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Dienophiles olefinic

Second, substituents on the dienophile (olefinic or azo) can adopt a position in the transition state either exo or endo to the diene system. It has been found that the endo transition state is favored significantly over the exo transition state. This preference has been attributed to secondary orbital interactions (attraction) between the diene and polar substituents on the dienophile. [Pg.316]

According to the mechanism proposed, the 1,4-cycloaddition (Diels-Alder reaction) of the diene part of the tetrazine to the dienophile (olefin) gives an unstable bicyclic intermediate, which spontaneously eliminates a molecule of nitrogen, forming a 4,5-dihydropyridazine, which in the absence of appropriate stabilization by gem disubstitution in positions 4 and 5, easily isomer-izes to the thermodynamically more stable 1,4-dihydropyridazine (38). The structure of the latter intermediates were unambiguously proved using NMR.91,92... [Pg.28]

The endo stereoselectivity ( > 95%), the preservation of dienophile olefin geometry, and the enhanced reactivity of ( )-l-ethoxypropene versus (2)-1-ethoxypropene [k(E) k(Z) = 35] in the [4 + 2] cycloadditions of 2-ace-tylcyclohex-2-enone with electron-rich olefins have been interpreted as being consistent and characteristic of a concerted inverse electron demand Diels-Alder reaction [Eq. (19)].104... [Pg.278]

The dienophile olefin geometry is maintained in the course of the cycloaddition reaction. [Pg.287]

One such strategy exploits 142 for construction of the heterodiene 149, derived from azide 148, in which the stereochemistry of the intramolecular Diels—Alder cycloadducts is controlled by the configuration of the dienophile olefin. Treatment of 142 with diphenylpho-sphoryl azide in the presence of diisopropyl azodicarboxylate and triphenylphosphine affords the epoxy azide 148 with inversion of chirality. This is then converted in six steps to the heterodiene intermediate 149, which undergoes an intramolecular cycloaddition to fiimish a single adduct that is subsequently converted to 150. Transformation of 150 into 151 in seven steps completes the synthesis [59] (Scheme 36). [Pg.339]

Path B works back through cyclohexene 111 and an intramolecular Diels-Alder reaction of dienamine 112. Once again, enamine tautomerization is likely to present big problems, but once again this can be overcome by using dienamide 113. This plan provides an opportunity to control absolute stereochemistry if 113 could be prepared in enantiopure form. The dienophile olefin geometry would afford the proper relative stereochemistry at C2 and C3, but stereochemistry relative to 5 and Cio is not guaranteed. [Pg.356]

The following compounds have been obtained from thiete 1,1-dioxide Substituted cycloheptatrienes, benzyl o-toluenethiosulfinate, pyrazoles, - naphthothiete 1,1-dioxides, and 3-subst1tuted thietane 1,1-dioxides.It is a dienophile in Diels-Alder reactions and undergoes cycloadditions with enamines, dienamines, and ynamines. Thiete 1,1-dioxide is a source of the novel intermediate, vinylsulfene (CH2=CHCH=SQ2). which undergoes cyclo-additions to strained olefinic double bonds, reacts with phenol to give allyl sulfonate derivatives or cyclizes unimolecularly to give an unsaturated sultene. - Platinum and iron complexes of thiete 1,1-dioxide have been reported. [Pg.215]

Diels-Alder is the preparation of cyclic olefins from dienes and a dienophile... [Pg.255]

The Diels-Alder reaction of a diene with a substituted olefinic dienophile, e.g. 2, 4, 8, or 12, can go through two geometrically different transition states. With a diene that bears a substituent as a stereochemical marker (any substituent other than hydrogen deuterium will suffice ) at C-1 (e.g. 11a) or substituents at C-1 and C-4 (e.g. 5, 6, 7), the two different transition states lead to diastereomeric products, which differ in the relative configuration at the stereogenic centers connected by the newly formed cr-bonds. The respective transition state as well as the resulting product is termed with the prefix endo or exo. For example, when cyclopentadiene 5 is treated with acrylic acid 15, the cw fo-product 16 and the exo-product 17 can be formed. Formation of the cw fo-product 16 is kinetically favored by secondary orbital interactions (endo rule or Alder rule) Under kinetically controlled conditions it is the major product, and the thermodynamically more stable cxo-product 17 is formed in minor amounts only. [Pg.91]

