Cyclohexenes retro-Diels-Alder reaction

Hydrocarbon Thermal Isomerization 4.6 Cyclohexene Retro Diels-Alder Reaction... 

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

Removal of the carbonate ring from 7 (Scheme 1) and further functional group manipulations lead to allylic alcohol 8 which can be dissected, as shown, via a retro-Shapiro reaction to give vinyl-lithium 9 and aldehyde 10 as precursors. Vinyllithium 9 can be derived from sulfonyl hydrazone 11, which in turn can be traced back to unsaturated compounds 13 and 14 via a retro-Diels-Alder reaction. In keeping with the Diels-Alder theme, the cyclohexene aldehyde 10 can be traced to compounds 16 and 17 via sequential retrosynthetic manipulations which defined compounds 12 and 15 as possible key intermediates. In both Diels-Alder reactions, the regiochemical outcome is important, and special considerations had to be taken into account for the desired outcome to. prevail. These and other regio- and stereochemical issues will be discussed in more detail in the following section. 

Retro-Diels-Alder reactions can be used to regenerate dienes or alkenes from Diels-Alder protected cyclohexene derivatives under pyrolytic conditions144. Most of the synthetic utility of this reaction comes from releasing the alkene by diene-deprotection. However, tetralin undergoes cycloreversion via the retro-Diels-Alder pathway to generate o-quinodimethane under laser photolysis (equation 89)145. A precursor of lysergic acid has been obtained by deprotection of the conjugated double bond and intramolecular Diels Alder reaction (equation 90)146. 

Alkenes have also been prepared by retro-Diels-Alder reaction of resin-bound cyclohexenes. Figure 3.40 depicts an interesting example of this strategy, in which the retro-Diels-Alder reaction is induced by conversion of a thermally stable, resin-bound 7-oxanorbornadiene into a thermally unstable 7-oxanorbornene [795]. 

In antithetical analyses of carbon skeletons the synthon approach described in chapter 1 is used in the reverse order, e.g. 1,3-difunctional target molecules are "transformed" by imaginary retro-aldol type reactions, cyclohexene derivatives by imaginary retro-Diels-Alder reactions. 

Strained cyclohexenes, such as norbornene derivatives, can undergo retro-Diels-Alder reactions even at relatively low temperatures, and this reaction can be used to prepare 1,3-dienes and alkenes (e.g. synthesis of cyclopentadiene by thermolysis of... 

The final series of five procedures presents optimized preparations of a variety of useful organic compounds. The first procedure in this group describes the preparation of 3-BROMO-2(H)-PYRAN-2-ONE, a heterodiene useful for (4+2] cycloaddition reactions. An optimized large scale preparation of 1,3,5-CYCLOOCTATRIENE, another diene useful for [4+2] cycloaddition, is detailed from the readily available 1,5-cyclooctadiene. Previously, the availability of this material has depended on the commercial availability of cyclooctatetraene at reasonable cost. A simple large scale procedure for the preparation of 3-PYRROLINE is then presented via initial alkylation of hexamethylenetetramine with (Z)-1,4-dichloro-2-butene. This material serves as an intermediate for the preparation of 2,5-disubstituted pyrroles and pyrrolidines via heteroatom-directed metalation and alkylation of suitable derivatives. The preparation of extremely acid- and base-sensitive materials by use of the retro Diels-Alder reaction is illustrated in the preparation of 2-CYCLOHEXENE-1.4-DIONE, a useful reactive dienophile and substrate for photochemical [2+2] cycloadditions. Functionalized ferrocene derivatives... 

This analysis is very satisfactory for the retro-Diels-Alder reactions, e.g. the decomposition of vinyl cyclohexene (1) i ) which are ground state... 

Retro Diels-Alder reaction (Section 16.14B) The reverse of a Diels-Alder reaction in which a cyclohexene is cleaved to give a 1,3-diene and an alkene. 

Only in the retro Diels-Alder reaction of cyclohexene at 1000 °C do isotope effects provide some support for part of the reaction not being concerted, see D. K. Lewis, B. Brandt, L. Crockford, D. A. Glenar, G. Rauscher, J. Rodriguez and J. E. Baldwin, J. Am. Chem. Soc., 1993,115, 11728. 

