Recent Advances in Inverse-Electron-Demand Hetero-Diels-Alder Reactions of 1-Oxa-1,3-Butadienes

Pd(II)-Catalyzed tandem selective dehydrogenative [4+2] annulation of 2-methyl-1,3-cycloalkanediones with olefins

A practical and effective palladium-catalyzed selective dehydrogenative [4+2] annulation of 2-methyl-1,3-cycloalkanediones with olefins was reported. The active 2-methylene-1,3-cycloalkanedione was in situ generated via Pd-catalyzed enolate oxidation processes, and it subsequently reacted with a wide variety of olefins to afford various polysubstituted dihydropyran derivatives in good to excellent yields.

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.

Synthesis of azepane-fused pyrano[3,2-b]indoles by Lewis acid-catalysed oxa Diels-Alder reactions

The dihydroazepino[1,2-a]indole diones 3 are tricyclic oxindole-type enones which are readily accessible by catalytic photooxygenation of cyclohepta[b]indoles 1 followed by dehydration. Lewis acid-catalysed oxa Diels-Alder reactions of enones 3 with enol ethers 4 were developed that lead to novel tetracyclic azepane-fused pyrano[3,2-b]indoles 5, with high stereoselectivity and under mild reaction conditions.

Total Synthesis of Alcyonolide

The total synthesis of alcyonolide, an antitumor xenicn diterpenoid, has been achieved. The inverse electron demand hetero-Diels-Alder reaction using a dienophile possessing an electron-withdrawing group provided the endo adduct which included a condensed lactone and dihydropyran rings with the desired three stereogenic centers. After introduction of the side chain by the Negishi coupling, the lactone ring was opened to form a Weinreb amide. The sequential transformation including conversion of Weinreb amide to aldehyde, PMB to acetate, and allylation of the aldehyde gave a mixture of separable four diastereomers. The desired stereoisomer was submitted to the 2,2,6,6-tetramethylpiperidine 1-oxyl oxidation, which afforded the δ-lactone and the methyl ketone side chain. Finally, the olefin metathesis of the desired isomer gave racemic alcyonolide.

Sustainable Solvent-Free Diels-Alder Approaches in the Development of Constructive Heterocycles and Functionalized Materials: A Review

The Diels-Alder reaction (DAR) is found in myriad applications in organic synthesis and medicinal chemistry for drug development, as it is the method of choice for the expedient synthesis of complex natural compounds and innovative materials including nanomaterials, graphene expanses, and polymeric nanofibers. Furthermore, the greatest focus of attention of DARs is on the consistent reaction procedure with stimulus yields by highly stereo- and regioselective mechanistic pathways. Therefore, the present review is intended to summarize conventional solvent-free (SF) DARs for the expedient synthesis of heterocyclic compounds and materials. In particular, this review deals with the DARs of mechanochemical grinding, catalysis (including stereoselective catalysts), thermal, and electromagnetic radiation (such as microwave [MW], infrared [IR], and ultraviolet [UV] irradiation) in SF procedures. Therefore, this comprehensive review validates the application of DARs to pharmaceutical innovations and biorenewable materials through consistent synthetic approaches.

Catalytic Enantioselective Diels Alder Reaction: Application in the Synthesis of Antiviral Agents

The Diels–Alder reaction (DAR) is one of the most effective and reliable strategies for the construction of six-membered carbocyclic and heterocyclic rings, and it is widely used in the synthesis of organic molecules and drugs. Due to the high regio- and stereo-selectivity and its versatility, DARs have represented a powerful tool for organic chemistry for many years. In addition, the asymmetric DAR has become a fundamental synthetic approach in the preparation of optically active six-membered rings and natural compounds. The COVID-19-related pandemic requires continuous research; DAR represents an useful method to obtain optically active intermediates for the synthesis of antiviral agents under different catalytic conditions. We would like to highlight an intriguing synthetic procedure applied to the development of novel synthetic protocols that are potentially useful against a large panel of viruses and other unmet diseases.

Palladium-catalysed diastereodivergent inverse-electron-demand oxa-Diels-Alder reactions of in situ formed cyclopentadienones via ligand-control

Here we report Pd(0)-catalysed asymmetric inverse-electron-demand oxa-Diels-Alder reactions between the carbonates of 4-hydroxy-2-cyclopentenones and α-cyano chalcones, by in situ generating η2-Pd(0)-cyclopentadienone complexes as HOMO-raised dienophiles, and diastereodivergent synthesis could be...

