Cyclizations to lactones. 18O mechanism study

18O was incorporated into the hydroxyl oxygen of 2-(1,1-diphenyl-1-hydroxymethyl)succinanilide (2), which was subsequently treated with concentrated sulfuric acid (a), refluxing acetic acid (b), and methanolic hydrogen chloride (c). The resulting cyclization product, 4,4-diphenyl-3-carboxanilidebutanolactone (3), exhibited complete elimination (a) or retention (b and c) of 18O. These results indicate that both a carbonium ion mechanism (a) or an intramolecular (A-2) acyl substitution mechanism (b and c) can account for the formation of lactone 3