Photochemical addition of dimethyl acetylenedicarboxylate to the enamine H2C=CE-NH-CE=CH2 (E = COOMe) yields the bridged cyclohexene (90), which decomposes at 90 °C in a retro-Diels-Alder reaction to ethylene and the ester (91). 

The graphitization of diamond may involve thermally-allowed six re-electron concerted processes an electrocyclic reaction converts a six-membered ring in the diamond lattice into three isolated double bonds as shown in Fig. 12. Subsequently, each of the three cyclohexene rings can undergo a retro-Diels-Alder reaction, multiplying the number of double bonds in a triple cascade reaction 23]. 

Cyclopentadiene is a popular moiety to release in a retro Diels-Alder reaction, as the cyclohexene portion of the bicyclo[2.2.1]heptene is locked in a boat conformation, as required in the transition state for the retro (and indeed forward Diels-Alder) reaction. Other bridged bicyclic systems are also more prone to undergo retro Diels-Alder reaction. The bridged adduct from fiiran and maleic anhydride readily undergoes the retro reaction and, although the endo isomer is formed at a much faster rate, the reversible nature of the reaction leads to the accumulation of the more stable exo isomer. 

Even while he was busy with the manufacture of ethylene oxide, Walter Reppe was also involved with the development of Buna synthetic rubber. In 1926, the newly formed I.G. Farben decided to embark on the industrial synthesis of rubber, despite the poor quality of the methyl rubber made during World War I. This time, however, it was agreed that butadiene would be used. Several routes to butadiene were investigated, including decyclization of cyclohexene (a retro-Diels-Alder reaction), but the so-called four-step process (Vierstufen Verfahren) soon won out. This was partly because it used acetylene, and hence surplus carbide from cyanamide manufacture, but also because it drew on the steps - and hence the momentum - of the BASF butanol synthesis. As a member of the former butanol group, Reppe was a natural candidate for the four-step process project. 

Diels-Alder reactions are reversible cyclohexenes can dissociate to a diene and an alkene by a retro Diels-Alder reaction. 

Vinylcyclohexene undergoes a retro Diels-Alder reaction to butadiene with logk= 15.2 - 62000/2.31 r or 15.7 - 61 S00/23RT At lower tempera-tures optically active 4-( x<9-dideuteriovinyl)cyclohexene undergoes both racemi-zation and deuterium scrambling to C-3. These processes occur with... 

Retro-Diels-Alder Reaction Cyclohexene (Figure 6.7) and its derivatives undergo a retro-Diels-Alder (RDA) reaction upon El, which is a special case of the cycloreversion reaction [27,28]. This reaction is the reverse of a well-known [4 -h 2] cycloaddition reaction between a conjugated diene and an olefin, named the Diels-Alder reaction, after its discoverers. The impetus for the RDA reaction is the ring double bond, which upon ionization creates a radical site and a charge site ... 

Another well-known process producing abundant rearrangement ions in the mass spectrum is the retro-Diels-Alder reaction of cyclic olefins. The simplest example of this is shown by cyclohexene ... 

A major problem in interpreting pericyclic reactions of radical ions is the difficulty of distinguishing between concerted and nonconcerted paths. The same difficulty occurs in the case of thermal reactions, but the problem here is more acute since there is less scope for the application of subtle experimental techniques. Thus cyclohexene (6) undergoes a retro-Diels-Alder reaction to (7) in the mass spectrometer but the products formed do not enable us to distinguish between a two-step process via the intermediate radical ion (8) and a pericyclic process involving the cyclic transition state (9). 

The parallel between retro-Diels-Alder reactions in the mass spectrometer and the corresponding thermal reactions of neutral cyclohexenes does, however, suggest that both proceed by a common mechanism, i.e., a one-step pericyclic process. This conclusion has been strongly supported by Loudon et al in a very detailed comparison of the thermal and mass spec-... 

The retro-Diels-Alder reaction often accounts for prominent fragments of cyclohexenes and 1,4-cyclohexadienes ... 

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