(−)-Thallusin, isolated from a marine bacterium, is the only known naturalproduct to act as an algal morphogenesis inducer. Because (−)-thallusin can only be obtained in exceedingly limited amounts from microbial cultivation, a synthetic supply of this compound is highly desirable. Here, we describe a novel synthetic pathway to (±)-thallusin and the first asymmetric synthesis of (−)-thallusin utilizing