We report a highly stereoselective Au-catalyzed synthesis of 9-oxabicyclo[3.3.1]nona-4,7-dienes from diverse 1-oxo-4-oxy-5-ynes. Formation of these highly strained anti-Bredt oxacycles implies the workability of an unprecedented 1,4-dipole of s-trans-methylene(vinyl)oxonium. This work reveals the feasibility of a formal [4 + 2] cycloaddition on an s-trans-heterodiene framework.