Cycloaddition reactions of (E)-l-acetoxybutadiene (18a) and (E)-l-methoxy-butadiene (18b) with the acrylic and crotonic dienophiles 19 were studied under high pressure conditions [9] (Table 5.1). Whereas the reactions of 18a with acrylic dienophiles regioselectively and stereoselectively afforded only ortho-enJo-adducts 20 in fair to good yields, those with crotonic dienophiles did not work. Similar results were obtained in the reactions with diene 18b. The loss of reactivity of the crotonic dienophiles has been ascribed to the combination of steric and electronic effects due to the methyl group at the )S-carbon of the olefinic double bond. [Pg.208]

Harano and colleagues [48] found that the reactivity of the Diels-Alder reaction of cyclopentadienones with unactivated olefins is enhanced in phenolic solvents. Scheme 6.28 gives some examples of the cycloadditions of 2,5-bis-(methoxycar-bonyl)-3,4-diphenylcyclopentadienone 45 with styrene and cyclohexene in p-chlorophenol (PCP). Notice the result of the cycloaddition of cyclohexene which is known to be a very unreactive dienophile in PCP at 80 °C the reaction works, while no Diels-Alder adduct was obtained in benzene. PCP also favors the decarbonylation of the adduct, generating a new conjugated dienic system, and therefore a subsequent Diels-Alder reaction is possible. Thus, the thermolysis at 170 °C for 10 h of Diels-Alder adduct 47, which comes from the cycloaddition of 45 with 1,5-octadiene 46 (Scheme 6.29), gives the multiple Diels-Alder adduct 49 via decarbonylated adduct 48. In PCP, the reaction occurs at a temperature about 50 °C lower than when performed without solvent, and product 49 is obtained by a one-pot procedure in good yield. [Pg.276]

Keywords Facial selection. Orbital phase, Secondary orbital interaction. Orbital unsymmetrization. Ketones, Olefins, Diels-Alder dienophiles, Diels-Alder dienes, Michael acceptor. Amine nitrogen atom... [Pg.129]

Diels-Alder cycloadditions involving norbomene 57 [34], benzonorbomene (83), 7-isopropylidenenorbomadiene and 7-isopropylidenebenzonorbomadiene (84) as dienophiles are characterized as inverse-electron-demand Diels-Alder reactions [161,162], These compounds react with electron-deficient dienes, such as tropone. In the inverse-electron-demand Diels-Alder reaction, orbital interaction between the HOMO of the dienophile and the LUMO of the diene is important. Thus, orbital unsymmetrization of the olefin it orbital of norbomene (57) is assumed to be involved in these top selectivities in the Diels-Alder cycloaddition. [Pg.163]

Reactive unsaturated nitroso compounds such as 1112 can also be readily prepared from a-halooximes such as 1111 on treatment with powdered Na2C03 in diethyl ether to give, in the presence of enoltrimethylsilyl ether 1113 or strained olefins such as norbomene and other dienophiles, hetero-Diels-Alder adducts such as 1114 and 1115 in moderate yields [150-155] (Scheme 7.47). [Pg.178]

Esters of a-diazoalkylphosphonic acids (95) show considerable thermal stability but react with acids, dienophiles, and triphenylphosphine to give the expected products. With olefinic compounds in the presence of copper they give cyclopropane derivatives (96), but with no such compounds present vinylphosphonic esters are formed by 1,2-hydrogen shift, or, when this route is not available, products such as (97) or (98) are formed, resulting from insertion of a carbenoid intermediate into C—C or C—H bonds. The related phosphonyl (and phosphoryl) azides (99) add to electron-rich alkynes to give 1,2,3-triazoles, from which the phosphoryl group is readily removed by hydrolysis. [Pg.116]

It is of interest to investigate the usefulness of this theory to the chemical change involving the interaction between the conjugated systems 56,62,145). Such a-n interactions are frequently stereoselective. The addition to olefinic double bonds and the a, -elimination are liable to take place with the fraMS-mode 146h The Diels-Alder reaction occurs with the cis-fashion with respect to both diene and dienophile. [Pg.73]

