Diastereo- and regioselectivity in Diels-Alder reaction of [1,4,2]diazaphospholo[4,5-a]pyridines

[1,4,2]Diazaphospholo[4,5-a]pyridines undergo diastereoselective Diels-Alder reaction at the >CP- functionality with 2,3-dimethylbutadiene and isoprene in the presence of sulfur or selenium. The reaction with isoprene occurs regioselectively. On carrying out the reaction with diene in presence of methyl iodide, the initially formed [2+4] cycloadduct is methylated regioselectively at the σ²,λ³-nitrogen. The results of the DFT calculations of the Diels-Alder reaction with isoprene are in accord with the observed regioselectivity. The relative stabilities of the two transition structures have been explained on the basis of NBO analysis.     

Origin of the stereo- and regioselectivities in the Diels-Alder reactions of azaphospholes: a DFT investigation

Computations of the Diels-Alder (DA) reactions of azaphosphole representative namely, thiazolo[3,2-d][1,4,2]diazaphosphole with 1,3-butadiene and isoprene at the density functional theory level reveal concerted mechanisms via asynchronous transition states. The activation energies (B3LYP/6-311++G**// B3LYP/6-311G**), 16-19 kcal mol⁻¹, are much smaller than the value (32.57 kcal mol⁻¹) calculated for the DA reaction of the non-phosphorus analogue, imidazo[2,1-b]thiazole with 1,3-butadiene. An electron-withdrawing group at the 3-position of the dienophile enhances both stereo- and regioselectivities, which agree nicely with the experimental values. Inclusion of solvent effect (PCM model) reveals that the stereo- and regioselectivities are not affected appreciably. The relative stabilities of the transition structures corresponding to the endo/exo stereoisomers and meta (P/Me, 1:3)/para (P/Me, 1:4) regioisomers have been rationalized on the basis of the secondary molecular orbital interactions.     

Diels-Alder reaction of 2-phosphaindolizines catalysed by organoaluminium reagent: theoretical and experimental results

Theoretical calculations at the DFT (B3LYP/6-31+G^**) level of the model Diels-Alder (DA) reactions of 1-methyl-3-(methoxycarbonyl)-2-phosphaindolizine with 1,3-butadiene in the presence of methylaluminium dichloride reveal that the co-ordination of organoaluminium reagent to the carbonyl oxygen increases the activation barrier compared to that for the uncomplexed 2-phosphaindolizine. On the other hand, co-ordination of the organoaluminium reagent to the σ², λ³ P atom lowers the activation barrier by ∼6 kcal mol⁻¹. 1-Methyl-2-phosphaindolizines having an electron-withdrawing group at the 3-position only undergo DA reaction with 2,3-dimethylbutadiene in the presence of the ethylaluminium dichloride catalyst in methylene chloride at a low temperature to afford [2+4] cycloadducts. The formation of an intermediate having the ethylaluminium reagent co-ordinated to σ², λ³ P atom has been detected by ³¹P NMR. The products have been characterized by ³¹P and ¹H NMR studies.     

Computational evaluation of asymmetric Diels-Alder reactions of vinylboranes with chiral dienes

The asymmetric Diels-Alder reactions of dichlorovinylborane and vinyl-9-BBN with two chiral dienes have been studied theoretically. Calculations suggest high reactivity and regio- and stereoselectivity could be achieved in these processes giving the anti para endo product. This class of reactions represents a potential stereoselective approach to a variety of chiral carbocycles.     

Regioselective Diels-Alder reactions of 3-indolylquinones

6-(3-Indolyl)quinolinequinone derivatives gave regioselective Diels-Alder reactions with a variety of dienophiles, yielding polycyclic carbazole derivatives. One-pot reactions, proceeding through a cascade of reactions including regioselective Michael and Diels-Alder steps, gave heptacyclic derivatives starting from indoles and 2,5,8(1H)-quinolinetriones. Double Diels-Alder reactions of 6-(3-indolyl)quinolinequinones and dihalobenzoquinones gave eleven-cycle products in one step.     

