Figure 5

Preparation of key intermediate 15: (a) t-BuLi, THF, -78 °C, then 1-bromo-4-chlorobutane, −78 °C – rt; (b) I₂, HgO, Ac₂O, DCM, reflux; (c) NBS, silica gel, BHT, protected from light, MeCN, −10 °C; (d) allyltributylstannane, Pd(PPh₃)₄, DMF, reflux; (e) Pd₂(dba)₃, SPhos, t-BuOK, THF, reflux; (f) NaI, PPh₃, EtOAc, reflux.