(±)-Isopodophyllotoxin (1e) and related lactones 12, 13, 14 were synthesized by a biomimetic procedure from the doubly unsaturated esters 11a and 11b by means of an oxidative free radical cycloaddition reaction utilizing a stable cation radical salt, Weitz' aminium salt, in one step. (±)-Isopicropodophyllin (1g), the trans-fused lactones, 12, 13, and 16, and the cis-fused lactone 17 were also synthesized from the esters 11c and 11b by the reaction with the same reagent.