Diels–Alder Reactions of 1-Azadienes

Synthetic techniques for thermodynamically disfavoured substituted six-membered rings

Six-membered rings are ubiquitous structural motifs in bioactive compounds and multifunctional materials. Notably, their thermodynamically disfavoured isomers, like disubstituted cyclohexanes featuring one substituent in an equatorial position and the other in an axial position, often exhibit enhanced physical and biological activities in comparison with their opposite isomers. However, the synthesis of thermodynamically disfavoured isomers is, by its nature, challenging, with only a limited number of possible approaches. In this Review, we summarize and compare synthetic methodologies that produce substituted six-membered rings with thermodynamically disfavoured substitution patterns. We place particular emphasis on elucidating the crucial stereoinduction factors within each transformation. Our aim is to stimulate interest in the synthesis of these unique structures, while simultaneously providing synthetic chemists with a guide to approaching this synthetic challenge.

Cationic palladium(ii)-catalyzed synthesis of substituted pyridines from α,β-unsaturated oxime ethers

An efficient method for the synthesis of multi-substituted pyridines from β-aryl-substituted α,β-unsaturated oxime ethers and alkenes via Pd-catalyzed C–H activation has been developed. The method, using Pd(OAc)₂ and a sterically hindered pyridine ligand, provides access to various multi-substituted pyridines with complete regioselectivity. Mechanistic studies suggest that the pyridine products are formed by Pd-catalyzed electrophilic C–H alkenylation of α,β-unsaturated oxime followed by aza-6π-electrocyclization. The utility of this method is showcased by the synthesis of 4-aryl-substituted pyridine derivatives, which are difficult to synthesize efficiently using previously reported Rh-catalyzed strategies with alkenes.

An efficient method for the synthesis of multi-substituted pyridines from α,β-unsaturated oxime ethers via cationic Pd(ii)-catalyzed C–H activation has been developed.

Enantioselective Inverse-Electron Demand Aza-Diels-Alder Reaction: ipso,α-Selectivity of Silyl Dienol Ethers

A highly efficient enantioselective inverse-electron-demand aza-Diels–Alder reaction between aza-sulfonyl-1-aza-1,3-butadienes and silyl (di)enol ethers has been developed. The presented methodology allows the synthesis of benzofuran-fused 2-piperidinol derivatives with three contiguous stereocenters in a highly selective manner, as even the hemiaminal center is completely stereocontrolled. Density functional theory (DFT) calculations support that the hydrogen-bond donor-based bifunctional organocatalyst selectively triggers the reaction through the ipso,α-position of the dienophile, in contrast to the reactivity observed for dienolates in situ generated from β,γ-unsaturated derivatives. Moreover, the calculations have clarified the mechanism of the reaction and the ability of the hydrogen-bond donor core to hydrolyze selectively the E isomer of the dienol ether. Furthermore, to demonstrate the applicability of silyl enol ethers as nucleophiles in the asymmetric synthesis of interesting benzofuran-fused derivatives, the catalytic system has also been implemented for the highly efficient installation of an aromatic ring in the piperidine adducts.

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.

Reactivity of steroidal 1-azadienes toward enamines: an approach to novel chiral penta- and hexacyclic steroids

The chemical behavior of steroidal N-sulfonyl-1-azadienes toward carbonyl compounds, in the presence of pyrrolidine, is described. With aldehydes, these azadienes participate in hetero-Diels-Alder reactions with the in situ generated enamines. The stereoselectivity results from the approach of the dienophiles from the less hindered α-face of the steroid, with the pyrrolidine moiety endo and retention of the enamine trans geometry. This diastereoselective synthetic methodology led to a new class of chiral pentacyclic steroids. Interestingly, the studied steroidal scaffolds follow a different mechanistic pathway with cyclic ketones. They undergo a diastereoselective annulation reaction, under enamine catalysis, affording chiral hexacyclic steroids.

Enamides and dienamides in phosphoric acid-catalysed enantioselective cycloadditions for the synthesis of chiral amines

This feature article describes how enamides and dienamides can participate in chiral phosphoric acid catalyzed enantioselective cycloadditions to prepare a wide range of cyclic amines.

Diversified Synthesis of Chiral Chromane-Containing Polyheterocyclic Compounds via Asymmetric Organocatalytic Cascade Reactions

Two different organocatalytic cascade reaction pathways have been developed toward the diversified synthesis of chromane-containing polyheterocyclic compounds from the readily available starting materials. The application of Hantzsch ester is proposed to be the key to achieve the switch between these two different cascade reaction pathways, and then the electron-deficient 1-aza-1,3-butadienes could be used as the four-atom and two-carbon unit, respectively, to react with 2-hydroxy cinnamaldehydes in a highly regio- and stereocontrolled manner. On the basis of larger-scale synthesis, further transformations of the obtained products have also been realized, leading to cycloadducts with high structural and stereogenic complexity bearing five stereogenic centers, and one is a tetrasubstituted stereocenter.

Reactivity of Steroidal 1-Azadienes toward Carbonyl Compounds under Enamine Catalysis: Chiral Penta- and Hexacyclic Steroids

The synthesis and reactivity of a steroidal N-sulfonyl-1-azadiene, derived from 16-dehydropregnenolone acetate, toward carbonyl compounds under enamine catalysis is disclosed. An unexpected annulation reaction was observed involving an initial stereoselective conjugate addition of the in situ generated enamine to the steroidal 1-azadiene. The developed diastereoselective synthetic methodology is a novel approach to a new class of chiral pentacyclic and hexacyclic steroids.

