Enantioselective synthesis of (+)-obolactone based on a symmetry-breaking Wacker monooxidation of a diene.

A concise synthesis of the dihydro-α-pyrone/dihydro-γ-pyrone natural product (+)-obolactone (13) is disclosed. The dienediol acetonide 23 (≥97% ee) was obtained from 1,5-dichloropentane-2,4-dione in four steps. A Wacker monooxidation of 23 furnished the monoketone 24 in 64% yield. The OH group of the ensuing dihydro-γ-pyrone 31 was esterified under Mitsunobu conditions with cinnamic acid (→ 80% inversion and 20% retention of configuration). A ring-closing metathesis formed the dihydro-α-pyrone moiety of the target in the terminating step.