Unexpected Prins cyclization: iron-promoted cyclization/hydration of alkynyl-dimethyl acetals

Unexpected products containing acyl-substituted unsaturated seven-membered carbocycle were synthesized by FeCl3-promoted intramolecular Prins cyclization of alkynyl-dimethylacetals in good yields. It is remarkable that the synthesized 7-exocyclic vinyl cations generated as a result of Prins-type cyclization were trapped by H2O (no halogen ion) in CH2Cl2 to give the corresponding enol derivatives. Those enol derivatives underwent enol-keto tautomerism and then eliminated one molecule of MeOH to give the corresponding acyl-substituted unsaturated seven-membered carbocycle.