[structure: see text] A stereocontrolled convergent synthesis of the annonaceous acetogenin pyragonicin (1) is presented. The keyintermediates were accessed using asymmetric Horner-Wadsworth-Emmons (HWE) methodology. A reagent controlled zinc-mediated stereoselective coupling, joining the two highly functionalized intermediates 3 and 4, then provided the core structure.
This paper describes a second-generation synthesis of an antitumor tetrahydropyran ( THP) acetogenin, pyragonicin. The key step involved an olefin cross-metathesis between the THP segment and the terminal gamma-lactone residue. The coupling reaction in the presence of Grubbs' second-generation catalyst resulted in an unseparable mixture of a desired coupling product and its one-carbon eliminated product while the use of Grubbs' first-generation catalyst afforded the former exclusively. A novel MOM-migrating reaction found in a cyclization reaction is also discussed.
Total Synthesis of the Proposed Structure for Pyragonicin
The total synthesis of acetogenin 1 reported for pyragonicin and its 10-epimer 32 is described. The common THP ring system was stereoselectively constructed through a SMI2-induced reductive cyclization of beta-alkoxy acrylate 5 followed by Mitsunobu inversion, and each chiral center at C-10 was created by Brown's asymmetric allylation. Compound 1 had spectroscopic data consistent with that of natural pyragonicin, but a different optical rotation.