Construction of 3,5-substituted 1,2,4-oxadiazole rings triggered by tetrabutylammonium hydroxide: a highly efficient and fluoride-free ring closure reaction of O-acylamidoximes
Tetrabutylammonium hydroxide (TBAH) is an efficient and mild alternative to tetrabutylammonium fluoride (TBAF) for base catalyzed cyclizations of 1,2,4-oxadiazoles from O-acylamidoximes. For most 3,5-substituted 1,2,4-oxadiazoles the reactions were dramatically accelerated by addition of 0.1 equiv of TBAH at room temperature. This method was also more generally applicable allowing for a wider range