Aromatic compounds reverse Diels-Alder reactions

The first step of this reaction is a thermally allowed [4+2] cycloaddition (Diels-Alder reaction) to produce an intermediate bicyclic compound. This intermediate rapidly undergoes a thermally allowed reverse Diels-Alder reaction to produce the stable aromatic diester and carbon dioxide. 

The application of thermolytic reactions (e.g. Diels-Alder and reverse Diels-Alder processes) for the synthesis of fluoro-organic compounds has been reviewed. The cycloaddition of the meso-ionic dithiolone or thiazolone compounds (209 X = S or NPh) with the Diels-Alder adduct of cyclo-octatetraene and dimethyl acetylenedicarboxylate occurs at the cyclobutene ring to give the exo-adducts (210). Pyrolysis of compounds (210) yields dimethyl phthalate and 7,9-diheteratricyclo[4,2, 1,0 ]non-3-en-8-ones (211 X = S or NPh) by retro-[4-t- 2] addition. The similar additions of (209) and related compounds to dimethyl 7-oxabicyclo[2,2,l]-hepta-2,5-diene-2,3-dicarboxylate afforded [4 -1-2] adducts which on pyrolysis suffered double thermal fragmentation e.g. loss of XCO and 3,4-dimethoxycarbonyl-furan) to afford five-membered aromatic heterocyclics. ... 

This experiment is a further example of the Diels-Alder reaction. For a discussion of the basic aspects of this reaction see Experiment [14]. In the present case, the central ring of anthracene is shown to possess the characteristic properties of a diene system. Thus, this aromatic compound reacts to form stable Diels-Alder adducts with many dienophiles at the 9 and 10 positions (the two positions on the central ring where new bonds can be made without destroying the aromaticity of the other two rings). Maleic anhydride, a very reactive dienophile, is used here in the reaction with anthracene. Note, that as this reaction is reversible, it is usually best carried out at the lowest possible temperatures consistent with an acceptable reaction rate (see Experiment [14]). 

The Diels-Alder reaction is a 1,4-addition of an olefinic compound to a conjugated diene. The diene system may be part of an aliphatic, aromatic, or heterocyclic nucleus such as furan. The olefinic compound usually contains one or more groups that activate the double bond, although this is not always necessary. For example, ethylene is condensed with butadiene at 200°C to give cyclohexene. Triple bonds may replace double bonds in both the diene and the dieneophile. Cis addition of the dienophile to the diene occurs and several reactions of the above type have been shown to he reversible. 

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