A DFT study of the polar Diels-Alder reaction between 4-aza-6-nitrobenzofuroxan and cyclopentadiene

The polar Diels-Alder reaction between 4-aza-6-nitrobenzofuroxan (ANBF) and cyclopentadiene has been studied using DFT procedures at the B3LYP/6-31G* level. Only one highly asynchronous transition state structure associated to the formation of the [4+2] adduct 13 is found. A further [3,3] sigmatropic shift on the [4+2] cycloadduct 13 allows its conversion into the thermodynamically more stable [2+4] cycloadduct 14. The analysis of the global and local electrophilicities of the reagents correctly explain the behaviour of ANBF as a strong electrophile in polar cycloadditions.     

A dimethylsulfonium methylide mediated highly regioselective olefination of conjugated polyolefin 1,1-dioates to conjugated polyene-2-yl-malonates and their applications in Diels-Alder reactions

The reactions between dimethylsulfonium methylide and 1,3-diene- or 1,3,5-triene-1,1-dioates under specific conditions enable the highly regioselective tandem ylide addition-eliminative olefination to provide 1,3-butadien-2-yl- or 1,3,5-hexatriene-2-yl-malonates. Alkylation at malonate methine carbon of 1,3-butadien-2-ylmalonates with a suitable alkyl halide having in-built functionalities for a dienophile generation led to quick assembly of precursors for type 2 intramolecular Diels-Alder reaction. Syntheses of functionalized bicyclo[n.3.1] alkenes (n=5 or 6) with the double bond at the bridgehead position have been achieved via the IMDA. An asymmetric version of this reaction has been developed using a MacMillan's imidazolidinone catalyst, which provided a bicyclo[5.3.1] alkene with very high enantioselectivity. In situ methylation at malonate methine carbon of 1,3,5-hexatriene-2-yl-malonates followed by intermolecular Diels-Alder reaction with N-methylmaleimide provided the cycloadduct with complete regiocontrol and high diastereoselectivity.     

Novel synthesis of o-naphthothiophenequinone derivatives via regioselective Diels-Alder reaction

A novel procedure to construct o-naphthothiophenequinones has been achieved from readily available o-benzothiophenquinones and N-dienes via Diels-Alder reaction-aromatization sequence as key steps. The absolute regioselectivity was established via Diels-Alder reaction of o-benzothiophenquinones with rich electron N-dienes.     

A theoretical study of the mechanism and stereoselectivity of the Diels-Alder cycloaddition between difluoro-2-methylencyclopropane and furan

A theoretical study of the molecular mechanism and stereoselectivity of the Diels-Alder cycloaddition reaction between difluoro-2-methylencyclopropane and furan has been carried out at the B3LYP/6-31G+^∗∗ level of theory. The calculation of activation and reaction energies indicates that the 3-endo cycloadduct is favored both kinetically and thermodynamically, which is in agreement with the experimental data. Analysis of the bond order and charge transfer in the transition states shows that this reaction takes place via a synchronous-concerted mechanism.     

Diels-Alder Reactivity of a Chiral Anthracene Template with Symmetrical and Unsymmetrical Dienophiles: A DFT Study

In this work, we used Density Functional Theory calculations to assess the factors that control the reactivity of a chiral anthracene template with three sets of dienophiles including maleic anhydrides, maleimides and acetoxy lactones in the context of Diels-Alder cycloadditions. The results obtained here (at the M06-2X/6-311++G(d,p) level of theory) suggest that the activation energies for maleic anhydrides and acetoxy lactones are dependent on the nature of the substituent in the dienophile. Among all studied substituents, only −CN reduces the energy barrier of the cycloaddition. For maleimides, the activation energies are independent of the heteroatom of the dienophile and the R group attached to it. The analysis of frontier molecular orbitals, charge transfer and the activation strain model (at the M06-2X/TZVP level based on M06-2X/6-311++G(d,p) geometries) suggest that the activation energies in maleic anhydrides are mainly controlled by the amount of charge transfer from the diene to the dienophile during cycloaddition. For maleimides, there is a dual control of interaction and strain energies on the activation energies, whereas for the acetoxy lactones the activation energies seem to be controlled by the degree of template distortion at the transition state. Finally, calculations show that considering a catalyst on the studied cycloadditions changes the reaction mechanism from concerted to stepwise and proceed with much lower activation energies.     

