A facile stereoselective synthesis of julolidine hybrid analogs via domino knoevenagel intramolecular hetero Diels–Alder reaction

We have reported here a facile stereoselective synthesis of julolidine hybrid analogs by employing domino Knoevenagel intramolecular hetero Diels–Alder reaction on symmetrical 1,3-diones and unsymmetrical 1,3-diones It was found that the cycloaddition proceeded efficiently under microwave irradiation to afford highly stereoselective cycloadducts in good yields     

Stereoselective synthesis of α-methylenecyclopentenones via a Diels–Alder/retro-Diels–Alder protocol

A new procedure for the stereoselective synthesis of cross-conjugated dienones is reported. This method makes use of the Diels–Alder adduct of anthracene and dimethyl fumarate, a precursor to a spirocyclopent-2-enone anthracene adduct as the key intermediate. The addition of propyllithium or octyllithium to the key intermediate followed by a retro-Diels–Alder reaction furnished α-methylenecyclopentenones bearing a γ-propyl or γ-octyl side chain, respectively, in moderate yields and as single geometric isomers.     

Stereoselective synthesis of chiral hydrocarbazoles via the catalytic Diels–Alder reaction of siloxyvinylindole and cyclic Z-olefin

The catalytic Diels–Alder reaction of siloxyvinylindole and cyclic Z-olefin derived from pyroglutamic acid gave optically active substituted hydrocarbazoles. The exo/endo selectivity of this reaction could be controlled by using an appropriate Lewis acid. Scandium triflate gave high exo-selectivity and copper triflate gave moderate endo-selectivity. Subsequent stereoselective alkylation of the cycloadduct led to the synthesis of highly substituted hydrocarbazoles with five continuous chiral centers including a quaternary carbon.     

Design and synthesis of oxa-bowls via Diels–Alder reaction and ring-rearrangement metathesis as key steps

Ring-rearrangement metathesis of norbornene derivatives prepared by Diels–Alder reaction has been used to design various oxa-bowls. Enyne ring-rearrangement metathesis of norbornene ring systems with propargyl moiety produced diene containing oxa-bowls, suitable for further expansion by Diels–Alder strategy.     

One-pot enyne ring-closing metathesis–Diels–Alder reactions for the synthesis of polycyclic sulfamides

Ring-closing metathesis (RCM) and sequential Yb(OTf)₃ promoted Diels–Alder reactions of sulfamide-linked enynes proceeded selectively in one-pot to afford a series of bicyclic and tricyclic sulfamides. Excellent levels of diastereoselectivity are observed for the cycloaddition step, with only the endo-adducts being isolated. The protocol was further extended to incorporate a one-pot RCM–cross metathesis (CM)–Diels–Alder sequence, permitting rapid access to high levels of molecular complexity from simple and easily accessible precursors.     

Recent advances of intermolecular Diels–Alder reaction in bio-inspired synthesis of natural products

The Diels–Alder(D–A)reaction is one of the most powerful reactions in organic synthesis.The intermolecular D–A reaction attracts much less attentions than its intramolecular counterpart in natural product synthesis,possibly due to the issues of reactivity and selectivity.In the past decade,the intermolecular D–A reaction has been increasingly utilized in the total synthesis of structurally complex natural products.In this article,we present a few examples for the elegant applications of the intermolecular D–A reaction that are inspired by biosynthetic hypotheses of the target natural products.These examples demonstrate that D–A reaction is not only useful for preparing building blocks but also powerful for coupling structurally complicated and sterically demanding segments in a highly chemo-and stereo-controlled fashion,which may inspire further developments of intermolecular D–A reaction from both strategy and methodology perspectives.     

Intramolecular Diels–Alder reaction for the synthesis of tetracyclic carbazoles and isocanthines

One pot intramolecular Diels–Alder reaction has been efficiently used as a new route for the synthesis of four tetracyclic carbazoles and four isocanthine analogues where a dialdehyde is utilised as a common intermediate for both the scaffolds. Biological activity was evaluated for some molecules, which demonstrated moderate activity against HeLa cervical cancer cell lines.     

Total synthesis of (±)-nicolaioidesin B via a highly regio- and diastereoselective Diels–Alder reaction

The total synthesis of (±)-nicolaioidesin B, a natural product that shows preferential cytotoxicity against pancreatic cancer cells under nutrient starvation conditions, is achieved. The key step is a thermally-induced, highly regio- and diastereoselective Diels–Alder reaction between (E)-N-methoxy-N-methylcinnamide and ocimene under solvent-free conditions.     

