Scheme 253. Iterative Deprotonation–Coupling Approach toward Thiophene-Based Double Helices^,

For all the racemates, the absolute configurations of the chiral axes of depicted enantiomers are shown to facilitate discussion.

Reagents and conditions: (a)⁴⁷⁴n-BuLi (2.1 equiv), THF, −78 °C, 2 h, then 60 °C, 2 h; (b) CuBr₂, Et₂O, −78 °C, 1 h, then rt, overnight; (c) ZnCl₂, −78 °C, then rt; (d) 253.6, Pd(PPh₃)₄, Et₂O, 120 °C, 48 h; (d) CuCl₂, Et₂O, −78 °C, 2 h, then rt, 24 h.