Retro Diels Lewis acid-catalyzed

Microwave heating has also been employed for performing retro-Diels-Alder cycloaddition reactions, as exemplified in Scheme 6.94. In the context of preparing optically pure cross-conjugated cydopentadienones as precursors to arachidonic acid derivatives, Evans, Eddolls, and coworkers performed microwave-mediated Lewis acid-catalyzed retro-Diels-Alder reactions of suitable exo-cyclic enone building blocks [193, 194], The microwave-mediated transformations were performed in dichloromethane at 60-100 °C with 0.5 equivalents of methylaluminum dichloride as catalyst and 5 equivalents of maleic anhydride as cyclopentadiene trap. In most cases, the reaction was stopped after 30 min since continued irradiation eroded the product yields. The use of short bursts of microwave irradiation minimized doublebond isomerization. 

The Lewis acid catalyzed reaction of furan (169) with ketovinylphosphonate 170 produced a mixture of adducts, both of which slowly underwent retro Diels-Alder reactions at room temperature121. When diethylaluminum chloride was used as the catalyst, the endo selectivity (with respect to the keto functionality) was enhanced from 171/172 = 58/42 to 78/22 by raising the reaction temperature from — 25 °C to 0°C (equation 47). This is in agreement with the FMO theory, since initial Lewis acid complexation is with the phosphonate group. 

In the presence of a more reactive dienophile, a retro Diels-Alder reaction can be carried out at or below room temperature when catalyzed by a Lewis acid.78 In fact, this process can be regarded as a trans-Diels-Alder reaction in which the C C bond is replaced by another more reactive functionality. Thus, when treated with fumaronitrile in the presence of EtAlCl2 at ambient temperature for 2 hours, compound 159 can easily be converted to compound 160 with the removal of cyclopentadiene (Scheme 5-48). 

The joining of a double or triple bond to an alkene reactant having a transferable allylic hydrogen is called an ene reaction ". The reverse process is called a retro-ene reaction. Like Diels-Alder reactions, Lewis acids such as AICI3 or BF3 can catalyze ene reactions. 

Most of the synthetic applications of retro-Diels-Alder reactions involve cyclopentadiene or furan derivatives, but there are other dienes that have appropriate functionality for this process. One example is illustrated by heating pyrone 102 in a sealed tube at 200°C with bis(trimethylsilyl)acetylene to give 103, after initial formation of cycloadduct 104 and loss of carbon dioxide via a retro-Diels-Alder reaction.The retro-Diels-Alder usually requires higher temperatures than the Diels-AIder reaction, and the normal Diels-AIder product can be obtained without competition from the retro reaction. When the retro Diels-AIder reaction is desired, flash vacuum pyrolysis is a common technique used in synthesis. " Retro-Diels-Alder reactions can also be catalyzed by Lewis acids (sec. 11.6.A).An example that uses furan as a retro-Diels-Alder synthon is taken from work by Cannone et al., who used the retro-reaction as a synthetic route to 4-substituted... 

The unactivated alkyne-oxazole 203 was considerably less reactive in the intramolecular Diels-Alder retro-Diels-Alder sequence. In ethyl benzene at 135°C, only trace amounts of the tricyclic furan 205, formed via the Diels-Alder adduct 204, were produced. Attempts to catalyze the reaction with Lewis acids were unsuccessful. 

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