Ethyl 7-aryl-6,8-dioxo-2-oxa-3,7-diazabicyclo[3.3.0]oct-3-ene-4-carboxylates and ethyl 7-aryl-6,8-dioxo-1-oxa-2,7-diazaspiro[4.4]non-2-ene-3-carboxylates were selectively reduced with sodium tetrahydridoborate at the carbonyl group in the β-position with respect to the oxygen atom in the isoxazole ring. © 2009 Pleiades Publishing, Ltd.