Recent Advances on Diels-Alder-Driven Preparation of Bio-Based Aromatics

Efficient synthesis of fully renewable, furfural-derived building blocks via formal Diels-Alder cycloaddition of atypical addends

Diels-Alder (DA) cycloaddition of furanics is emerging as a key transformation in circular chemistry, providing access to highly versatile, biobased platform molecules. Further development of this technology into viable industrial applications faces major challenges, a notorious one being the lack of reactivity of the most readily available furans, i.e. the furfural derivatives. Herein we describe the remarkably-facile intramolecular DA reaction of allyl acetals of different furfurals to efficiently afford formal DA adducts with the atypical, unreactive dienophile allyl alcohol. Our methodology gives access to unprecedented oxanorbornene derivatives in high chemo-, regio- and stereoselectivity, which can be readily diversified into valuable products. This offers the potential of scalable production of renewable chemical building blocks from cheap, bioderived platform molecules.

Substituted anilides from chitin-based 3-acetamido-furfural

The synthesis of aromatic compounds from biomass-derived furans is a key strategy in the pursuit of a sustainable economy. Within this field, a Diels-Alder/aromatization cascade reaction with chitin-based furans is emerging as a powerful tool for the synthesis of nitrogen-containing aromatics. In this study we present the conversion of chitin-based 3-acetamido-furfural (3A5F) into an array of di- and tri-substituted anilides in good to high yields (62-90%) via a hydrazone mediated Diels-Alder/aromatization sequence. The addition of acetic anhydride expands the dienophile scope and improves yields. Moreover, replacing the typically used dimethyl hydrazone with its pyrrolidine analogue, shortens reaction times and further increases yields. The hydrazone auxiliary is readily converted into either an aldehyde or a nitrile group, thereby providing a plethora of functionalized anilides. The developed procedure was also applied to 3-acetamido-5-acetylfuran (3A5AF) to successfully prepare a phthalimide.

Understanding the reactivity and selectivity of Diels-Alder reactions involving furans

The reactivity and endo/exo selectivity of the Diels-Alder cycloaddition reactions involving furan and substituted furans as dienes have been computationally explored. In comparison to cyclopentadiene, it is found that furan is comparatively less reactive and also less endo-selective in the reaction with maleic anhydride as the dienophile. Despite that, both the reactivity and the selectivity can be successfully modified by the presence of substituents at either 2- or 3-positions of the heterocycle. In this sense, it is found that the presence of strong electron-donor groups significantly increases the reactivity of the system while the opposite is found in the presence of electron-withdrawing groups. The observed trends in both the reactivity and selectivity are analyzed quantitatively in detail by means of the activation strain model of reactivity in combination with the energy decomposition analysis methods.

π-Facial selectivity in the Diels-Alder reaction of glucosamine-based chiral furans and maleimides

Furans derived from carbohydrate feedstocks are a versatile class of bio-renewable building blocks and have been used extensively to access 7-oxanorbornenes via Diels-Alder reactions. Due to their substitution patterns these furans typically have two different π-faces and therefore furnish racemates in [4 + 2]-cycloadditions. We report the use of an enantiopure glucosamine derived furan that under kinetic conditions predominantly affords the exo-product with a high π-face selectivity of 6.5 : 1. The structure of the product has been resolved unequivocally by X-ray crystallography, and a multi-gram synthesis (2.8 g, 58% yield) confirms the facile accessibility of this multifunctional enantiopure building block.

Targeting Valuable Chemical Commodities: Hydrazine-mediated Diels-Alder Aromatization of Biobased Furfurals

A hydrazine-mediated approach towards renewable aromatics production via Diels-Alder aromatization of readily available, biobased furfurals was explored as alterative to the more classical approaches that rely on reactive but uneconomical reduced dienes (e. g., 2,5-dimethylfuran). To enable cycloaddition chemistry with these otherwise unreactive formyl furans, substrate activation by N,N-dimethyl hydrazone formation was investigated. The choice of the reaction partner was key to the success of the transformation, and in this respect acrylic acid showed the most promising results in the synthesis of aromatics. This strategy allowed for selectivities up to 60 % for a complex transformation consisting of Diels-Alder cycloaddition, oxabridge opening, decarboxylation, and dehydration. Exploration of the furfural scope yielded generic structure-reactivity-stability relationships. The proposed methodology enabled the redox-efficient, operationally simple, and mild synthesis of renewable (p-disubstituted) aromatics of commercial importance under remarkably mild conditions.

Tandem Thio-Michael Addition/Remote Lactone Activation of 5-Hydroxymethylfurfural-Derived δ-Lactone-Fused Cyclopentenones

The creation of structurally diverse chemical entities from fairly simple biorefinery products remains a challenge. In this work 5-hydroxymethylfurfural (HMF) was identified as a key synthon for preparing highly complex cyclopentenones (CP) via tandem 1,4-addition/elimination/remote lactone activation to external O- and N-nucleophiles in δ-lactone-fused-CPs hotspots. This scaffold was also reactive enough to be incorporated into model cysteine-peptides in low concentrations, paving the way to a potential translation generating complexity in the synthesis of small peptides. The new enones also exhibited activity against intraerythrocytic Plasmodium falciparum (IC₅₀ =1.32 μm).

