The first enantioselective total synthesis of (-)-sinularianin B, a structurally unique sesquiterpenoid isolated from the Formosan soft coral Sinularia sp., has been accomplished in 16 steps with 39.5% overall yield. Key transformations include formation of a cyclopentane possessing a tertiary hydroxy group via a tandem intermolecular-intramolecular alkylation involving a 5-endo cyclization.