Triazolidinediones react with each enantiomeric bornanesultam derivative of tiglic acid to produce the appropriate ene adduct in high yield and with excellent regioselectivity and diastereoselectivity. The optically pure products, viz. N-[(3R)-3-(4methyl-3,5-dioxo-1,2,4-triazolidin-1-yl)-2-methylenebutanoyl ](1S,2R)-bornane-10,2-sultam 0.15-hydrate, C(18)H(26)N(4)O(5)S(.)0.35H(2)O, and its enantiomer N-[(3S)-3-(4-methyl-3,5-dioxo1,2,4-triazolidin-1-yl)-2-methylenebutanoyl ]-(1R,2S)-bornane10,2-sultam0.35-hydrate, C(18)H(26)N(4)O(5)S(.)0.35H(2)O, have been characterized by spectroscopy and single- crystal X- ray analysis. Their structures are the result of C ss-re attack of the enophile on the double bond of the alkene.