4-Aroyl-4-oxazolines undergo stereospecific [2 + 3] cycloaddition to olefinic dipolarophiles with the formation of 3-aroyl-4,5-dihydrofurans. The latter are also found to be reactive towards further [2 + 3] cycloaddition reactions. Possible mechanisms for these reactions are discussed and the synthesis of several new furans and dihydrofurans reported.