Rearrangement of 5-Substituted 5-Aminopentadienals

Strong nucleophiles are needed for inducing ring opening 3 → 4 since the pyrylium salt intermediates 3 , formed from 5-X-substituted 5-aminopenta-2,4-dienals 2 upon treatment with acid, are quite unreactive due to “aromatic” stabilization. However, this allows an easy access to a variety of 2-aminopyrylium salts 3 from “push–pull” enynes 1 . X=OAc, F, Cl, Br, I, OPh.