Scheme 1. Synthesis of building block 7. Reagents and conditions: a) Ph₂SO, Tf₂O, TTBP, DCM, preactivation of compound 2 at −72 °C for 15 min, then addition of compound 3, 85%; b) NaOMe, DCM/MeOH, 82%; c) 1,3-propanedithiol, Et₃N, DCM/MeOH, 86%; d) TrocCl, NaHCO₃, THF, 64%.