A four-step domino Knoevenagel–hetero-Diels–Alder reaction

A new, catalyst-free, domino Knoevenagel–hetero-Diels–Alder elimination oxidation reaction is reported. 5-Aryl-2H,6H-chromeno[4′,3′:4,5]thiopyrano[2,3-d][1,3]thiazol-2-ones are formed in moderate to good yields via reaction of 2-(3-aryl-3-chloro-2-propenyloxy)benzaldehydes with 4-thioxo-1,3-thiazolidin-2-one.     

Green chemistry for the synthesis of methacrylate-based hydrogels crosslinked through Diels–Alder reaction

This study describes an environmentally friendly and green synthetic approach for the preparation of poly(aminoethylmethacrylate)-based hydrogels crosslinked through Diels–Alder (DA) reaction in water. This “click” reaction offers the possibility of preparing chemically crosslinked hydrogels in the absence of any catalyst, initiator or coupling agent, thus preserving the biocompatibility of the material. The suitable furan diene was obtained by modifying a methacrylate polymer by its reaction with furfural, a first generation compound derived from renewable resources. Methacrylate-based complementary polymeric dienophiles were also prepared by introducing maleimide groups into the structure. The products obtained at different steps were characterized by FTIR, NMR and TGA techniques. The study of the rheological properties of the hydrogels proved the success of this green “click” synthetic strategy confirming the formation of chemically crosslinked networks by the use of the Diels–Alder reaction. Furthermore, SEM studies revealed promising morphological properties of the ensuing hydrogels in terms of biomedical applications.     

Construction of dihydropyran-bridged macrocycles by inverse-electron-demand Diels–Alder reaction

A variety of dihydropyran-bridged macrocyclic structures were constructed by the inverse-electron-demand Diels–Alder reaction of 2-oxo-4-aryl-but-3-enoates. Controlling of the tether length and the position of the activating substituent in the substrates would guide the reaction to the formation of four different types of polycyclic frames, namely bicyclic [n.3.1], bicyclic [n.2.2], tricyclic [n.3.1.1], and tricyclic [n.2.2.2] macrocycles. The intermolecular/intramolecular selectivity of the Diels–Alder reaction was virtually governed by the tether length. The reactions were carried out rapidly under mild conditions, and offered a practical method for creating bridged polycyclic structures with large rings from acyclic precursors.     

Asymmetric synthesis and the novel retro Diels–Alder reaction of a P-chiral sulfonyl-substituted phosphanorbornene

An asymmetric Diels–Alder reaction between phenylvinylsulfone and 1-phenyl-3,4-dimethylphosphole in the presence of a chiral organometallic reaction promoter generates the corresponding P-chiral sulfonyl-substituted phosphanorbornene which is found to undergo the facile cycloreversion reaction under mild conditions.     

Synthesis of isoindolinones via inverse-electron demand Diels–Alder cycloadditions

A new synthetic method has been developed for the synthesis of 2,4,5-substituted isoindolinones by means of intramolecular inverse-electron demand Diels–Alder cycloaddition reactions of substituted pyridazines. By taking advantage of the inherent reactivity of an intermediate chloropyridazine, a diverse set of analogues were prepared from common reagents and intermediates.     

Enantioselective synthesis of 1-nitrotricyclo[5,2,2,02,6]undeca-3,8-dienes via tandem consecutive asymmetric Diels–Alder reaction–Cope rearrangement

The 1-glyco-2-nitrocyclohexa-2,4-dienes 1a and 5a react with cyclopentadiene to yield, almost exclusively, the 10-glyco-1-nitrotricyclo[5,2,2,0^2,6]undeca-3,8-dienes 4a and 8a. Formation of these products is explained as the result of a tandem consecutive asymmetric Diels–Alder reaction–Cope rearrangement. Periodate oxidation of deprotected sugar side-chains, followed by sodium borohydride reduction yielded enantiomerically pure 10-formyl- and 10-hydroxymethyl-1-nitrotricyclo[5,2,2,0^2,6]undeca-3,8-dienes. Structures have been determined by X-ray crystallographic and spectroscopic analyses, and chemical correlation.     

Site-selective and regioselective Diels–Alder reaction of allenyl aryl ethers

Abstract  

The site-selectivity and regioselectivity of Diels–Alder reactions of allenyl aryl ethers with cyclopentadiene and acrolein were studied. While cyclopentadiene (as an electron-rich diene) only reacted with the external double bond of allenyl aryl ethers to provide the site-selective normal electron demand Diels–Alder cycloadducts, acrolein (as an electron-deficient diene) reacted with the C₁–C₂ π bond of allenyl aryl ethers to provide the site- and regioselective hetero-Diels–Alder cycloadducts as exclusive products.     

