8-(Azidomethyl)-1-naphthoic acid was elaborately prepared, and its coupling with amines provided the corresponding 8-(azidomethyl)-1-naphthamides. The Staudinger reactions of 8-(azidomethyl)-1-naphthamides with phosphine produced iminophosphoranes, and easy intramolecular cyclization of the iminophosphoranes afforded 2,3-dihydro-1H-benzo[de]isoquinolin-1-one leaving amines with almost quantitative conversion rates for the ipsilateral effect of 1,8-substituents on naphthalene. The protocol exhibits some advantages, including a readily available protecting group, cleavages of amides in almost quantitative conversion rates, an aqueous medium, reactions at room temperature, a broad substrate scope, wide functional group tolerance, and suitable scale-up reactions.