Convergent Total Syntheses of Anti ‐Diol Diastereomers of Tonkinelin Toward Its Absolute Structure Assessment

The stereoselective syntheses of (17 S ,18 R ,36 S )‐tonkinelin and its diastereomer (17 R ,18 S ,36 S )‐tonkinelin via a convergent strategy have been achieved. The strategy involved the stepwise construction of two key fragments—a chiral ester‐derived phosphonium salt and a requisite aldehyde for a convergent Wittig olefination. These are the anti ‐diol isomers of tonkinelin and were synthesized to gain more insight on the absolute configuration of the natural isolate. Asymmetric dihydroxylation of cis ‐olefin, Wittig olefination, ring‐closing metathesis, and chemoselective diimide olefin reduction were key steps employed. The synthesis of all four diastereomers helped to establish the natural isolate to have the (17 S ,18 S ,36 S ) absolute configuration.