Oxazolium salts Diels-Alder reactions

Cycloaddition of 1050 with DMAD gave the tetrasubstituted bispyrrole 1058. In this case, loss of 2 mol HCIO4 from the bis-oxazolium salt was presumed to generate a mesoionic species (not shown) that underwent cycloaddition with DMAD, followed by a retro Diels-Alder reaction and loss of 2 mol of HNCO to yield 1058. 

Wenkert and co-workers prepared a series of 5-alkenyl-substituted oxazolium salts and investigated their intramolecular Diels-Alder reactions as a means to construct alkaloids (Scheme 1.427). The starting oxazoles 1662a-c, 1663, and 1664 were prepared from an acyclic methyl ester and lithiomethyl isocyanide. Methylation of 1662a-c, 1663, and 1664 with methyl trillate then afforded the corresponding iV-methyloxazolium salt quantitatively. The oxazolium salts 1665, 1666, and 1667 were stable for up to 3 h at 90°C. 

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