Scheme 8a.
a Reagents and conditions: (a) CCl3CN, DBU, CH2Cl2 (95%); (b) Cs2CO3, allyl-Br, DMF (70%); (c) (B(C6F5)3), CH2Cl2 (59%, 1:7, α: β; plus 15% 34 and 21% 68); (d) NaOH, 1,4-dioxane; then Cs2CO3, H2O, MeOH; (e) KHCO3, BnBr, DMF (92%, 2 steps); (e) HCO2H, Pd(OAc)2), Et3N, PPh3, 1,4-dioxane (70%).