unsaturated imines, N-aryl imines, o-benzoqinone imides, and other aza-olefins.

Brønsted acid-catalyzed stereoselective [4+3] cycloadditions of ortho-hydroxybenzyl alcohols with N,N′-cyclic azomethine imines

The first [4+3] cycloaddition of ortho-hydroxybenzyl alcohols has been established to construct biologically important seven-membered heterocyclic scaffolds in good yields and excellent diastereoselectivities (up to 92% yield, most >95 : 5 dr).

Asymmetric Inverse-Electron-Demand Oxa-Diels-Alder Reaction of Allylic Ketones through Dienamine Catalysis

A remote β,γ-regioselective asymmetric inverse-electron-demand oxa-Diels-Alder reaction between allylic ketones and α-cyano-α,β-unsaturated ketones has been developed through induced extended dienamine catalysis of a cinchona-derived primary amine. A spectrum of densely substituted dihydropyran frameworks were efficiently produced with excellent enantioselectivity and fair to exclusive diastereoselectivity.

Organocatalytic oxa-Diels–Alder reaction of α,β-unsaturated ketones under non-classical conditions

Non-classical activation techniques improve the organocatalysed oxa-Diels–Alder reaction of aldehydes with enones.

The catalytic asymmetric synthesis of tetrahydropyridazines via inverse electron-demand aza-Diels–Alder reaction of enol ethers with azoalkenes

A highly efficient catalytic asymmetric IEDDA reaction of azoalkenes with enol ethers is reported. This methodology provides a facile entry to biologically important tetrahydropyridazines in good yield with good to excellent ee.

An organocatalytic cascade reaction of 2-nitrocyclohexanone and α,β-unsaturated aldehydes with unusual regioselectivity

An organocatalytic cascade reaction of 2-nitrocyclohexanone and α,β-unsaturated aldehydes was developed.

Efficient construction of highly functionalized spiro[γ-butyrolactone-pyrrolidin-3,3'-oxindole] tricyclic skeletons via an organocatalytic 1,3-dipolar cycloaddition

Highly functionalized spiro[γ-butyrolactone-pyrrolidin-3,3'-oxindole] tricyclic skeletons were delivered successfully with high optical purity using an effective yet simple procedure.

Catalytic asymmetric β,γ activation of α,β-unsaturated γ-butyrolactams: direct approach to β,γ-functionalized dihydropyranopyrrolidin-2-ones

Skeleton in the closet: The title reaction enables the development of the first catalytic β,γ-selective Diels-Alder [4+2] annulation of α,β-unsaturated γ-butyrolactams (see scheme; Boc=tert-butoxycarbonyl, Ts=4-toluenesulfonyl). This process provides a direct method for the enantioselective construction of bi- or tricyclic dihydropyranopyrrolidin-2-one skeletons in only one step.