Dynamics of Diels–Alder reactions involving 2-trialkylsilyloxyfurans

Increasing the size of the silyl group on 2-trialkylsilyloxyfurans reduces the rate of Diels–Alder reactions with maleic acid derivatives. While the exo-adduct resulted from the reaction between 2-silyloxyfurans and maleic anhydride, endo-adducts resulted from the reactions with maleate esters. Analysis of transition state structures for the cycloaddition, calculated at the B3LYP/6-31G1 level of theory, revealed significant stretch-mode asynchronicity in the forming bonds, with selectivity arising from steric interactions that affect torsional strain about the shorter of the forming bonds.     

Diels–Alder reactions of five-membered heterocycles containing one heteroatom

Diels–Alder reactions of five-membered heterocycles containing one heteroatom with an N-arylmaleimide were studied. Cycloaddition of 2,5-dimethylfuran (4) with 2-(4-methylphenyl)maleimide (3) in toluene at 60 °C gave bicyclic adduct 5. Cycloadditions of 3 with 2,5-dimethylthiophene (11) and 1,2,5-trimethylpyrrole (14) were also studied. Interestingly, the bicyclic compound 5 cleanly rearranged, with loss of water, when treated with p-toluenesulfonic acid in toluene at 80 °C to give 4,7-dimethyl-2-p-tolylisoindoline-1,3-dione (6).     

Asymmetric Diels–Alder reactions of optically active oxazolidinone-derived vinylsulfonamides

Chiral vinylsulfonamides bearing an oxazolidin-2-one moiety derived from (R)-2-phenylglycinol and (1R,2S)-norephedrine have been employed as dienophiles in asymmetric Diels–Alder reactions at normal pressure affording d.e.s of up to 99% when EtAlCl₂ or Et₂AlCl were employed as Lewis acids. Theoretical calculations justify the observed enantio- and diastereoselectivity.     

Enantioselective Diels–Alder reactions with left-handed G-quadruplex DNA-based catalysts

Since the discovery of left-handed G-quadruplex(L-G4)structure formed by natural DNA,there has been a growing interest in its potential functions.This study uti...     

Palladium-catalyzed asymmetric Diels–Alder reactions with novel chiral imino-phosphine ligands

Palladium-catalyzed asymmetric Diels–Alder reactions have been achieved with considerably high enantioselectivity by using chiral imino-phosphine ligands derived from (1S,4R)-(+)-fenchone, (1R,2R,5R)-(+)-2-hydroxy-3-pinanone derivatives, (1S,5R)-(−)-menthone, (1R,4R)-(+)-camphor, and (1S)-(+)-ketopinic acid. A mechanism for the asymmetric induction is proposed on the basis of the stereochemical outcome of the reactions.     

Diels–Alder reactions of directly C3-dieneylated chlorophyll derivatives

The [4π?+?2π] cycloaddition of methyl 3-(1,3-butadien-1/2-yl)pyropheophorbides-a with tetracyanoethylene, dimethyl acetylenedicarboxylate, and naphthoquinone gave the corresponding Diels–Alder reaction adducts. The trans-1-substituted 1,3-butadiene was more reactive than its regioisomeric 2-substituent. The oxidation of some cycloadducts gave C3-arylated chlorophyll derivatives, whose ortho-substitution blue-shifted the corresponding Qy absorption maxima in dichloromethane because of the steric repulsion between the aryl and chlorin π-planes.     

Enhanced reactivity of tuned imidazolidin-2-one chiral auxiliaries in Diels–Alder reactions

Readily accessible alkyl 1-N-benzoyl-2-oxoimidazolidin-4-carboxylates display enhanced reactivity as efficient chiral auxiliaries in the Diels–Alder reaction of their 3-N-enoyl derivatives.     

The Diels–Alder reactions of para-benzoquinone nitrogen-derivatives: an experimental and theoretical study

An experimental and theoretical study of the comparative reactivity and selectivity of the Diels–Alder reactions of para-benzoquinones and three nitrogen derivatives have been performed. The mono-oximes derivatives do not react under the tested reaction conditions, whereas the tosylated mono-oximes react slowly. However, the mono N-tosyl imines show excellent reactivity, and superior to the parent para-benzoquinones. DFT calculations support these experimental results.     

