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Cycloaddition Diels-Alder process

Because the fluorine substituents both inductively and hyperconjugatively withdraw electron density from the C(2)-C(3) tt bond, the LUMO is located there, and Diels-Alder reactions take place exclusively with this bond [25] 1,1 -Difluoro allene and fluoroallene reaet readily with a large selection of cyclic and acyclic dienes, and acyclic dienes, [2+2] cycloadditions compete with the Diels-Alder processes As shown in the example in equation 79, a significantly different regiochemistry is observed for the [2+4] cycloaddition compared with the [2+2]... [Pg.824]

V-Acyliminium ions act as dienophiles in [4 + 2] cycloaddition reactions with conjugated dienes13, while A-acylimimum ions that (can) adopt an x-cis conformation are able to act as heterodienes in an inverse electron demand Diels-Alder process with alkenes or alkynes3 (see Section D. 1.6.1.1.). [Pg.804]

Cycloaddition of 125 with buckminsterfullerene (Ceo) at 3 kbar allowed the adduct [48] to be obtained, preventing a retro Diels-Alder process (Scheme 5.19). Cycloadditions of tropone (125) with furans 134 gave mixtures of 1 1 endo-dcad exo-monocycloadducts 135 and 136, respectively [49a], together with some bisadducts. In this case furan reacts solely as the 27t component in spite of its diene system. Whereas 2-methoxy furan gave mainly the kinetically controlled product 135 (R= OMe Ri =R2 =H), under the same conditions 3,4-dimethoxy furan afforded the thermodynamically controlled cycloadduct 136 (R=H Ri =R2 =OMe) as the major product (Scheme 5.19). [Pg.228]

Subsequently to the intermolecular Diels-Alder reaction, a new diene is produced which can then be utilized in a second cycloaddition process. The feasibility of the second Diels-Alder process was demonstrated by the thermal cycloaddition of 44 with a variety of dienophiles to afford the cycloadducts 47 in high yields, albeit with moderate diastereoselectivity (Scheme 8.8). Additional investigations will be necessary to delineate further the scope and limitations of this rapid increase in molecular complexity. [Pg.168]

Several examples of allowed syn cycloadditions follow, but the reader may recall the 4 + 2 Diels-Alder and retro Diels-Alder processes in equations (20) and (21), parts c and d, and the 4 + 2 dipolar cycloaddition in (26). [Pg.217]

These results indicate that the sulfinyl group seems to be much more efficient in the control of the stereoselectivity of 1,3-dipolar cycloadditions (endo or exo adducts are exclusively obtained in de> 80%) than in Diels-Alder processes (mixtures of all four possible adducts were formed). Additionally, complete control of the regioselectivity of the reaction was observed. Despite these clearly excellent results, the following paper concerning asymmetric cycloaddition of cyclic nitrones and optically pure vinyl sulfoxides was reported nine years later [154]. (Meanwhile, only one paper [155], related to the synthesis of /1-nicotyri-nes, described the use of reaction of nitrones with racemic vinyl sulfoxides, but these substrates were merely used as a masked equivalent of acetylene dipolaro-phile). In 1991, Koizumi et al. described the reaction of one of the best dipolarophiles, the sulfinyl maleimide 109, with 3,4,5,6-tetrahydropyridine 1-oxide 194 [154]. It proceeded in CH2C12 at -78 °C to afford a 60 20 10 6 mixture of four products in ca. 90 % yield (Scheme 92). [Pg.98]

Here again, the cycloaddition is endo-selective, with only regioisomers 79 being formed, and, when using 2- and/or 3-substituted furans, only the unsubstituted furan double-bond reacts in these inverse electron-demand Diels-Alder processes [134-136]. Indoles, pyrroles, and thiophenes can also be made to react as dienophiles with ortho-quinone monoketals... [Pg.556]

Since perfluoroalkyl-substituted olefins and alkynes possess low-lying frontier orbitals, [4 + 2] cycloaddition reactions to oxazoles and thiazoles without strongly electron-donating substituents are unfavorable. On the other hand, five-membered heteroaromatic compounds possessing an electron-rich diene substructure, like furans, thiophenes, and pyrroles, should be able to add perfluoroalkyl-substituted olefins as well as alkynes in a normal Diels-Alder process. A reaction sequence consisting of a Diels-Alder reaction with perfluoroalkyl-substituted alkynes as dienophile, and a subsequent retro-Diels-Alder process of the cycloadduct initially formed, represents a preparatively valuable method for regioselective introduction of perfluoroalkyl groups into five-membered heteroaromatic systems. [Pg.44]

