As a continuation of our study on the biogenetically patterned transformation leading to the natural terpenoids, conversion of an eudesmanolide directing to the related eremophilanolide was investigated and the object was accomplished. After several attempts, treatment of 5α, 6α-epoxy-dihydroalantolactone (15) with HCOOH-acetone (1 : 2) at reflux was found to afford eight products (A-H). Among them, two major products (products A and B) and one minor product (product D), which were obtained in a combined yield of 46%, were elucidated to be the desired eremophilanolides (17, 18, and 23) on the basis of the physicochemical evidence and the chemical conversion of 23 to tetrahydroligularenolide (34). The structure of the product C was assigned 20 on the basis of its physical properties and the chemical derivation. Afterwards, a better reaction condition transforming 15 to give 17 and 18 in a combined 68% yield was found.
