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The total syntheses of dimeric guaianolide sainsliadimer B (5b) and gochmatiolides A-B (6a-6b) from α-santonin have beenaccomplished in 25 steps with approximately 1％ overall yield.A well-designed Diels-Alder reaction of natural dehydrozaluzanin C (3) with the precisely modified monomeric diene 2 allows stereoselective assembly of a dimeric gochnatiolide-type skeleton 4 with the required stereochemistry and preinstalled functionalities, which guarantees the synthesis of dimeric guaianolides (5b, 6a-6b) via a multi-step functionality manipulation.[1] Further assembly of dimeric gochnatiolide B with diene 2 via a Diels-Alder reaction, followed by epoxidation, deprotection, oxidation and dehydration provides epi-ainsliatrimerB.