A Brønsted acid-catalyzed formal [5+2+1] cycloaddition of ynamides and isoxazoles with water is described. This process provides atom-economical access to oxygen-bridged tetrahydro-1,4-oxazepines, where the bridged oxygen atom originates from water. The unique property of the Brønsted acid shows distinct chemoselectivity from the corresponding gold-catalyzed cycloadditions.