A metal-free tandem reaction, initiated by the generation of a diazonium cation and followed by cycloetherification, was developed. Acid-promoted de-tert-butylation of N-nitroso N-tert-butylamine was used to generate a diazonium cation in situ, demonstrating a new application of nitroso chemistry. This reaction was employed in the synthesis of substituted benzofuran-3(2H)-ones and dihydrobenzo[d][1,3]oxaphosphole 3-oxides.