ORGANOSELENIUM-MEDIATED AND PROTON-MEDIATED CYCLIZATION REACTIONS OF ALLYLIC AMIDES AND THIOAMIDES - SYNTHESES OF 2-OXAZOLINES AND 2-THIAZOLINES

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.