Hexadienes 1,2-cycloaddition with

The vinylogous 3,5-hexadien-2-one (16) adds in a 1,4 cycloaddition with zl -dehydroquinolizidine (17) to form compound 18 (26). A similar 1,4-cycloaddition reaction takes place between pyrylium salts and the pyrrolidine or morpholine enamines of cycloalkanones (26a). 

If the reaction is carried out at a pressure of 12 kbar, the mechanism changes, and cycloaddition with ci5,traro-2,4-hexadiene proceeds stereo specifically yielding the traws-selenapyrans as the major products along with the corresponding tetraarylethylenes. 

Diels-Alder reactions. Although 2,4-hexadiene shows only slight regioselec-tivity in Diels-Alder reactions, the 1-acetoxy derivative (1) undergoes regioselective cycloaddition with a variety of dienophiles.1... 

As both, the cycloaddition of trans-piperylene and trans,trans-hexadiene implies a connection of a methylated unsaturated carbon with the more bulky part of the standard silaethene, a decrease of reactivity of trans.trans-hexadiene on sterical grouds is ruled out. On the other hand, an intermediate formation of a weak 7i-complex of trans-piperylene but not of bulkier trans.trans-hexadiene with the unsaturated silicon atom of silaethene would explain an increased reactivity of trans-piperylene (Scheme 12). In a way, the silaethene would then catalyze its own [4+2] cycloaddition with trans-piperylene but not with trans.trans-hexadiene. This mechanism can also explain the transformation of the [4+2] into a [2+2] cycloaddition by going from trans- to cis-piperylene but not fi om trans,trans- to cis.trans-hexadiene. 

Starting from ( + )-8a-cyano-octahydro-2(l/f)-naphthalenone oxime, the corresponding (2/ ,4a/ ,8a.V)-2-chloro-2-nitmso derivative was prepared and employed in the cycloaddition with (2 , 4 )-2,4-hexadiene. The dihydro-1,2-oxazine 1 was obtained in good yield, but with only 39% ee. The configuration at C-6 was established as S, by correlation with known compounds40. 

Moderate asymmetric induction and low yields are observed when the compound 5, prepared from (A, )-hexadienal and (15,25)-pseudoephedrine, undergoes cycloaddition with the nitroso derivative 6. The major cycloadduct 7 is obtained together with minor amounts of diastereomers (d.r. 62 24 11 3) and after chromatography is isolated in 32% yield. Its configuration is first assigned by H NMR, and then by conversion into the known (+ )-(S)-2-methyl-l-(4-toluenesulfonyl)piperidine. Compound 7 is the key intermediate for the total synthesis of an amino allose derivative135. 

The formation of cyclobutanes from alkenes by radical pathways was established by the work of Bartlett and co-workers. For example, l,l-dichloro-2,2-difluoroethene undergoes nonstereospecific cycloaddition with frflus,frans-2,4-hexadiene at 100°C to give an 82 18 mixture of diastereomeric cyclobutanes ... 

Retention of stereochemistry also occurs for the diene. For example, (2 , 4 )-hexadiene reacts with maleic anhydride to give a product with cis methyl groups. Many examples of this type lead to the conclusion that the Diels-Alder and other [4 + 2] cycloaddition reactions occur suprafaciaUy on each component. 

The first studies on cation-radical Diels-Alder reactions were undertaken by Bauld in 1981 who showed [33a] the powerful catalytic effect of aminium cation radical salts on certain Diels-Alder cycloadditions. For example, the reaction of 1,3-cyclohexadiene with trans, iraw5-2,4-hexadiene in the presence of Ar3N is complete in 1 h and gives only the endo adduct (Equation 1.14) [33]. 

A one-pot procedure [9] based on the cycloaddition of 4-aryl-2-silyloxybuta-dienes 7 and bisdiene 8 with alkynes, followed by oxidative aromatization of the cycloadducts, opened a route to polycyclic phenols without isolating the cyclo-hexadiene derivative intermediates (Scheme 2.5). 

A strong acceptor TCNE undergoes [2+2] rather than [4+2] cycloaddition reactions even with dienes. 1,1-Diphenylbutadiene [20] and 2,5-dimethyl-2,4-hexadiene (Scheme 5) [21] afford mainly and exclusively vinyl cyclobutane derivatives, respectively. In the reactions of 2,5-dimethyl-2,4-hexadiene (1) the observed rate constant, is greater for chloroform solvent than for a more polar solvent, acetonitrile (2) the trapping of a zwitterion intermediate by either methanol or p-toluenethiol was unsuccessful (3) radical initiators such as benzyl peroxide, or radical inhibitors like hydroquinone, have no effect on the rate (4) the entropies of activation are of... 

As shown in Table VII, [2 + 4] cycloaddition is the most common reaction pathway followed by Me2Si=C(SiMe3)2, but it is usually accompanied by significant quantities of the product of an ene reaction. As the diene becomes more sterically hindered in its s-cis conformation, as in cis/trans-2,4-hexadiene, the product of an ene reaction predominates. With butadiene, where minimal steric effects are to be expected, the exclusive product of the reaction was found to be the [2 + 4] cycloaddition... 

