Scheme 23.

Porco’s branching strategy from the Michael adduct 85. Reagents and conditions: (1) Zn, AcOH-THF, then Na₂CO₃ (aq.) [R¹ = C≡CMe, R² = H], 92%; (2) Grubbs’ 1st-gen. cat., ethylene, MW, 150 W, 50 °C, CH₂Cl₂ [R¹ = allyl, R² = C≡CCH₂OMe], ≥86%; (3) Co₂(CO)₈, MW, 150 W, 80 °C, CH₂Cl₂ [R¹ = C≡CH, R² = allyl], 67%.