Organoboron-based allylation approach to the total synthesis of the medium-ring dilactone (+)-antimycin A(1b)

The stereoselective synthesis of (+)-antimycin A1b has been accomplished in 12 linear steps and 18% overall yield from (-)-ethyl lactate. A robust, scalable, and highly diastereoselective montmorillonite K10-promoted allylation reaction between an α-silyloxy aldehyde and a substituted potassium allyltrifluoroborate salt provides a general approach to the core stereochemical triad of the antimycin A family. The requisite (Z)-substituted potassium allyltrifluoroborate salt was synthesized using a syn-selective hydroboration/protodeboration of an alkynylboronate ester, followed by a Matteson homologation reaction. The total synthesis leverages an MNBA (Shiina's reagent)-mediated macrolactonization to generate the 9-membered dilactone ring and a late-stage PyBOP-mediated amide coupling employing an unprotected 3-formamidosalicylic acid fragment, thereby shortening the longest linear sequence and, perhaps most notably, generating the antimycin A C7-C8-C9 stereotriad in a single step using a single chiral pool-derived stereocenter.