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Here, we report the first asymmetric total synthesis of (+)-talassimidine and (+)-talassamine, two hetidine-type C 20 -diterpenoid alkaloids. A highly regio- and diastereoselective 1,3-dipolar cycloaddition of an azomethine ylide yielded a chiral tetracyclic intermediate in high enantiopurity, thus providing the structural basis for asymmetric assembly of the hexacyclic hetidine skeleton. In this key step, the introduction of a single chiral center induces four new continuous chiral centers. Another key transformation is the dearomative cyclopropanation of the benzene ring and subsequent S N 2-like ring opening of the resultant cyclopropane ring with water as a nucleophile, which not only establishes the B ring but also precisely installs the difficult-to-achieve equatorial C7-OH group.