The term Diels-Alder reaction in a general sense refers to the reaction between a diene and a dienophile. Retro Diels-Alder reaction is a process that, under certain conditions, produces diene and olefin or a compound containing a C=C bond. The application of flash vacuum pyrolysis to effect the retro Diels-Alder reaction, as shown in Schemes 5-46 and 5-47, has become the standard procedure since the introduction of the method by Stork et al.74 in the 1970s. Therefore, alkenes that are difficult to access by conventional methods may be obtained via retro Diels-Alder reactions.75 In particular, this reaction allows the preparation of thermodynamically less stable compounds such as 4,5-dialkyl cyclopenta-2-en-one. In this case, the alkene functional group can be regarded as being protected by cyclopentadiene (as shown in 154 or 157), which, after subsequent reaction, can easily be removed through quick pyrolysis. [Pg.306]

Helmchen and co-worker investigated the use of phosphinooxazolines as ligands for copper(II) catalyzed Diels-Alder reactions (Scheme 19) (214). Optimal selectivities are found for a-naphthyl-substituted phosphinooxazoline (299). These catalysts require 2.5 h to induce complete conversion to cycloadduct, compared to 18 h using the triflate complex 269c under identical conditions. Helmchen invokes a square-planar metal geometry to explain the stereochemistry of the adducts, similar to the model proposed by Evans. He suggests that the bulky phosphine substituents are required to orient binding of the dienophile in such a way as to place the olefin directly below the terf-butyl substituent on the oxazoline. [Pg.104]

Af-Acyliminium ions are known to serve as electron-deficient 4n components and undergo [4+2] cycloaddition with alkenes and alkynes.15 The reaction has been utilized as a useftil method for the construction of heterocycles and acyclic amino alcohols. The reaction can be explained in terms of an inverse electron demand Diels-Alder type process that involves an electron-deficient hetero-diene with an electron-rich dienophile. Af-Acyliminium ions generated by the cation pool method were also found to undergo [4+2] cycloaddition reaction to give adduct 7 as shown in Scheme 7.16 The reaction with an aliphatic olefin seems to proceed by a concerted mechanism, whereas the reaction with styrene derivatives seems to proceed by a stepwise mechanism. In the latter case, significant amounts of polymeric products were obtained as byproducts. The formation of polymeric byproducts can be suppressed by micromixing. [Pg.205]

Reich and coworkers130 demonstrated that the reactions of vinylallenes with unsym-metrical dienophiles proceed predominantly via a transition state in which the largest substituents on both the allene moiety and the olefin are furthest apart. The regiochemistry is governed by these steric interactions, because the HOMO coefficients of the vinylallene at the sites of bond formation are very similar. [Pg.374]

Cycloaddition Reactions with Alkenes Olefins can react with electrogenerated radicals, cationic species or dienophiles. [Pg.355]

Joining olefin metathesis on the very short list of exchange reactions involving carbon-carbon bonds, the Diels-Alder reaction was studied in 2005 by Lehn and colleagues [52]. As the authors note, most Diels-Alder reactions proceed only in the forward direction at room temperature, with retro Diels-Alder reactions typically requiring elevated temperatures. Careful tuning of the diene and dienophile, however, can alter this significantly. In particular, reactions of substituted fulvenes (32) with diethylcyanofumarate (33) were... [Pg.20]

Although pyrroles do not generally participate in Diels-Alder reactions with olefinic dienophiles, the very reactive hexafluoro-Dewar benzene with pyrrole gives the 1 1 and 1 2 adducts, 27 and 28, both of which probably have all-ea o stereochemistry. Some other 7-azabicyclo-heptene derivatives have been obtained via cycloaddition reactions of 7-azaquadricyclanes (see Section II, F). [Pg.92]


See other pages where Dienophiles olefinic is mentioned: [Pg.348]    [Pg.274]    [Pg.283]    [Pg.175]    [Pg.195]    [Pg.141]    [Pg.60]    [Pg.348]    [Pg.274]    [Pg.283]    [Pg.175]    [Pg.195]    [Pg.141]    [Pg.60]    [Pg.335]    [Pg.88]    [Pg.53]    [Pg.43]    [Pg.446]    [Pg.459]    [Pg.19]    [Pg.568]    [Pg.97]    [Pg.60]    [Pg.374]    [Pg.413]    [Pg.458]    [Pg.39]   
See also in sourсe #XX -- [ Pg.43 ]




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