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Organoselenium- and Proton-Mediated Cyclization Reactions of Allylic Amides and Thioamides. Syntheses of 2-Oxazolines and 2-Thiazolines
Engman, Lars
, p. 3425 - 3430 (2007/10/02)
A variety of allylic amides and thioamides were treated with phenylselenenyl bromide in chloroform to give, via 5-exo cyclization, 2-oxazolines and 2-thiazolines, respectively, carrying a (phenylselenenyl)methyl substituent in the 5-position.In some cases (N-crotyl- and N-cinnamylamides/thioamides), dihydro-1,3-oxazines/-thiazines were formed via 6-endo cyclization.The phenylselenenyl group of the cyclofunctionalization products was slowly eliminated by treatment with m-chloroperbenzoic acid to introduce unsaturation in the resulting oxazoline/thiazoline.Reductive removal of the phenylselenenyl group was effected by treatment with triphenyltin hydride.This reaction was sometimes accompanied by a rearrangement of the heterocyclic ring.Proton-induced cyclizations of allylic thioamides to give 2-thiazolines was slowly but efficiently effected in boiling toluene containing a catalytic amount of p-toluenesulfonic acid.
