 The mechanism of the classical <unk> <unk> reaction was first demonstrated experimentally by Shoppe to be an intramolecular <unk> and is outlined below . Activation of the ketone by the acid catalyst generates a <unk> cation which undergoes a thermally allowed 4π <unk> <unk> as dictated by the Woodward @-@ Hoffman rules . This generates an <unk> cation which undergoes an elimination reaction to lose a β @-@ hydrogen . Subsequent <unk> of the <unk> produces the <unk> product . 
