A total of eleven naturally occurring guaianolides, 1 iP,13-dihydrokauniolide, estafiatin, isodehydrocostuslactone, 2-oxodesoxyligustrin, arborescin, 1 ß,13-dihydroludartin, 8-deoxy-11 ß, 13-dihydrorupicolin B, ludartin, kauniolide, dehydroleucodin, and leucodin, and three related compounds, 1,10-epiarborescin, 8-deoxyrupicolin B, and 3,4-epiludartin, were synthesized through a common cationic intermediate A, which was derived from ( 1151)-1 ß-(mesyloxy)eudesm-3-eno-12,6a-lactone [2] by solvolytic rearrangement.The guaianolides represent one of the largest groups of sesquiterpene lactones with over 500 known naturally occurring compounds (2). Some of them have been shown to possess high antitumor (3-15), antishistosomal (3,16,17), anthelmintic (18), contraceptive (19,20), root-growth stimulatory (3,21,22), root-growth and germination inhibitory activities (3,4,(13)(14)(15)23,24), and preventive or curative activities for crop diseases (4,15). Because of the diverse biological activities of the guaianolides and since they are