Abstract: The total synthesis of two guaianolide sesquiterpenes,(A)-dehydrocostus lactone and (f)-estafiatin, is described. The synthesis starts with 2, 4, 6-cycloheptatrien-l-one (tropone) and introduces the elements of the five-membered ring through a 1, 8-addition of the Grignard reagent derived from 2-(2-bromoethyl)-l, 3-dioxolane. A stereocontrolled Lewis acid mediated cyclization reaction generates the requisite cis-fused hydroazulene ...