Total Synthesis of the Cytotoxic Threo, Trans, Erythro, Cis, Threo Annonaceous Acetogenin Trilobin

A synthesis of trilobin, a new stereochemical varient of the adjacent bis-tetrahydrofuran subgroup of the Annonaceous acetogenins, is described. The synthesis involves three stereochemically defining carbon-carbon bond-forming steps. The first of these introduces the C23-C24 stereocenters and the left side chain by means of an S(E)2' addition of the nonracemic 11-carbon gamma-oxygenated allylic indium reagent derived from the alpha-oxygenated allylic stannane 4 to a C24-C16 core aldehyde 3. The second develops the C15-C16 stereocenters and a segment of the right chain through BF(3)-promoted S(E)2' addition of the nonracemic 6-carbon gamma-oxygenated allylic stannane 11 to the C16-C34 aldehyde 10. The third employs the addition of the dialkylzinc reagent 17 to the C10-C34 aldehyde 15 in the presence of a chiral bis-sulfonamide catalyst to establish the C10 stereocenter while adding the C1-C9 residue of the right chain. The C36 stereocenter and the butenolide are appended through condensation of the C1-C34 ester with the protected (S)-lactaldehyde 23.