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Manzamine alkaloids constitute a novel family of several marine sponge metabolites that exhibit significant cytotoxic activity against leukemia and antibiotic activity. The unprecedented structures of these highly functionalized heterocyclic ring system and remarkable biological properties have attracted much attention as a challenging synthetic targets. While the simplest congener manzamine C (3) and related compounds have been previously synthesized by us and Langlois' group, the more complex manzamine A (1) has been more challenging target. Quite recently, Winkler, Martin and their coworkers have succeeded in total synthesis of manzamine A and its related compounds. We have also been interested in developing efficient routes to tetraazacyclic intermediate 5 based on the initial construction of tricyclic intermediate 20 by an intermolecular Diels-Alder reaction of functionalized dihydropyridinone 10 as a dienophile with siloxydiene, leading to the construction of a cis relationship in the central AB ring system of this unique structure. Using this strategy, we could synthesize advanced key intermediates 30 and 36. Details of the synthsis and further conversion to ircinal A (4) and manzamine A will be discussed.