Domino aza/oxa-hetero-Diels–Alder reaction for construction of novel spiro[pyrido[3′,2′:5,6]pyrano[2,3-d]pyrimidine-7,5′-pyrimidine]

The unprecedented reaction of α,β-unsaturated N-arylaldimines with two molecular 5-arylidene-1,3-dimethylbarbituric acids in methylene dichloride at room temperature afforded unique spiro[pyrido[3′,2′:5,6]pyrano[2,3-d]pyrimidine-7,5′-pyrimidines] in moderate to good yields and with high diastereoselectivity.

The first organocatalytic, ortho-regioselective inverse-electron-demand hetero-Diels–Alder reaction

The development of the unprecedented ortho-regioselective inverse-electron-demand hetero-Diels–Alder (IEDHDA) reaction is described.

Organocatalytic regioselective, diastereoselective, and enantioselective annulation of cyclic 1-azadienes with γ-nitro ketones via 3,4-cyclization

A regio-, diastereo-, and enantioselective [2 + 4] annulation reaction of cyclic 1-azadienes with γ-nitro ketones is reported.

A [3 + 2]–[4 + 2]–[3 + 2] cycloaddition sequence of isoquinolinium ylide

A unique [3 + 2]–[4 + 2]–[3 + 2] cycloaddition sequence of isoquinolinium ylide is investigated to afford polycyclic compounds with ten stereogenic centers.

Metal- and Protection-Free [4 + 2] Cycloadditions of Alkynes with Azadienes: Assembly of Functionalized Quinolines

A base promoted, protection-free, and regioselective synthesis of highly functionalized quinolines via [4 + 2] cycloaddition of azadienes (generated in situ from o-aminobenzyl alcohol) with internal alkynes has been discovered. The reaction tolerates a wide variety of functional groups which has been successfully extended with diynes, (2-aminopyridin-3-yl)methanol, and 1,4-bis(phenylethynyl)benzene to afford (Z)-phenyl-2-styrylquinolines, phenylnaphthyridine, and alkyne-substituted quinolines, respectively. The proposed mechanism and significant role of the solvent were well supported by isolating the azadiene intermediate and deuterium-labeling studies.

Synthesis of Enantiopure 3-Hydroxypiperidines from Sulfinyl Dienyl Amines by Diastereoselective Intramolecular Cyclization and [2,3]-Sigmatropic Rearrangement

The highly diastereoselective base-promoted intramolecular cyclization of a variety of enantiopure sulfinyl dienyl amines provides novel sulfinyl tetrahydropyridines that are readily converted to 3-hydroxy tetrahydropyridines via sigmatropic rearrangement. The influence of N- and C- substituents on the process has been studied. Procedures to shorten the sequence such as the tandem cyclization followed by [2,3]-sigmatropic rearrangement, as well as cyclization of the free amine, under Boc- or ArSO- deprotection conditions have been examined. Good to excellent levels of selectivity are generally observed for the reported transformations (dr: 75/25 to >98/2). A novel protocol to access substituted amino dienyl sulfoxides is also reported.

Zn-catalyzed enantio- and diastereoselective formal [4 + 2] cycloaddition involving two electron-deficient partners: asymmetric synthesis of piperidines from 1-azadienes and nitro-alkenes

We report a catalytic asymmetric synthesis of piperidines through [4 + 2] cycloaddition of 1-azadienes and nitro-alkenes. The reaction uses earth abundant Zn as catalyst and is highly diastereo- and regioselective. A novel BOPA ligand (F-BOPA) confers high reactivity and enantioselectivity in the process. The presence of ortho substitution on the arenes adjacent to the bis(oxazolines) was found to be particularly impactful, due to limiting the undesired coordination of 1-azadiene to the Lewis acid and thus allowing the reaction to be carried out at lower temperature. A series of secondary kinetic isotope effect studies using a range of ligands implicates a stepwise mechanism for the transformation, involving an initial Michael-type addition of the imine to the nitro-alkene followed by a cyclization event. The stepwise mechanism obviates the electronic requirement inherent to a concerted mechanism, explaining the successful cycloaddition between two electron-deficient partners.

Direct access to pyrimidines through organocatalytic inverse-electron-demand Diels–Alder reaction of ketones with 1,3,5-triazine

An organocatalytic inverse-electron-demand Diels–Alder reaction of ketones with 1,3,5-triazine through enamine catalysis has been developed.

Gold(I)-catalyzed intermolecular cycloaddition of allenamides with α,β-unsaturated hydrazones: efficient access to highly substituted cyclobutanes

α,β-Unsaturated N,N-dialkyl hydrazones undergo a mild [2 + 2] cycloaddition to allenamides when treated with a suitable gold catalyst. The method, which represents the first application of N,N-dialkyl hydrazones in gold catalysis, is compatible with a wide variety of substituents at the alkenyl moiety of the hydrazone component, proceeds with excellent levels of regio- and diastereoselectivity, and provides densely substituted cyclobutanes with good to excellent yields.

The diene-transmissive hetero-Diels–Alder reaction of 2-vinyl α,β-unsaturated aldimines: stereoselective synthesis of hexahydroquinazolin-2-ones

The tandem Diels–Alder methodology of cross-conjugated 1-azatrienes for synthesis of crossed bis-cycloadducts with high chemo-, regio- and stereoselectivities is described.

1-Azadienes as regio- and chemoselective dienophiles in aminocatalytic asymmetric Diels-Alder reaction

Electron-deficient 1-aza-1,3-butadienes containing a 1,2-benzoisothiazole-1,1-dioxide or 1,2,3-benzoxathiazine-2,2-dioxide motif act as regio- and chemoselective dienophiles in normal-electron-demand Diels-Alder reactions with HOMO-raised trienamines, rather than typical 4π-participation in inverse-electron-demand versions. The enantioenriched cycloadducts could be efficiently converted to spiro or fused frameworks with high structural and stereogenic complexity by a sequential aza-benzoin reaction or other transformations.