Reactivity switch-over of 4-hydroxydithiocoumarins under various conditions and their application in organic synthesis

4-Hydroxydithiocoumarin is a valuable organic precursor to architect important heterocycles. The three different reactive nucleophilic sites in 4-hydroxydithiocoumarins display intriguing regioselectivity in their reaction towards various electrophiles. Previously, 4-hydroxydithiocoumarins have been used in the synthesis of heterocycles using Claisen and thio-Claisen reactions. Hetero-Diels-Alder reactions involving 4-hydroxydithiocoumarins have proven to be very convenient in assembling complex molecular organic frameworks from readily available feedstocks. Development of multicomponent reactions employing 4-hydroxydithiocoumarins has given rise to several important reaction protocols to access polycyclic compounds. Recently, this moiety has been used in forging some unusual S-S, S-N and S-O bonds under oxidative conditions which further explores its hidden reactivity in organic synthesis. Besides that, hydrothiolation of various alkynes and alkenes using 4-hydroxydithiocoumarins has led to the synthesis of some potential lead molecules. This mini-review provides an account of the reactivity pattern of 4-hydroxydithiocoumarins and their strategic applications in various reactions for the synthesis of several heterocycles and other important organic syntheses.

Enantioselective access to tricyclic tetrahydropyran derivatives by a remote hydrogen bonding mediated intramolecular IEDHDA reaction

Inverse-electron-demand-hetero-Diels-Alder reactions of alkenes with α,β-unsaturated keto compounds allow rapid access to the tetrahydropyran ring found in numerous natural products and bioactive molecules. Despite its synthetic interest, catalytic asymmetric versions of this process remain underdeveloped, especially regarding the use of non-activated alkenes reacting with α,β-unsaturated ketone or aldehyde, for which no report can be found in the literature. Herein, we describe the catalytic inverse-electron-demand-hetero-Diels-Alder reactions between neutral alkenes and an α,β-unsaturated ketones or aldehydes to produce a variety of trans-fused [5,6,8] tricyclic structures containing a central, chiral tetrahydropyran ring. This complex transformation, which is achieved using a chiral phosphoric acid, allows for the formation of four stereogenic centers in a single step with high regio-, diastereo- and enantioselectivity (up to 99% ee). Such level of stereocontrol could be achieved by a key remote double hydrogen atom bonding interaction between the linear substrate and the catalyst.

Although the hetero-Diels–Alder reaction is a staple of organic chemistry, catalytic asymmetric versions of the inverse-electron demand variant often require specially engineered substrates for the reaction to work. Here the authors cyclize non-activated alkenes with α,β-unsaturated ketones or aldehydes to form chiral fused heterocycles using a chiral phosphoric acid catalyst.

Can Primary Arylamines Form Enamine? Evidence, α-Enaminone, and [3+3] Cycloaddition Reaction

The formation of enamine from primary arylamines was detected and confirmed by nuclear magnetic resonance spectroscopy. The presence of a radical quencher, e.g., (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl, was found to be essential for the detection of enamine formation. A direct synthesis of α-enaminones from primary arylamines and ketones was also developed. Mechanistic investigation of α-enaminone formation suggests that an amine radical cation generated through O₂ singlet energy transfer was involved in initiating α-enaminone formation. The reactivity and utility of α-enaminones were explored with a [3+3] cycloaddition reaction of enones affording dihydropyridines in good yields (58-85%). α-Enaminones displayed a set of reactivities that is different from that of enamines. The knowledge gained in this work advances our basic understanding of organic chemistry, providing insights and new opportunities in enamine catalysis.

Alternating Current Electrolysis as Efficient Tool for the Direct Electrochemical Oxidation of Hydroxamic Acids for Acyl Nitroso Diels-Alder Reactions

The acyl nitroso Diels-Alder reaction of 1,3-dienes with electrochemically oxidised hydroxamic acids is described. By using alternating current electrolysis, their typical electro-induced decomposition could be suppressed in favour of the 1,2-oxazine cycloaddition products. The reaction was optimised using Design of Experiments (DoE) and a sensitivity test was conducted. A mixture of triethylamine/hexafluoroisopropanol served as supporting electrolyte in dichloromethane, thus giving products of high purity after evaporation of the volatiles without further purification. The optimised reaction conditions were applied to various 1,3-dienes and hydroxamic acids, giving up to 96 % isolated yield.

