A one-pot three-component route towards fluorinated five/five-annulated nitroso acetals bearing two fused isoxazolidine rings is described. Target nitroso acetals were obtained with complete regioselectivity and high stereoselectivity. The mechanism of this tandem [4 + 1]/[3 + 2] cycloaddition was proposed to include the intermediate formation of elusive 3-fluoro-isoxazoline-N-oxides as key intermediates. This method is applicable for the synthesis of a broad scope of nitroso acetals starting from various fluoronitroalkenes, halogenated dicarbonyl compounds and dipolarophiles of different electronic nature.