Scheme 2.

(a) CuCN (5 eq.), Et₂O (0.6 M), MeLi (1.6M in Et₂O; 10 eq.), 0 °C; 27 (1 eq.), Et₂O (0.2 M), 71%; (b) CH₂Cl₂ (0.07 M), trifluoroacetic acid (TFA) (10% v/v), 0 °C; (c) Boc-Ala-OSu (1.1 eq.), DIPEA (3.1 eq.), MeCN (0.2 M), 0 °C, 50% over two steps; (d) CH₂Cl₂ (0.07 M), trifluoroacetic acid (TFA) (10% v/v), 0 °C; (e) 30 (1 eq.), DIPEA (3 eq.), MeCN (0.2 M), 0 °C, 72% over two steps; (f) 18 (1 eq.), SnCl₄ (2 eq.), toluene (0.2 M), reflux.