Iminium salts, addition Diels-Alder reaction

Other reactions are possible with iminium salts derived from a,p-unsaturated carbonyl compounds. The Baylis-Hillman reaction (chapter 18) also uses conjugate addition as a first step and we shall deal with that immediately. Much more work has been done on the Diels-Alder reaction that will follow. The Baylis-Hillman reaction between an electrophilic a,p-unsaturatcd carbonyl compound and an... 

Cycloadditions of iminium compounds with 1,3-dienes to afford Diels-Alder adducts seems to be a reaction of general applicability.46 On the other hand, simple electron-rich imino species are less reliable dienophiles. Some electron-rich imines will undergo Diels-Alder reactions under acid catalysis and thus are probably actually reacting as iminium salts. However, inverse electron demand [4 + 2] cycloadditions of neutral imines with electron-deficient dienes have been reported, as have additions to exceptionally reactive dienes (vide infra). 

A few additional imino Diels-Alder reactions have been reported which involve iminium salts. Bohme and Ahrens found that the perchlorate salt in Eq. (11)... 

---