Scheme 4. Synthesis of (a) Fmoc–Q5CO2(tBu)–OH (31) and (b) Fmoc–QCO2(tBu)–OH (36). Reagents and conditions: (a) ethylvinylether, H2SO4, AcOH, 70 °C to 120 °C, 30 min; (b) SeO2, pyridine, 80 °C, 2 d; (c) benzyl bromide, K2CO3, DMF, r.t., 12 h; (d) 2-tert-butoxy-2-oxoethyl zinc(ii) bromide, bis(dibenzylidene acetone)-palladium [0], 1,2,3,4,5-pentaphenyl-1′-(di-tert-butylphosphino)ferrocene, THF, 70 °C, 12 h; (e) H2, Pd/C, THF, r.t., 1 d; (f) Fmoc–Cl, dioxane/H2O, NaHCO3 (10% w/vol), 0 °C to r.t., overnight; (g) LiOH·H2O, THF/H2O, r.t., 1 h; (h) 2-tert-butoxy-2-oxoethyl zinc(ii) bromide, bis(dibenzylidene acetone)-palladium [0], 1,2,3,4,5-pentaphenyl-1′-(di-tert-butylphosphino)ferrocene, anhydrous THF, r.t., 4 h. (c) Possible decarboxylation mechanism.
