Cyclic imino Diels-Alder reaction

Cyclic N-acylimines have been used effectively as dienophiles in imino Diels-Alder reactions. The most common type of cyclic imino dienophile is dehydrohydantoin [Eq. (5)],... 

Fig. 8.78 InCb catalyzed imino Diels-Alder reaction of N-arylaldimines and cyclic dienes.

A number of diverse stmctural types of cyclic imino dienophiles have been used in cycloadditions. For instance, dehydrohydantoins are useful partners in hetero Diels-Alder reactions. Two methods have been developed for in situ generation of these species. In one tqjproach, methoxyhydantoins such as (24) (equation S) are heated or are treated with acid to promote elimination of methanol, affording dienophile (25).22- This intermediate can be trapped regio- and stereo-selectively with 1,3-dienes. For example, with 1,3-cyclohexadiene only endo adduct (26) is formed. There is no ambiguity in this case concerning the dienophile configuration, and thus product (26) clearly derives from an endo transition state. 

Another general approach towards imino sugars is based on the formation of cyclic imines and their subsequent reactions. The versatility of imines results from a number of possible transformations they can undergo reduction to amines/ nucleophilic addition/ and aza-Diels-Alder cycloadditions are among the most common. 

---