Cyclopentadiene (6) reacted with 105 at 0°C to give 108 (entry 1). At 100°C, butadiene (12) afforded 109 (entry 2). No [2 + 2] cycloadduct was formed in either reaction. Perfluoromethylenecyclopropane (105) failed to react with cis,cis- or cis,trans-2,4-hexadiene at 100 °C, although 110 was readily formed from trans,frans-2,4-hexadiene (106) under these conditions [29] (entry 3). Anthracene (107) added to 105 at 100 °C. The dienophilicity of 105 is exceptional when compared with the reactivity of simple fluoroolefins, such as perfluoro-isobutylene, which require 150 and 200 °C to undergo cycloaddition to cyclopentadiene [30] and anthracene, respectively. 

The conjugation of oligonucleotides with peptides also can be done using Diels-Alder cycloadditions in water (Tona and Haner, 2005). Marchan et al. (2006) used the same 3,5-hexadiene phosphoramidite derivative as Hill et al. (2001), but in this case used a maleimide-modified... 

Wilker and Erker92 have studied the stereochemistry of the [4+2] cycloaddition of diarylthioketones and corresponding diaryselenoketones with Irons, trans- and cis,trara,s-2,4-hexadiene, (see later Scheme 30), and found an analogous behaviour between S and Se. 

Six-membered rings. Selenoketones react as dienophiles with conjugated die-nes24,89,223 233 234 in Diels-Alder [4+2] cycloadditions to form 3,6-dihydro-2H-selenapyrans. Reaction of arylselenoketones92with /ra ,v,/ra .v-2,4-hexadiene proceeds in a completely stereospecific way to yield the corresponding cis-selenapyran, while with cw,tra i-2,4-hexadiene it proceeds stereoselectively, forming cis-selenapyran with small amounts of the /nmv-selenapyran isomers, Scheme 30. 

Some reactions via intermediate alkylideneallyl cations have been reported. Solvolysis of 3-bromo-2,5-dimethyl-2,4-hexadiene in ethanol at 100 °C for 80 min gives 5-ethoxy-2,5-dimethyl-2,3-hexadiene in quantitative yield (Scheme 2) (5). This indicates that ethanol selectively attacks the sp2 carbon of the intermediate alkylideneallyl cation. A similar selectivity has been observed in the solvolysis of 2,3-dienyl alcohols (6), and is in agreement with the charge distribution. A cycloaddition reaction via an alkylideneallyl cation intermediate has been reported as illustrated in Scheme 3(7). 

The reaction of thioacetyl cations with 2,5-dimethyl-l,5-hexadiene under low-pressure conditions in an FT-ICR mass spectrometer leads to elimination of propene. At variance from the [4 + 2+] polar cycloadditions observed under high-pressure conditions in the QqQqQ instrument, Caserio and coworkers220 invoked electrophilic attack of the CE CS"1"... 

Depending on the trapping reagents, various reactions have been observed an ene reaction between propene, isobutene, and 2231 or 27,34 a [2 + 4] cycloaddition between butadiene and 12,21 22,31 and 27,34 or both an ene reaction and a [2 + 4] cycloaddition between 2,3-dimethylbutadiene and 2231 or 27,34 and between piperylene or hexadiene and 22.31 Some of these reactions are summarized in Scheme 8. By contrast, 27 does not react with the C = C double bond of an alkene such as CH2 = CH-OMe.34... 

This section is devoted to cyclizations and cycloadditions of ion-radicals. It is common knowledge that cyclization is an intramolecular reaction in which one new bond is generated. Cycloaddition consists of the generation of two new bonds and can proceed either intra- or intermolecularly. For instance, the transformation of 1,5-hexadiene cation-radical into 1,4-cyclohexadienyl cation-radical (Guo et al. 1988) is a cyclization reaction, whereas Diels-Alder reaction is a cycloaddition reaction. In line with the consideration within this book, ring closure reactions are divided according to their cation- or anion-radical mechanisms. 

Backvall and Plobeck reported a formal synthesis of ellipticine (228) starting from indole (707). The [4+2] cycloaddition of 1-indolylmagnesium iodide (1208) with 3-(phenylsulfonyl)-2,4-hexadiene (1209) afforded the tetrahydrocarbazole... 

Pyridine 20 has been prepared by the silver- or acid-catalyzed isomerization of 1-aza-bicyclo[2.2.0]hexadiene 22, which in turn was obtained in a [2 + 2]-cycloaddition reaction of tris(to7-butyl)azete with hexafluorobut-2-yne. Irradiation of Dewar pyridine 22 gives a high yield of azaprismane 23, which isomerizes thermally to rearranged pyridine 24. Finally, pyridine 24 yields 1-aza Dewar pyridine 25 photolytically.34... 

Figure 21-17 Schematic representation of the energy of the stepwise addition of CH2=CF2 to fra/7s,frans-2,4-hexadiene. The lower diagram is a schematic cross section" that shows everything in two dimensions. If 48 and 49 dissociate to CF2=CF2 and diene, 48 will return the trans.trans isomer, but 49 will go to the cis,trans diene through the transition state represented by the dashed line. In some [2 + 2] cycloadditions of this type, trans.trans to cis.cis isomerizations are observed in competition with cycloaddition, as expected for breaking apart the intermediate corresponding to 49.

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