Scheme 4.
Reagents and conditions: (i) propargyl alcohol, AgOTf, 4Å molecular sieves, CH2Cl2, 0 °C, 2 h; (ii) NaOMe, MeOH; (iii) TiBr4, CH2Cl2/EtOAc; (iv) Zn dust, NH4Cl, EtOH/H2O, 90 °C.
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