ABSTRACT The conformation of some pentaoxacyclopentadecanediones «ere investigated by 'H-'3C NMR and conformations were found as regular anti, gouch, anti (a, g+ ,a) units. However the role of a side group was observed that the rate of interconvertion was strongly influenced so that the polyoxa-lactone ring could excist as diastereomere of in equilibrium.