Catalyst-free and additive-free reactions enabling C-C bond formation: a journey towards a sustainable future

This review focuses on the catalyst- and additive-free C-C bond forming reactions reported mostly from the year 2005 to date. C-C bond forming reactions are highly important as large and complex organic molecules can be derived from simpler ones via these reactions. On the other hand, catalyst- and additive-free reactions are economical, environmentally friendly and less sensitive to air/moisture, allow easy separation of products and are operationally simple. Hence, a large number of research articles have been published in this area. Though a few reviews are available on the catalyst-free organic reactions, most of them were published a few years ago. The current review excludes catalysts as well as additives and is specific to only C-C bond formation. Besides many organic name reactions, catalyst/additive-free C-H functionalizations, coupling reactions and UV-visible-light-promoted reactions are also discussed. Undoubtedly, the contents of this review will motivate readers to do more novel work in this area which will accelerate the journey towards a sustainable future.

Generation and trapping of electron-deficient 1,2-cyclohexadienes. Unexpected hetero-Diels-Alder reactivity

Keto-substituted 1,2-cyclohexadienes were generated by base-mediated (KOt-Bu) elimination, and found to dimerize via an unprecedented formal hetero-Diels-Alder process, followed by hydration. These highly reactive cyclic allene intermediates were also trapped in Diels-Alder reactions by furan, 2,5-dimethylfuran, or diphenylisobenzofuran to afford cycloadducts with high regio- and diastereoselectivity, and could also be intercepted in a hetero-Diels-Alder process with enamine dienophiles. Endo/exo stereochemistry was unambiguously determined via X-ray crystallography in the case of nitrile-substituted 1,2-cyclohexadiene. DFT calculations indicate that the novel hetero-Diels-Alder processes observed with these allenes occur via a concerted asynchronous cycloaddition mechanism.

Gold(I)-Catalyzed Domino Reaction for Furopyrans Synthesis

We report herein an efficient synthesis of furopyran derivatives through a gold(I)-catalyzed domino reaction. The cascade reaction starts with two regioselective cyclizations, a 5-endo-dig and a 8-endo-dig, followed with a Grob-type fragmentation and a hetero Diels–Alder. The obtained furopyran derivatives contain fused and spiro-heterocycles. During this one-pot process, four bonds and four controlled stereogenic centers including a quaternary center are formed.

Organophotoredox-Catalyzed Intermolecular Oxa-[4+2] Cycloaddition Reactions

An intermolecular oxa-[4+2] cycloaddition reaction promoted by a thioxanthylium photoredox catalyst under irradiation with green light has been developed. The reaction of ortho-quinone methides with styrenes smoothly affords the desired cycloadducts. Especially styrenes bearing electron-donating groups are efficiently transformed in this reaction. This method represents a sustainable way to carry out oxa-[4+2] cycloaddition reactions using only a catalytic amount of a photocatalyst and visible light.

Novel Synthesis of Substituted 2-Trifluoromethyl and 2-Perfluoroalkyl N-Arylpyridinium Compounds-Mechanistic Insights

We report a new one-pot synthesis of 2-trifluoromethylated/2-perfluoroalkylated N-aryl-substituted pyridiniums, 5,6,7,8-tetrahydroquinoliniums and 6,7,8,9-tetrahydro-5H-cyclohepta[b]-pyridinium compounds starting from an activated β-dicarbonyl analogue (here a perfluoro-alkylated gem-iodoacetoxy derivative), an aromatic amine and a (cyclic or acyclic) ketone. The key step of this multicomponent reaction, involves the formation of a 3-perfluoroalkyl-N,N’-diaryl-1,5-diazapentadiene intermediate, various examples of which were isolated and characterized for the first time, together with investigation of their reactivity. We propose a mechanism involving a concurrent inverse electron demand Diels-Alder or Aza-Robinson cascade cyclisation, followed by a bis-de-anilino-elimination. Noteworthy, a meta-methoxy substituent on the aniline directs the reaction towards a 2-perfluoroalkyl-7-methoxyquinoline, resulting from the direct cyclization of the diazapentadiene intermediate, instead of pyridinium formation. This is the first evidence of synthesis of pyridinium derivatives from activated β-dicarbonyls, ketones, and an aromatic amine, the structures of which (both reactants and products) being analogous to species involved in biological systems, especially upon neurodegenerative diseases such as Parkinson’s. Beyond suggesting chemical/biochemical analogies, we thus hope to outline new research directions for understanding the mechanism of in vivo formation of pyridiniums, hence possible pharmaceutical strategies to better monitor, control or prevent it.

