Synthesis and Structural Confirmation of Secalosides A and B

Secalosides A and B are a pair of glycosides isolated from rye pollen (<i>Secale cereale</i>) that exhibit <i>in vivo</i> anticancer activity against various cancer cell lines. The underlying mode of action was proposed to be immunomodulatory. However, their three-dimensional chemical structures remained ambiguous for nearly 30 years, impeding further biological investigation. We report the definitive structural determination of the secalosides, achieved by total synthesis. The formation of the highly strained ten-membered bis-lactone and the indane core required the development of a late-stage ring contraction strategy enabled by an unprecedented transannular oxidative enolate coupling. Our structural assignment and synthetic route provide a foundation for exploring the molecular mechanism of action and structural-activity relationship of secalosides A and B.