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