Organocatalytic Asymmetric Approach to γ,δ-Functionalization of 3-Cyano-4-styrylcoumarins via Bifunctional Catalysis.

A novel organocatalytic reaction cascade between 3-cyano-4-styrylcoumarins and 2-mercaptoacetophenones is described. It is based on stereocontrolled functionalization of cyanocoumarins proceeding in a sequence of thia-Michael/aldol/annulation reactions. This highly diastereo- and enantioselective reaction is realized by employing enantioselective bifunctional catalysis and exhibits a broad substrate scope and excellent functional group tolerance. The synthetic application involves the transformation of the imidoester group, thus opening access to biologically relevant coumarin and δ-lactone-fused products.

Dearomative Michael addition involving enals and 2-nitrobenzofurans realized under NHC-catalysis.

In this manuscript, the first enantioselective dearomative Michael addition between α,ß-unsaturated aldehydes and 2-nitrobenzofurans realized under N-heterocyclic carbene activation has been described. The reaction proceeds via addition of homoenolate to Michael acceptors leading to the formation of biologically important heterocycles with high yields and stereoselectivities. Their functionalization potential has been confirmed in selected, diastereoselective transformations.

Asymmetric Dearomative (3+2)-Cycloaddition Involving Nitro-Substituted Benzoheteroarenes under H-Bonding Catalysis.

In our studies, the organocatalytic 1,3-dipolar cycloaddition between 2-nitrobenzofurans or 2-nitrobenzothiophene and N-2,2,2-trifluoroethyl-substituted isatin imines has been developed. The reaction has been realized by employing bifunctional organocatalysis, with the use of squaramide derivative being crucial for the stereochemical efficiency of the process. The usefulness of the cycloadducts obtained has been confirmed in selected transformations, including aromative and non-aromative removal of the nitro group.

Enantioselective H-bond-directed vinylogous iminium ion strategy for the functionalization of vinyl-substituted heteroaryl aldehydes.

In this work the first H-bond-directed vinylogous iminium ion strategy has been developed as a convenient strategy for the γ,δ-functionalization of vinyl-substituted heteroaromatic aldehydes. Their reaction with α-mercaptoketones proceeds in a cascade manner involving 1,6-addition followed by intramolecular aldol reaction. Excellent stereoselectivities have been obtained as a result of the H-bond interactions controlling the outcome of the cyclization step. The application of the strategy for the synthesis of tricyclic compounds bearing furan, tetrahydrothiophene and dihydropyran moieties has also been demonstrated.

Doubly vinylogous and doubly rearomative functionalization of 2-alkyl-3-furfurals.

The manuscript describes a straightforward functionalization of 2-alkyl-3-furfurals via simple aminocatalytic conjugate addition. The reaction proceeds through the formation of dearomatized dienamine-like intermediate that undergoes 1,6-addition to 4-alkylidene-2,6-dialkylcyclohexa-2,5-dienones. This process can be described as doubly rearomative as it proceeds with the re-formation of both furan and phenyl aromatic moieties. Target products have been obtained in a highly stereoselective manner, providing an interesting example of 2-alkyl-3-furfural functionalization via doubly vinylogous Michael addition. The mechanism of the reaction has been studied by means of computational methods.

The Game of Electrons: Organocatalytic Higher-Order Cycloadditions Involving Fulvene- and Tropone-Derived Systems.

The great progress that took place in the field of higher-order cycloadditions involving fulvene- and tropone-derived systems in the last few years is astonishing. By application of organocatalytic activation modes, new higher-order reactivities have been identified and described in the literature. These approaches take advantage of the high reliability of organocatalysis, at the same time expanding its potential and paving new directions for its further evolution. In this Minireview, the progress in the field of organocatalytic higher-order cycloadditions involving fulvene- and tropone-derived systems is summarized and insights into mechanistic aspects of the developed reactivities are provided. Furthermore, the discussion on the nomenclatural issues related to cycloaddition reactions has been conducted and solutions to clarify the picture proposed.

Inverting the reactivity of troponoid systems in enantioselective higher-order cycloaddition.

