Reagents and conditions: a) AcSH (2.0 eq.), PPh3 (2.0 eq.), DIAD (2.0 eq.), THF, 0 °C – rt, 3 h, 98%; b) NaI (10.0 eq.), AcCl (10.0 eq.), MeCN, reflux, 7 h, 85%; c) 1) NCS (4.0 eq.), 2N HCl/MeCN (v/v, 1/4), 10 °C, 1 h; 2) 0.1N NaOH/dioxane (1: 1, v/v), rt, 0.5 h; d) 1) 6N HCl, reflux, 12 h; 2) FmocOSu, 0.1N NaOH/dioxane, overnight, 14% for four steps.