The Delta (3)-imidazoline 3-oxides I undergo diastereoselective cycloaddition with dimethyl acetylenedicarboxylate 2 to give 3a,4,5,6-tetrahydroimidazo[1,5-b]isoxazoles 3. Thermally and base induced ring-opening reactions of compounds 3 were demonstrated. cis-6-Phenyl substituted adducts 3d,e undergo ring opening with secondary but not with ternary amines. The same adducts undergo regio- and diastereoselective Michael addition with sodium methoxide to give 2-methoxy-3a,5,6-triphenyl-hexahydro-imidazo[1,5-b]isoxazole-2,3-dicarboxylic acid dimethyl esters 6. (C) 2001 Elsevier Science Ltd. All rights reserved.