Synthetic approaches to [5,6]-benzannulated spiroketal natural products

Studies toward the synthesis of three biologically active [5,6]-benzannulated spiroketal natural products are described. The first total synthesis of paecilospirone is reported, employing a late-stage, pH-neutral spiroketalization. A formal synthesis of γ-rubromycin is described, where the spiroketal moiety is formed by delicate manipulation of the electronic properties of the spirocyclization precursor. Finally, model work toward the total synthesis of berkelic acid is summarized, introducing a novel Horner–Wadsworth–Emmons/oxa-Michael (HWE/oxa-M) cascade to access the spiroketal precursor.