A strategy for the synthesis of isoxazolidine/1,2-oxazinane-fused isoquinolin-1(2H)-ones from alkyne-tethered N-alkoxyamides is described, in which cheap Mn(acac)2 is used as a catalyst to facilitate a radical cascade annulation. The method features mild conditions, additive-free reaction and broad substrate scope. It is the first example via manganese/air catalytic systems to construct isoquinolin-1(2H)-one heterocycles.