Stereochemical and Theoretical Aspects of Hetero Diels-Alder Reactions

Stereochemical and Theoretical Aspects of Hetero Diels-Alder Reactions 

From the transition structures, it can be seen that the ds-pro duct can be formed either by an endo-E-syn or an exo-Z-syn orientation, whereas the trans-product is obtained either by an exo-E-anti or an endo-Z-anti transition structure. For intramolecular reactions the situation is simplified since calculations have 

In contrast to the great number of calculations concerning the all-carbon Diels-Alder reaction [39], there are only a few theoretical studies on the hetero Diels-Alder reaction [41,42,45 - 53 ]. The general mechanism of the Diels-Alder reaction is still in discussion however, in most cases a concerted reaction is assumed,but there is also evidence for a two-step path. The ab initio calculations carried out for the butadiene/ethene system by Houk, Ortega, Bernardi und Gajewski gave a symmetrical transition structure only using the semiempirical AM1/CI method (half electron approximation) an unsymmetrical diradicaloid intermediate was found [40]. 

For hetero Diels-Alder reactions it has been shown by calculations that the transition structures are usually less symmetric than for the all-carbon Diels-Alder reactions also a change from a concerted non-synchronous to a stepwise mechanism depending on the substituents at the reacting species and the reaction conditions can occur. 

One of the first calculations on hetero Diels-Alder reactions was done in our group in collaboration with Anders on the 1-oxa-1,3-butadiene (acrolein)/ethene system [41, 42]. The performed ab initio und semiempirical calculations show 

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Stereochemical and theoretical aspects of hetero Diels-Alder reactions have been intensively studied and recently reviewed. A clear definition for the endo- and exo-orientation regarding intermolecular all-carbon and hetero Diels-Alder reactions was recently suggested by Tietze in a review covering the literature of the last decade. The reaction mechanisms of several hetero Diels-Alder reactions have been investigated and calculated and concerted processes as well as stepwise reaction pathways have been proposed depending on the compounds employed and the reaction conditions. 

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