The Interplay between Kinetics and Thermodynamics in Furan Diels-Alder Chemistry for Sustainable Chemicals Production

Biomass‐derived furanic platform molecules have emerged as promising building blocks for renewable chemicals and functional materials. To this aim, the Diels–Alder (DA) cycloaddition stands out as a versatile strategy to convert these renewable resources in highly atom‐efficient ways. Despite nearly a century worth of examples of furan DA chemistry, clear structure–reactivity–stability relationships are still to be established. Detailed understanding of the intricate interplay between kinetics and thermodynamics in these very particular [4+2] cycloadditions is essential to push further development and truly expand the scope beyond the ubiquitous addend combinations of electron‐rich furans and electron‐deficient olefins. Herein, we provide pertinent examples of DA chemistry, taken from various fields, to highlight trends, establish correlations and answer open questions in the field with the aim to support future efforts in the sustainable chemicals and materials production.

Atom-economic Approach to the Synthesis of α-(Hetero)aryl-substituted Furan Derivatives from Biomass

An atom-economic ring construction approach to the synthesis of α-(hetero)arylfurans based on renewable furanic platform chemicals has been developed. Corresponding compounds have been prepared in good to excellent yields via [2+2+2] and [4+2] cycloaddition reactions using metal-catalyzed or photoredox protocols. Easily available HMF-based 2-hydroxymethyl-5-ethynylfuran and 2-hydroxymethyl-5-cyanofuran were used as starting materials. A synthetic route with an improved carbon economy factor has been implemented to achieve sustainability aim. The possible application of arylfurans as molecular conductors has been investigated by DFT calculations, which revealed excellent charge transfer properties. As a future perspective, integration of biomass processing strategy into manufacturing of molecular electronics was pointed out to achieve the aim of sustainability.

Advances in Diels-Alder/aromatization of biomass furan derivatives towards renewable aromatic hydrocarbons

The effective upgrading of renewable resources into high value-added chemicals is of great significance to achieve the sustainable economic development, as well as the implementation of carbon neutral technologies practically....

Diels–Alder Cycloadditions of Bio-Derived Furans with Maleimides as a Sustainable «Click» Approach towards Molecular, Macromolecular and Hybrid Systems

This mini-review highlights the recent research trends in designing organic or organic-inorganic hybrid molecular, biomolecular and macromolecular systems employing intermolecular Diels–Alder cycloadditions of biobased, furan-containing substrates and maleimide dienophiles. The furan/maleimide Diels–Alder reaction is a well-known process that may proceed with high efficiency under non-catalytic and solvent-free conditions. Due to the simplicity, 100% atom economy and biobased nature of many furanic substrates, this type of [4+2]-cycloaddition may be recognized as a sustainable “click” approach with high potential for application in many fields, such as fine organic synthesis, bioorganic chemistry, material sciences and smart polymers development.

Production of Copolyester Monomers from Plant-Based Acrylate and Acetaldehyde

PCTA is an important copolyester that has been widely used in our daily necessities. Currently, its monomers are industrially produced from petroleum-derived xylene. To reduce the reliance on fossil energy, we herein disclose an alternative route to access PCTA monomer (terephthalate/isophthalate=2.4/1) in 61 % overall yield using plant-based acrylate and acetaldehyde as the feedstocks. The process includes Morita-Baylis-Hillman (MBH) reaction of acetaldehyde with acrylate, subsequent one-step dehydration/Diels-Alder reaction with acrylate over H₂ SO₄ /SiO₂ catalyst, and final Pd/C-catalyzed dehydrogenation. Besides, when varying the final step to hydrogenation, another important monomer UNOXOL™ diol (1,4-trans/1,4-cis/1,3-trans/1,3-cis=5.2/2/2.5/1) can be produced in 67 % overall yield.

Intermolecular Diels-Alder Cycloadditions of Furfural-Based Chemicals from Renewable Resources: A Focus on the Regio- and Diastereoselectivity in the Reaction with Alkenes

A recent strong trend toward green and sustainable chemistry has promoted the intensive use of renewable carbon sources for the production of polymers, biofuels, chemicals, monomers and other valuable products. The Diels-Alder reaction is of great importance in the chemistry of renewable resources and provides an atom-economic pathway for fine chemical synthesis and for the production of materials. The biobased furans furfural and 5-(hydroxymethyl)furfural, which can be easily obtained from the carbohydrate part of plant biomass, were recognized as "platform chemicals" that will help to replace the existing oil-based refining to biorefining. Diels-Alder cycloaddition of furanic dienes with various dienophiles represents the ideal example of a "green" process characterized by a 100% atom economy and a reasonable E-factor. In this review, we first summarize the literature data on the regio- and diastereoselectivity of intermolecular Diels-Alder reactions of furfural derivatives with alkenes with the aim of establishing the current progress in the efficient production of practically important low-molecular-weight products. The information provided here will be useful and relevant to scientists in many fields, including medical and pharmaceutical research, polymer development and materials science.