Peripherally Diels-Alder reaction

Benzenoid hydrocarbons, which like perylene (10) contain a peripheral cisoid C4 arrangement (a bay region [71]), react with maleic anhydride in the presence of a suitable dehydrogenating agent to form fully aromatic dicarboxylic acid anhydrides 12 ( benzogenic Diels-Alder reaction ) [72, 73], Since in this process the ratedetermining step is the exocyclic Diels-Alder reaction leading to 11, the reactivity behaviour of the system is dominated by the Jt-electronic properties of the hydrocarbon centres at which this primary reaction occurs [74]. 

An excellent example is the synthesis of mevinolin 126, an HMG-CoA reductase inhibitor that lowers the cholesterol level, by Hirama and Iwashita.19 Their idea was to use a Diels-Alder reaction to close both rings in one step and they therefore disconnected various peripheral fragments to leave the simpler ketone 127. Reversing the Diels-Alder reveals 128 and the starting material was actually one diastereoisomer to ensure stereochemical control in the ring closure. 

Similar applications of chiral titanium Lewis acid catalysts to asymmetric [2 + 2] cycloadditions, with up to nearly quantitative asymmetric induction, have employed 4-benzoquinones as additions and substituted styrene-type substrates11. In all of these asymmetric [2 + 2]-cvcload-dition reactions, the Lewis acid catalyst presumably is attached to peripheral functional groups and thus, similar to Lewis acid catalyzed Diels Alder reactions (see Section 1.5.8.3.5.4.), is only indirectly involved in the reaction course7. 

Similar to the above-described phthalocyanines, the hemiporphyrazinatometal complexes can be functionalized with dienophilic peripheral substituents, thereby enabling an oligomerization by repeated Diels-Alder reactions [219,220]. To increase the solubility of the resulting ladder polymers, additional peripheral substituents can be introduced into the monomeric subunits. The hemiporphyrazinatometal ladder polymers obtained by Diels-Alder couplings must also be converted into a ir-electron-containing system to achieve electrical conductivity. 

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