对映选择催化 Diels-Alder 反应

本文综述了对映选择催化 Diels-Alder 反应的进展。     

Chemo-, regio- and stereoselective Diels-Alder reactions of EWG bearing thiophene-1,1-dioxides

EWG-containing thiophene-1,1-dioxides were found to be very active dienophiles in reactions with 1,3-dienes to afford corresponding cycloadducts in mild conditions. A chemo-, regio- and stereoselective cycloaddition was observed. Reaction resulted in formation of derivatives of tetrahydrobenzothiophene-1,1-dioxides in high yields.     

Domino Diels-Alder reactions of N-methoxyethyl-7-oxa-norbornadiene-2,3-dicarboximide: an elusive, highly reactive dienophile

Flash vacuum pyrolysis (FVP) has been used to generate the novel 7-oxa-norbornadiene-2,3-dicarboxylic imide that in situ gave an unprecedented cycloaddition reaction cascade with the imidofuran, a side-product of FVP. Stereoselectivity of cycloadditions was studied with the aid of density functional calculations, which fully support observed exo/endo-selectivity.     

Diels-Alder reactions of highly pyramidalized tricyclo[3.3.0.0]oct-1(5)-ene derivatives: further chemistry of pentacyclo[6.4.0.0.0.0]dodeca-5,8,11-triene

A study of the trapping of highly pyramidalized tricyclo[3.3.0.0^3,7]oct-1(5)-ene derivatives (generated from a 1,2-diiodo precursor on reaction with t-BuLi, 0.45% sodium amalgam and molten sodium) with different dienes (11,12-dimethylene-9,10-dihydro-9,10-ethanoanthracene, 1,3-diphenylisobenzofuran, 2,5-dimethylfuran and furan) is presented. Byproducts from the trapping of pentacyclo[6.4.0.0^2,10.0^3,7.0^4,9]dodeca-5,8,11-triene with 1,3-diphenylisobenzofuran have been synthesized and fully characterized, including an X-ray diffraction analysis. Also, the above triene has been cross coupled with 3,7-dimethyltricyclo[3.3.0.0^3,7]oct-1(5)ene to give a tetrasecododecahedratetraene derivative.     

2-Nitrofurans as dienophiles in Diels-Alder reactions

α-Nitrofuran derivatives are studied in Diels-Alder reactions under thermal conditions. In contrast to α-acylfurans, they proved to be efficient dienophiles.     

High viscosity of ionic liquids causes rate retardation of Diels-Alder reactions

Second order rate constants, k2 have been determined for three bi-molecular Diels-Alder reactions to demonstrate that the high viscosity of ionic liquids can be a detrimental property in carrying out Diels-Alder reactions, if ionic liquids are employed as solvent media. It is possible to enhance the reaction rates of the reaction if a co-solvent is mixed in pure ionic liquid used as a solvent.     

An approach to the synthesis of furanoheliangolides through Diels-Alder reactions

The core structure of the natural sesquiterpene lactones furanoheliangolides, an 11-oxabicyclo[6.2.1]undecane system, was synthesized through a pathway involving two Diels-Alder reactions.     

Chemo- and regioselective synthesis of 2-alkylidenetetrahydrofurans bearing a chiral sulfur atom by domino reactions of sulfoximines

The addition of the dianion of Johnson’s sulfoximine to α,ω-halogenoesters evolves by an intramolecular heterocyclization to provide a direct access to 2-alkylidenetetrahydrofurans bearing a chiral sulfur atom via domino addition-elimination/S_N reactions.     

Contrasteric Diels-Alder reactions of 5-methyl-5-phenylcyclopentadiene

The frontier molecular orbital of 5-phenylcyclopentadiene was predicted on the basis of the orbital mixing rule to deform to favor the Diels-Alder reaction in a syn contrasteric manner. The prediction was substantiated experimentally by the reactions of 5-methyl-5-phenylcyclopentadiene with dienophiles to afford the syn attack products, exclusively.