AbstractA synthesis of C‐chlorinated analogues of 1,5‐diaza‐2,4‐diphosphorinan‐6‐ones is described. The P‐chlorophosphine 3, a key compound for all reported substitution reactions, reacts in an unusual way with N,N′‐dimethyl‐N,N′‐bis(trimethylsilyl)urea to give the unsymmetrical product 5, the formation of which is accounted for by a silatropy of the intermediate compound 4. Compound 5 is stable in solution but rearranges quantitatively into isomer 6 without solvent at room temperature. Compound 3, its fluoro derivative 9, and the alkoxy derivatives 10a–d exist as cis‐and trans‐isomers. Some stereochemical aspects, as well as the possibility of 1,2‐chlorotropy, are discussed.