γ-lactone in high yield. The remarkable solvent effect of water was observed in this reaction, although the medium effect is believed to be small in radicalreactions. Powerful solvent effects also operate in the preparation of medium- and large-ring lactones. Water as a reaction solvent strikingly promoted the cyclization reaction of large-membered rings. Stirring a solution of 3,6-dioxa-8-nonenyl iodoacetate