Diels-Alder reactions retrograde

Alder/retrograde Diels-Alder reaction sequence of a diaryl alkyne with a 3,6-dicarbomethoxy tetrazine. The resulting diazine (14) is then reduced, cleaved and cyclized with Zn/acetic acid to the 2,3,4,5-tetrasubstituted pyrrole (15), which is then N-alkylated with a-bromo-4-methoxyacetophenone to give a pentasubstituted pyrrole (16). The synthesis of lukianol A is completed by ester hydrolysis, decarboxylation, ring closure and deprotection. 

Scheme 6.85 Formation of l,2-dioxa-3,4-cyclohexadienes (415) by addition of singlet oxygen to conjugated enynes and retrograde dehydro Diels-Alder reaction of415.

Retrograde Diels-Alder reactions are unlikely reactions in coal liquefaction chemistry due, again, to the lack of suitable molecular structures. Decomposition of cyclohexene to ethylene and 1,3-butadiene, for instance, is 40 kcal/mol endothermic and at 450°C proceeds at a rate such that the half-life of cyclohexene is 13 hours (11a). For tetralin to react by an analogous reaction, resonance stability would be lost, and the reaction would be many orders of magnitude slower than cyclohexene. 

Some pyrolytic reactions can be seen as a reverse (retrograde) addition. Diels-Alder reaction for example is known to be reversible and retro Diels-Alder reactions are rather common. The retro-ene reaction (retro hydro-allyl addition), for example, takes place by the following mechanism ... 

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