Gold- and Silver-Catalyzed [4+2] Cycloadditions of Ynamides with Oxetanes and Azetidines

Gold-catalyzed [4+2] cycloadditions between ynamides and oxetanes are described; these reactions involve oxetanes and gold-p-ynamides as nucleophiles and electrophiles, respectively. Excellent cycloaddition regioselectivities are achieved over a reasonable range of ynamide and oxetane substrates. For azetidines, their [4+2] cycloadditions with ynamides are implemented more efficiently with silver hexafluoroantimonate, which is also compatible with various ynamides and azetidines. These two cycloadditions provide facile accesses to six-membered heterocycles such as 6-amino-3,4-dihydro- 2H-pyrans and 2-amino-1,4,5,6-tetrahydropyridines.