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Structures of the main sesquiterpene lactone-type skeletons: germacranolide isomers (i-a,i-b); elemanolide (ii); eudesmanolide isomers (iii-a,iii-b); eremophilanolide (iv); guaianolide isomers (v-a,v-b); pseudoguaianolide isomers (vi-a,vi-b); and hypocretenolide (vii).
