The reactions of 3,4-dichloro-5-methoxyfuran-2(5H)-one with 1,2-ethylenebis(diphenylphosphine) and 4,5-bis(diphenylphosphinyl)-2-phenylpyridazin-3(2H)-one involve substitution of both chlorine atoms with σ3-P atoms to form unstable diphosphonium bicyclic salts which in the course of the reaction undergo an unusual fragmentation to give finally mono-and tricyclic diphosphoniacyclohexane structures
