Studies on the Synthesis of the ABC Rings of (±)-Hexacyclinic Acid

A synthesis of the ABC-rings of the polyketide natural product hexacyclinic acid has been achieved. The B-ring was formed first via an intramolecular ester-tethered Diels−Alder reaction, and the A-ring was annulated to this by means of a SmI2 mediated reductive 5-exo-trig cyclization of a samarium−ketyl radical onto a vinyl group. Finally, the C-ring was closed using olefin metathesis. Interestingly, use of enyne metathesis resulted in the synthesis of a more functionalized 5-membered C-ring in a model system, but an undesired 6-membered C-ring in the actual system. An investigation of this change in selectivity is discussed.