Ruthenium carbenes as catalysts in stereoselective ene–yne metathesis/Diels–Alder and ene–yne metathesis/Diels–Alder/cross coupling multicomponent reactions

An ene–yne cross metathesis of silyl substituted alkynes and alkenes followed by a Diels–Alder reaction of the metathesis product 2-silyl-1,3-dienes has been developed. The dienes thus prepared in situ were shown to participate in highly diastereoselective Diels–Alder reactions. In one case the silicon substituted Diels–Alder cycloadduct was subsequently used without isolation and purification in a Hiyama cross coupling reaction. The cross coupling reactions enable these silicon dienes to be used as synthons for a variety of other dienes.     

Structure–reactivity relationship in Diels–Alder reactions obtained using the condensed reaction graph approach

By the structural representation of a chemical reaction in the form of a condensed graph a model allowing the prediction of rate constants (logk) of Diels–Alder reactions performed in different solvents and at different temperatures is constructed for the first time. The model demonstrates good agreement between the predicted and experimental logk values: the mean squared error is less than 0.75 log units. Erroneous predictions correspond to reactions in which reagents contain rarely occurring structural fragments. The model is available for users at https://cimm.kpfu.ru/predictor/.     

Ring opening of dihydro-2-pyridones and intramolecular Diels–Alder reactions

A new ring-opening reaction of 5,6-dihydro-2-pyridones was discovered. Compounds 1 and 7 were converted to the dienoic amides 2 and 8 by reaction with sodium hydride at room temperature. N-Allylation of compounds 2 and 8 followed by IMDA reaction provided the cis-fused hexahydro-1-indolones 5 and 10, respectively. Treatment of compounds 5 and 10 with DBU in refluxing ethyl acetate gave the conjugated products 6 and 11, which were further transformed to the amides 12–15. The phenylthio group of compound 11 was substituted by a methyl group to give product 16.     

Superelectrophilic Diels–Alder reactions and oxidations leading to heterocyclic biaryl compounds

Heterocyclic imines provide biaryl products by a two-step transformation. The first transformation involves a Diels–Alder reaction with a multiply protonated imine to give a tetrahydroquinoline product, whereas the second step involves oxidation with elemental sulfur at 260?°C.     

Sustainable Diels–Alder syntheses in imidazolium ionic liquids

An effective and simple protocol for the Diels–Alder reactions catalyzed and mediated by ionic liquid in conjunction with Lewis acid was studied. Diels–Alder adduct of the reaction of 1,4-benzoquinone with isoprene and 2,3-dimethyl-1,3-butadiene was obtained in good to excellent yields within the time span of 5.00–4.00 min under microwave irradiation. Due to the excellent activity of 2,3-dimethyl-1,3-butadiene as a diene, its efficiency was further analyzed by performing against four different dienophiles under various reaction conditions. The ionic liquid, 3-methyl-1-octyl-imidazolium tetrachloroaluminate mixed with Lewis acids, could be recycled and reused for consecutive cycles. The recovered ionic liquid in conjunction with Lewis acid displayed almost similar activity without any significant loss. This system is an essential upgrade and related to the reactions consuming common organic solvents at high temperatures and longer duration. Diels–Alder reactions under solventless conditions have also been investigated for the aforesaid reactions.     

Cycloaddition of keteniminium with terminal alkynes toward cyclobuteniminium and their use in Diels–Alder reactions

An efficient access to cyclobutanone derivatives has been developed via a ‘one-pot’ [2+2]/[4+2] sequence involving keteniminium and cyclobuteniminium salts as key intermediates. A broad range of novel cyclobuteniminium salts have been prepared via [2+2] cycloaddition of keteniminium salts and alkynes. The resulting [2+2] adducts were then further transformed by Diels–Alder reaction with various dienes to afford cyclobutanone derivatives in good yields. A density functional theory (DFT) based computational study has been performed to obtain insight into the nature of the cycloaddition reactions and to investigate the difference in reactivity of cyclobuteniminium salts. Finally, the usefulness of cyclobutanone derivatives has been demonstrated by ring expansion reactions affording lactone, lactame, and cyclopentanone derivatives.     