Inverse-Electron-Demand oxa-Diels-Alder Reactions of α-Keto-β,γ-unsaturated Esters and α,β-Unsaturated Hydrazones

A concise synthetic method for dihydropyrans has been developed by inverse-electron-demand oxa-Diels-Alder reaction of α-keto-β,γ-unsaturated esters with α,β-unsaturated hydrazones as electron-rich olefins. This reaction is catalyzed by Eu(hfc)₃ and proceeds in an endo-selective manner. This umpolung cycloaddition affords a variety of substituted dihydropyrans stereoselectively in high yields. In addition, indirect synthesis of formyl-substituted dihydropyran was achieved by dehydrazonation of the cycloadduct. This method is expected to be useful for the synthesis of dihydropyrans and tetrahydropyrans with unusual substitution patterns.

A General Biomimetic Hetero-Diels-Alder Approach to the Core Skeletons of Xenovulene A and the Sterhirsutins A and B

A biomimetic, regio- and stereoselective approach to the 5,6,11-tricyclic core skeleton of xenovulene A, as well as sterhirsutins A and B, is described. The key steps are a biomimetic inverse-electron-demand hetero-Diels-Alder cycloaddition of α-humulene and a ribose-derived vinyl ketone, followed by acid-catalyzed rearrangement of the 1,3-dioxolane that neighbors the resultant cyclic enol ether.

Factors Controlling the Diels-Alder Reactivity of Hetero-1,3-Butadienes

We have quantum chemically explored the Diels-Alder reactivities of a systematic series of hetero-1,3-butadienes with ethylene by using density functional theory at the BP86/TZ2P level. Activation strain analyses provided physical insight into the factors controlling the relative cycloaddition reactivity of aza- and oxa-1,3-butadienes. We find that dienes with a terminal heteroatom, such as 2-propen-1-imine (NCCC) or acrolein (OCCC), are less reactive than the archetypal 1,3-butadiene (CCCC), primarily owing to weaker orbital interactions between the more electronegative heteroatoms with ethylene. Thus, the addition of a second heteroatom at the other terminal position (NCCN and OCCO) further reduces the reactivity. However, the introduction of a nitrogen atom in the backbone (CNCC) leads to enhanced reactivity, owing to less Pauli repulsion resulting from polarization of the diene HOMO in CNCC towards the nitrogen atom and away from the terminal carbon atom. The Diels-Alder reactions of ethenyl-diazene (NNCC) and 1,3-diaza-butadiene (NCNC), which contain heteroatoms at both the terminal and backbone positions, are much more reactive due to less activation strain compared to CCCC.

Synthesis of symmetrical and unsymmetrical triarylpyrylium ions via an inverse electron demand Diels-Alder reaction

BF₃·OEt₂ mediated inverse electron demand Diels-Alder (IEDDA) reaction of chalcones with aryl acetylenes is reported for the synthesis of symmetrical and unsymmetrical 2,4,6-triarylpyrylium ions. The protocol provides an effective one-pot method for the utilization of readily available simple substrates under mild reaction conditions leading to a diverse array of pyrylium ions in moderately good yield.

Covalent modification of biological targets with natural products through Paal-Knorr pyrrole formation

Covering: up to June 2017Natural products and endogenous metabolites engage specific targets within tissues and cells through complex mechanisms. This review examines the extent to which natural systems have adopted the Paal-Knorr reaction to engage nucleophilic amine groups within biological targets. Current understanding of this mode of reactivity is limited by only a few examples of this reaction in a biological context. This highlight is intended to stimulate the scientific community to identify potential research directions and applications of the Paal-Knorr reaction in native and engineered biological systems.

A droplet-chip/mass spectrometry approach to study organic synthesis at nanoliter scale

A droplet-based microfluidic device with seamless hyphenation to electrospray mass spectrometry was developed to rapidly investigate organic reactions in segmented flow providing a versatile tool for drug development. A chip-MS interface with an integrated counterelectrode allowed for a flexible positioning of the chip-emitter in front of the MS orifice as well as an independent adjustment of the electrospray potentials. This was necessary to avoid contamination of the mass spectrometer as well as sample overloading due to the high analyte concentrations. The device was exemplarily applied to study the scope of an amino-catalyzed domino reaction with low picomole amount of catalyst in individual nanoliter sized droplets.