Nitroso and Nitro Diels-Alder Reactions

The generation of an 1,2-oxazine 4-1 by hetero Diels-Alder reaction is a transformation which opens a versatile array of highly functionalised acyclic and cyclic structures. Thus, pyrrolidine derivatives 4-2, amino alcohols 4-3 and aza sugars 4-4 are easily available from these cycloadducts. Cyclic dienes, e.g. 4-5 are converted into bicyclic adducts 4-6 representing straightforward intermediates for aminocyclitols 4-7 (Fig. 4-1). 

Since numerous applications of nitroso dienophiles in natural product synthesis [8] and, in particular, aza sugars [332] have been reviewed in the last years, these topics will only be discussed briefly here and only some exemplary and very recent transformations are presented in this article. 

Lewis acid catalysis, apparently dispensible due to the very high reactivity of nitroso dienophiles, has not yet been investigated although such a study has been suggested by Streith and Defoin [8]. Thus, examples of asymmetric catalysis lack completely in this area of hetero Diels-Alder chemistry. Nevertheless, cycloadditions involving nitroso dienophiles have reached an advanced level concerning stereoselectivity and therefore much attention has been paid towards the preparation and application of chiral, enantiopure dienophiles and dienes for these reactions. 

In some recent investigations carried out by Streith s [346, 347] and Wyatt s groups [348], also chiral dienes turned out to exhibit noteworthy potential for asymmetric cycloadditions to achiral nitroso compounds. 

However, the often excellent diastereoselectivity is not always accompanied with comparable high regioselection. The following example dealing with structurally more complex substrates might illustrate this problem. 

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