Towards the total synthesis of calyculin C: preparation of the C₉–C₂₅ spiroketal-dipropionate unit

An asymmetric synthesis of the C₉–C₂₅ spiroketal fragment of calyculin C is described. Key steps include two crotylation reactions using successively Brown's reagent and (Z)-crotyltrifluorosilane for the formation of the anti, anti, anti stereotetrad, ynone formation by a Pd-catalyzed coupling of a thiol ester with a terminal alkyne and a double intramolecular hetero-Michael addition for the stereoselective construction of the spiroketal framework.