Conclusion It is shown with 31P NMR that the pseudorotation of the oxyphosphoranes 4 and 5 involves intermediate structures with a diequatorial orientation of the dioxaphospholene ring. Thus it appears that the pseudorotation of trigonal bipyramidal phosphoranes that are stabilized by a dioxaphospholene ring may very well involve intermediates which are in conflict with the well-known ringstrain rule.