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.     

Asymmetric Diels–Alder and ene reactions promoted by a Ti(IV) complex bearing a C2-symmetric tridentate ligand

A new Ti(IV) complex obtained from the C₂-symmetric amino diol (1R,5R)-3-aza-3-benzyl-1,5-diphenyl pentan-1,5-diol, (1R,5R)-1, is used effectively as a Lewis acid promoter in asymmetric Diels–Alder reactions. Using various Evans' oxazolidinones as dienophiles and cyclopentadiene as the diene high yields of the adducts with moderate enantioselectivity, under different reaction conditions are achieved. The effects of solvent, temperature and ligand on the enantioselectivity of the Diels–Alder products are reported. Molecular modelling studies provide an understanding of the diastereofacial selectivity of the Diels–Alder reactions. Asymmetric carbonyl-ene reactions between various glyoxylate esters and α-methyl styrene are also described.     

Two-Way Catalysis in a Diels–Alder Reaction Limits Inhibition Induced by an External Electric Field

Oriented external electric fields (EEFs) act as catalysts that can induce selectivity in chemical reactions. The responses of the Diels–Alder (DA) reaction between butadiene and ethylene (BDE-DA) as well as cyclopentadiene and ethylene (CPDE-DA) towards EEF stimuli are investigated here using density functional theory (B3LYP) calculations. EEF is a vector that catalyzes the reaction in one direction while inhibiting it in the opposite direction. Here we report that the inhibitive direction becomes rate-enhancing after some increase in the EEF. The EEF value that brings about the maximum possible inhibition for the reaction is defined as the electrostatic resistance point (ERP). The possibility of both normal and inverse electron-demand DA reactions causes catalytic activity in both directions of the EEF starting at a unique ERP value. The C5 substituents of cyclopentadiene control the ERP values depending upon the resistance power that the functional group provides against the EEF. The endo and exo diastereomeric transition states of the DA reaction have distinct ERP values and the difference (ΔERP) provides the through-space electrostatic contribution to the stereoselectivity on a relative scale. Thus, the ERP values can be used as a gauge for the electrostatic interactions between substituent groups and external stimuli.     

Diels–Alder Additions to 2,2’-Biaceanthrylene

A series of Diels–Alder reactions between the diene 2,2’-biaceanthrylene and several dienophiles is presented. The diene is a cyclopenta-fused polycyclic aromatic hydrocarbon with anthracene units linked by two cyclopentene rings. Depending on the dienophile, the major product was the result of a single addition (dimethyl acetylenedicarboxylate) or double addition (quinone, benzyne) to the diene. Single crystal X-ray analysis of the quinone-derivative shows a propeller-like structure composed of mixed enantiomers. The synthesis and photophysical properties of these compounds are presented.     

How Lewis Acids Catalyze Diels–Alder Reactions

The Lewis acid(LA)-catalyzed Diels–Alder reaction between isoprene and methyl acrylate was investigated quantum chemically using a combined density functional theory and coupled-cluster theory approach. Computed activation energies systematically decrease as the strength of the LA increases along the series I₂<SnCl₄<TiCl₄<ZnCl₂<BF₃<AlCl₃. Emerging from our activation strain and Kohn–Sham molecular orbital bonding analysis was an unprecedented finding, namely that the LAs accelerate the Diels–Alder reaction by a diminished Pauli repulsion between the π-electron systems of the diene and dienophile. Our results oppose the widely accepted view that LAs catalyze the Diels–Alder reaction by enhancing the donor–acceptor [HOMO_diene–LUMO_dienophile] interaction and constitute a novel physical mechanism for this indispensable textbook organic reaction.