Stereodivergent Synthesis of 6,12-Guaianolide C1 Epimers via a Rationally Designed Oxy-Cope/Ene Reaction Cascade

Nature synthesizes epimeric C1 guaianolide congeners, key components of major natural product classes, through a single structurally flexible macrocyclic germacranolide core. Our rationally designed elemanolide-type scaffold (5) now mimics this natural process, enabling the stereodivergent synthesis of both C1 epimers of 6,12-guaianolide lactone motifs. An oxy-Cope/ene cascade acts as the key step of this process, generating two distinct conformers of an intermediate germacranolide, each leading to a specific C1 epimer. Highly stereoselective redox manipulations follow, culminating in the efficient syntheses of diverse osmitopsin-type guaianolides.