The domino process probably involves the chiral enamine intermediate 2-817 formed by reaction of ketone 2-813 with 2-815. With regard to the subsequent cycloaddition step of 2-817 with the Knoevenagel condensation product 2-816, it is interesting to note that only a normal Diels-Alder process operates with the 1,3-bu-tadiene moiety in 2-817 and not a hetero-Diels-Alder reaction with the l-oxa-1,3-butadiene moiety in 2-816. The formed spirocyclic ketones 2-818/2-819 can be used in natural products synthesis and in medicinal chemistry [410]. They have also been used in the preparation of exotic amino acids these were used to modify the physical properties and biological activities of peptides, peptidomimetics, and proteins... [Pg.175]

When the tetramethylene intermediate involved in [2 + 2] cycloadditions lacks stabilizing functionality at both termini, and the diene is free to assume, or is locked into, the (S)-cis conformation, the Diels-Alder process dominates ethylene and butadiene give vinylcyclobutane and cyclohexene in 1 5000 pro-... [Pg.70]

Dienophiles substituted with appropriate heteroatoms may offer a number of advantages such as (1) provide dienophilic equivalents of C-—C and moieties which do not undergo [4 + 2] cycloadditions to 1,3-dienes because of low (e.g. isolated alkenes, alkynes, allenes) or different reactivities (e.g. ketenes) (2) enhance or invert the regiochemistry of the Diels-Alder process (3) permit facile removal of the activating and/or regiocontrolling group after cycloaddition with or without introduction of further functionalities. The use of nitroalkenes as dienophiles demonstrates these issues most strikingly. [Pg.320]

While metallocenes are ubiquitous in organometallic and polymer chemistry, few such complexes have been reported to catalyze the Diels-Alder process in high enantioselectivity [127,128,129]. Thebis(tetrahydroindenyl)zirconium tri-flate 60 and the corresponding titanocene are electrophilic to the extent that they catalyze the low-temperature cycloadditions of acrylate and crotonate imides with cyclopentadiene with good diastereoselectivity and excellent enantioselec-tion (Scheme 48). The reactivity of 60 is noteworthy since the corresponding reaction using the crotonyl imide with highly reactive catalysts 31a or 44 requires temperatures of -15 and 25 °C, respectively. [Pg.1154]

The cycloaddition is ascribable to the oxyanion of hydrogen-bonded enolate (ArO —HNEt3 ) rather than to the hydrogen-bonded enol (ArOH—NEtj). An enantioselective version of the reaction was achieved by using a homochiral amine [27]. Similarly the reactions with less reactive dienophiles such as dimethyl fumarate, fumaronitrile, maleonitrile and methyl acrylate give the Diels-Alder adducts quantitatively when the cycloadditions are carried out in THF or CHCI3 in the presence of EtjN, while in MeOH Michael adducts were isolated. Experimental evidence supports the hypothesis that the base-catalyzed cycloadditions of anthrone with dienophiles are concerted Diels-Alder processes [25b]. [Pg.8]

Boger and co-workers have recently determined the regioselectivity of acylnitroso compound additions to some 2-substituted 1,3-cyclohex-adienes.5 In the cases of both electron-rich and electron-deficient substituted dienes the same regioisomer predominated by about 3 1 (Scheme 3-IX). It was suggested that the first case is consistent with a normal (HOMOdiene controlled) Diels-Alder process, whereas the second reaction is consistent with either a HOMOdiene or inverse electron demand (LUMOdiene controlled) cycloaddition. [Pg.229]

In another area, the phosphoryl group activates carbon-carbon multiple bonds towards cycloaddition reactions, as in the Diels-Alder process (Section V.D), in a way reminiscent of carbonyl-containing or other electron-withdrawing functions. [Pg.501]

The stereoselective synthesis of spirolactones is achieved by [4+2]cycloaddition of 3-methylene-4-isopropyloxetan-2-one (21) with 1,3-dienes (93CB1481). The addition of (21) to enan-tiomerically pure (20) gives the separable adducts (22) and (23) which, on separate thermolysis, undergo a retro-Diels-Alder process to yield the (5)- and (R)-forms of (21), respectively, each in 99% e.e. (93CB1509). [Pg.67]


See other pages where Cycloaddition Diels-Alder process is mentioned: [Pg.18]    [Pg.8]    [Pg.351]    [Pg.361]    [Pg.28]    [Pg.31]    [Pg.37]    [Pg.112]    [Pg.9]    [Pg.393]    [Pg.760]    [Pg.19]    [Pg.22]    [Pg.555]    [Pg.333]    [Pg.3]    [Pg.44]    [Pg.250]    [Pg.484]    [Pg.697]    [Pg.37]    [Pg.37]    [Pg.1162]    [Pg.665]    [Pg.665]    [Pg.537]    [Pg.196]    [Pg.13]   
See also in sourсe #XX -- [ Pg.35 ]




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Alder Cycloaddition

Diels cycloaddition

Diels-Alder cycloaddition

Diels-Alder cycloadditions

Diels-Alder processes

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