On treatment of 2-hydro-2-oxo-5,6-benzo-1,3,2λ4-dioxaphosphorin-4-one (1) with the inline trimers trim ethyl- and tribenzyltriazine and the (monomeric) methylene-tert-butylimine, an insertion reaction of the imine molecule into the P-H bond of 1 took place, forming com pounds 2 -4 . Similar results were obtained when 1 was allowed to react with aldimines, asymmetric at carbon, leading to 5-7 . The diastereotopic properties of 5 - 7 were observed by 1H NMR spectroscopy. NMR and IR spectroscopic investigations on 5 - 7 indicated > C = O···H-N < interactions. Upon reaction of 1 with hexafluoroacetone, trifluoroacetophenone or phenyltrifluoromethylketimine, the derivatives 8 -1 0 , involving a P-O-C fragment, were obtained. The formation of the isomeric “alcohol form ”, bearing a P-C-OH fragment, could not be proved. In the reaction of 1 with various o-quinones. one of the oxygen atom s of the quinone was found to be bonded to phosphorus, whereas the hydrogen atom was found to migrate to the second oxygen atom . Upon reaction of 1 with chloral, an unusual ring expansion took place with formation of the benzodioxaphosphepinone (14). The reaction of 1 with other aldehydes could not be realized. In the reaction of 14 with triethylamine, the ammonium salt 15 was formed. For the first time, an X-ray crystal structure determination of a dioxaphosphepinone ring system was conducted; in 15 the seven-membered ring displays a pseudo tub conformation.