New evidence on the structure of the product from the reaction of cyclic 2,4,6-trialkylphenylphosphine oxides with dimethyl acetylenedicarboxylate (DMAD); formed by an inverse Wittig reaction type protocol

New evidence based on spectroscopy, quantum chemical calculations and reactivity suggest that the spirocyclic oxaphosphetes () formed by the [2+2] cycloaddition of the title P-heterocycles () and DMAD are intermediates to afford a stabilised phosphonium ylide () existing as two conformers ().