Imino dienophile mechanism

As alluded to above, no detailed studies of the mechanism of the imino Diels-AIder reaction have been reported to date. However, it has been suggested that [4 + 2] cycloadditions of most C=N, N=N, 0=0 and N=0 dienophiles are HOMOdiene-LUMOdienophiie controlled.3 Whether, in fact, these reactions are concerted pericyclic processes has yet to be firmly established. In any case, a simple dipolar mechanistic model (Scheme 2-1) can often be used to qualitatively predict the regiochemistry of imino dienophile cyclizations with unsymmetrical dienes, which usually proceed with high selectivity.2 This model may, in fact, be indicative... 

Thus, a new type of Lewis acid, lanthanide triflates, is quite effective for the catalytic activation of imines, and has achieved imino Diels-Alder reactions of imines with dienes or alkenes. The unique reactivities of imines which work as both dienophiles and azadienes under certain conditions were also revealed. Three-component coupling reactions between aldehydes, amines, and dienes or alkenes were successfully carried out by using Ln(OTf)3 as catalysts to afford pyridine and quinoline derivatives in high yields. The triflates were stable and kept their activity even in the presence of water and amines. According to these reactions, many substituted pyridines and quinolines can be prepared directly from aldehydes, amines, and dienes or alkenes. A stepwise reaction mechanism in these reactions was suggested from the experimental results. 

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