Asymmetric synthesis of isoquinuclidines by Diels–Alder reaction of 1,2-dihydropyridine utilizing a chiral Lewis acid catalyst

The chiral isoquinuclidine derivative, 2-azabicyclo[2.2.2]octane ring system, endo-(7R)-3 was obtained in good yield with excellent diastereoselectivity (up to 92% de) by Diels–Alder reaction of 1-(phenoxycarbonyl)-1,2-dihydropyridine 1 with N-acryloyl-(4S)-4-benzyloxazolidin-2-one (4S)-2 using titanium-(2R,3R)-TADDOLate 4 as a chiral Lewis acid catalyst in toluene at 0 °C. On the other hand, endo-(7S)-3 was obtained in good yield with excellent diastereoselectivity (up to 97% de) by Diels–Alder reaction of 1 with (4R)-2 using Cu(OTf)₂/(4S,4′S)-bis(oxazoline) catalyst 8 as a chiral Lewis acid catalyst in dichloromethane at 0 °C. In these reactions, the choice of solvent and the combination of titanium-(2R,3R)-TADDOLate 4 {or Cu(II)/(4S,4′S)-bis(oxazoline) 8} and dienophile (4S)-2 {or (4R)-2} are very important. The stereochemistry of endo-(7R)-3 has been established to be (1R,4S,7R) and the reaction mechanism is proposed.     

Thermal cross-link between 2,5-furandicarboxylic acid-based polyimides and bismaleimide via Diels–Alder reaction

A series of cross-linked polyimides (PIs) were prepared via two-step solution polycondensation from 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA) and 2,5-furandicarboxylic acid-based diamines, N,N′-bis(4-amino-2-(trifluoromethyl)phenyl)furan-2,5-dicarboxamide (TFFDA) and N,N′-bis(4-aminophenyl)furan-2,5-dicarboxamide (p-FDDA), followed by thermal crosslinking reaction with bismaleimide. The thermal crosslinking reaction and its mechanism were studied by FTIR spectra and model reaction analysis, which showed Diels–Alder reaction between furan group and maleimide group played a main role in the thermal treatment. The properties of cross-linked PIs were characterized using dynamic mechanical thermal analysis, thermogravimetric analyses, tensile testing, ultraviolet-visible spectra, and wide-angle X-ray diffraction. The cross-linked polyimide film showed improved solvent-resistance, thermal and mechanical properties with T_g values of 234–306^oC, tensile strengths of 82–98 MPa and moduli of 2.3–3.0 GPa.     

Aminocatalytic asymmetric Diels–Alder reaction of phosphorus dienophiles and 2,4-dienals

Chiral compounds bearing a phosphorus functional group are valuable because of their wide application in biological studies and asymmetric catalysis. Here we present an asymmetric Diels–Alder cycloaddition between phosphor-containing dienophiles and 2,4-dienals under the catalysis of chiral amine 1 via the intermediacy of trienamine species (trienamine catalysis). A spectrum of densely functionalized phosphonocyclohexene derivatives was efficiently constructed in excellent enantioselectivity (up to 99% ee) and with good to high diastereoselectivity (up to >19:1).     

Synthesis of tetrahydroanthracen-9-ones by the domino aldol condensation/Diels–Alder cycloaddition

A facile protocol toward several substituted 3-aryl-3,4,4a,10-tetrahydro-2H-anthracen-9-ones 1 starting with substituted 2-allylbenzaldehydes 2 was described. The overall synthetic process was carried out by the domino aldol condensation/Diels–Alder cycloaddition of skeleton 2 with substituted cinnamaldehydes 3 in an alkalic aqueous-methanolic solution followed by base-induced double migration of the resulting cycloadducts. Skeleton 2 was prepared from isovanillin (4) in moderate yield in five-steps via the known procedures with the synthetic sequence of O-allylation, Claisen rearrangement, O-methylation, Grignard methylation, and PCC-mediated oxidation.     

Water-soluble graphene dispersion functionalized by Diels–Alder cycloaddition reaction

Water-soluble graphene dispersions were fabricated by the exfoliation of graphite functionalized with furfuryl alcohol by Diels–Alder cycloaddition reaction. The pristine graphite was firstly heat-treated in N-methyl-2-pyrrolidone (NMP) before it was functionalized with furfuryl alcohol, and then, the increased interlayer spacing is propitious for furfuryl alcohol to enter into the lattice and react with graphite. High-resolution transmission electron microscopy and Raman spectroscopy indicate that the functional graphene is a high-quality product without any significant defects, and atomic force microscopy shows that the functional graphene consists of single to few layers graphene. Moreover, the grafting ratio onto graphene is up to 1.52 mmol/g. Therefore, the method provides a feasible route to produce functional graphene.