Diels-Alder reactions special features

Chiral LA are rarely used in the constmction of chiral six-membered cyclic nitronates by the Diels-Alder reaction of olehns with a-nitoralkenes (96, 158), in spite of the potential efficiency of the process. Apparently, this is associated with the absence of known common features of the process and, as a consequence, with the necessity to perform special investigations for optimization in each particular case. 

Diels-Alder reactions of heterodienophiles have been known for decades, but only recently has this methodology become widely accepted by the synthetic community. There is enormous diversity in the structural types of compounds which can act as heterodienophiles, and a wide array of heterocyclic adducts can be prepared via these [4 + 2] cycloadditions. It seems clear that hetero Diels-Alder reactions span a range of mechanism from concerted to stepwise ionic processes. In many instances, mechanistic information is totally lacking. The discussion below therefore classifies heterodienophiles by structural rather than mechanistic class. Only the major types of synthetically useful heterodienophiles have been included. Moreover, the significant regio- and stereo-chemical features of the reactions have been exemplified as much as possible using recently reported cases. Other more comprehensive and more specialized reviews should be consulted for older material and more obscure hetero Diels-Alder cycloadditions. 

B. Some Special Features of the Diels-Alder Reaction... 

The Diels-Alder reaction is the oldest and best studied pericyclic reaction. Here we will consider some special features in the light of PMO theory. 

The theoretical principles of cycloaddition reactions are well understood and there have been many computational studies (see Pericyclic Reactions The Diels-Alder Reaction). Often the hetero-cycloaddition reaction shows similar characteristics to the carbocyclic analog, but a number of special features have been noted. In heterocyclic chemistry the cycloaddition reactions are often dipolar computational studies show that a concerted mechanism is followed in the gas phase. However, a number of studies have noted that these dipolar cycloaddition reactions become stepwise when solvent effects are included (via the reaction field method), with a consequent loss of stereospecificity." Other characteristics of hetero-cycloaddition reactions which have been studied include the endo/exo selectivity" and the regiose-lectivity (for example, [2-1-2] vs. [2-1-4])." High levels of electron correlation are generally required in order to establish these selectivities. 

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