Scheme 2 Synthesis of compounds 12a-d. Compound 7a was used for the synthesis of 8a, while compound 7b was employed for the synthesis of 8b-d. Reagents and conditions: (a) MeI, Ag₂O, DMF; (b) H₂SO₄, dioxane; (c) PDC, 4 Å molecular sieves; (d) (i) LDA, tripyrrolidinophosphine oxide, EtBr or 2-iodopropane, THF (dry), -68 °C or (ii) LDA, BnBr, THF (dry), -78 °C; (e) (i) H₂/Pd(OH)₂-C, AcOH, EtOH or (ii) H2/Pd-C, EtOH; (f) Bu₂SnO, toluene, reflux; (g) tetrabutylammonium bromide, 4-OMeBnBr, toluene, 90 °C; (h) PPh₃, DIAD, DPPA, THF (dry); (i) DDQ, H₂O, DCM.