Stereochemistry and the Retro Diels-Alder Reaction

Earlier in this chapter, a mention was made of the steric requirements of the diene reaction. It was shown that by virtue of rotation around a C-C single bond, an equilibrium can exist between a cisoid and a transoid isomer. Only the cisoid form is able to participate in the reaction. Transoid dienes are less reactive and generally give rise to polymers in preference to the DA 

Stereochemistry is an important tool for the understanding of reaction mechanisms as indicated by Martin and Hill. However, before the steric course of the reaction can be used as a guide to the mechanism of the DA reaction of MA, a number of relevant factors have to be considered including  

In addition, before the routine use of instrumentation such as gas-liquid chromatography, infrared spectroscopy, nuclear magnetic resonance spectrometry, etc., the identity of components and compositions were determined by isolation. The latter involved the problems of separating compounds of closely related properties. This often explains conflicting and inaccurate data published in the literature. The modern techniques of determining compositions have resolved some of these problems. 

During the study of the diene synthesis, it was observed quite early that the reaction exhibited pronounced steric selectivity. Through the efforts of Alder and Stein some confirmed generalizations have evolved which are useful guides. 

Cis Principle The addition of a dienophile to a diene is a purely cis addition. Thus, the adduct from anthracene and maleic anhydride (a c/5-dienophile) is c/5-anhydride. Fumaric acid (a // 2 5-dienophile), on the other hand, adds to yield a trans-dicarhoxylic acid.  

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