By the replacement of the oxygen with the sulfur atom in tropone, a novel reactivity pattern of troponoid systems in enantioselective transformation was achieved. It utilizes tropothione as an 8π-component in the [8+2]-cycloaddition involving α,ß-unsaturated aldehydes. The stereochemistry of the process is governed through the aminocatalytic LUMO-activation of the carbonyl reactant. It is in marked contrast to recently developed aminocatalytic higher-order cycloadditions proceeding via HOMO-activation strategies.

Site-Selective and Enantioselective α,ß,γ-Functionalization of 5-Alkylidenefuran-2(5 H)-ones: A Route to Polycyclic γ-Lactones.

A new strategy for a direct α,ß,γ-functionalization of the γ-lactone framework in the corresponding 5-alkylidenefuran-2(5 H)-ones is reported. The developed approach is based on a stereocontrolled cascade reaction with 2-mercaptocarbonyl compounds proceeding in a sequence of thia-Michael/aldol/oxa-Michael reactions. Such a synthetic strategy allows for a construction of a unique polycyclic architecture containing γ-lactone, tetrahydrofuran, and tetrahydrothiophene ring systems. Excellent enantioselectivities and diastereoselectivities have been obtained in the presence of bifunctional catalyst derived from cinchona alkaloids.

Breaking Aromaticity with Aminocatalysis: A Convenient Strategy for Asymmetric Synthesis.

Since the turn of the millennium, catalytic reactions promoted by primary or secondary amines have emerged as a highly useful method for organic chemistry. Very recently, the potential of such synthetic strategies has been vastly expanded by incorporating the principle of aromaticity breaking. Novel reaction pathways can now be accessed through the intermediacy of polyenamine intermediates derived from (hetero)aromatic systems. The proper design of carbonyl starting materials, combined with the high stereochemical efficiency provided by well-established aminocatalysts, has enabled the synthesis of products with (hetero)aromatic frameworks in highly enantio- and diastereomerically enriched form. In this Minireview, we provide an overview of recent advances in this field of research and familiarize the reader with new synthetic opportunities offered by these interesting methodologies.

Synthesis of γ,γ-Disubstituted Butenolides through a Doubly Vinylogous Organocatalytic Cycloaddition.

A novel organocatalytic approach to γ,γ-disubstituted butenolides is described. It is based on a fully site-selective functionalization of 5-alkylidenefuran-2(5H)-ones via trienamine-mediated [4+2]-cycloaddition with α,ß,γ,δ-diunsaturated aldehydes. The developed methodology proceeds with excellent stereocontrol and constitutes a unique example of trienamine chemistry with vinylogous dienophiles. Importantly, the reaction has very broad scope and allows for the introduction of substituents also in the α- or the ß-position of the butenolide ring. Usefulness of the products obtained has been confirmed in the intramolecular Stetter reaction leading to polycyclic product.

The Application of 2-Benzyl-1,4-naphthoquinones as Pronucleophiles in Aminocatalytic Synthesis of Tricyclic Derivatives.

This study demonstrates an unprecedented reactivity of 2-substituted-1,4-naphthoquinones. By applying the principle of vinylogy, they have been employed as vinylogous pronucleophiles in the organocatalytic cascade reaction for the first time. This novel catalytic activation of 1,4-naphthoquinones enables access to carboannulated naphthalen-1(4 H)-one derivatives of biological importance. The site-selectivity and stereoselectivity of a process proved possible to control by the proper choice of reaction conditions.

Organocatalytic Nonclassical Trienamine Activation in the Remote Alkylation of Furan Derivatives.

A new approach for the stereoselective remote alkylation of furan derivatives is reported. The reaction of 5-alkylfurfurals with nitroolefins as electrophilic counterparts occurs at their exocyclic ε-position and proceeds through the intermediacy of a nonclassical catalytic trienamine intermediate. The aminocatalyst bearing a H-bonding unit is used to control the stereochemical reaction outcome confirming the usefulness of such catalytic systems for the remote functionalizations of carbonyl compounds. Target products with two adjacent stereogenic centers are obtained in excellent yields and with good to moderate stereoselectivities.