Nucleophilic Addition to Oxazolium Salts: Stabilized Azomethine Ylides via 2-Substituted 4-Oxazolines

Article abstract of DOI:10.1021/jo00244a008

Treatment of oxazolium salts with cyanide generates 4-oxazolines 2 in situ.Ring opening to azomethine ylides 3 occurs spontaneously and 2 + 3 cycloadducts are obtained in the presence of acrylate, propiolate, or acetylenedicarboxylate dipolarophiles.In the case of acetylenic dipolarophiles, loss of HCN occurs under the reaction conditions and leads directly to pyrroles 5.The propiolate experiments are complicated by the formation of six-membered adducts 17 in some cases.This reaction pathway is explained by the addition of the acetylide anion derived from propiolate to the dipole, followed by cyclization.Sulfide nucleophiles can also be used to generate 4-oxazolines, but the yields of cycloadducts are lower.