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The reaction of monocyclic oxyphosphorane compounds 1a,1b,1c with ethylene glycol in pyridine was studied by 31P NMR.The results show that compound 1a the an unsaturated five-membered ring reacts faster than compound 1b the a saturated ring attached by two trans p-nitro phenyl groups,which in turn reacts greatly faster than the Cis- compound 1c. To interpret the mechanism, a hexacoordinate intermediate whose five-membered ring occupied the e-,e- position was proposed.