A series of novel enantiomerically pure spiro-isoxazolidines and spiro-isoxazolines were synthesized regioselectively by 1,3-dipolar cycloaddition using respectively two dipoles, nitrones and nitrile oxides, on the exocyclic double bond of the B ring of tomentosin (α-methylene-γ-butyrolactone), a sesquiterpene lactone extracted from Dittrichia viscosa.