Treatment of acylnitroso hetero Diels-Alder cycloadducts 2 with iron(III) or copper(II) in an alcohol solvent induces ring opening to afford predominantly monocyclic anti-1,4-hydroxamic acids 3. However, treatment of cycloadducts 2 with copper(II) in toluene reverses the stereoselectivity of the ring opening to afford syn-1,4-hydroxamic acids 4. These regio- and stereoselective processes separately provide anti-1,4- and syn-1,4-disubstituted cyclopentenes while regenerating a hydroxamic acid moiety, thus enhancing the chemical versatility of the Diels-Alder cycloadducts.