Protecting-Group Free Total Synthesis of β-Levantenolide through Late Stage Bromo-Spirocyclization.

We describe a concise asymmetric total synthesis of naturally occurring [5,5]-oxaspirolactone β-levantenolide through an adventitious bromo-spirocyclization reaction of a -decalin appended γ-alkylidenebutenolide. The γ-alkylidenebutenolide core was constructed through "Pd-Cu" bimetallic cascade lactonization reaction of properly functionalized enantiopure alkyne. (+)-Sclareolide was used as a chiral pool starting material to access the desired alkyne.