An expeditious stereoselective synthesis of natural (-)-Cassine via cascade HWE [3 + 2]-cycloaddition process.

l-Rhamnose is transformed to (-)-Cassine via a remarkable four step one pot reaction. The Horner-Wadsworth-Emmons [3 + 2]-1,3-dipolar cycloaddition reaction cascade is the pivotal step in this reaction sequence and makes the synthesis highly efficient.