Scheme 2. Reagents: (a) PTSA (0.2 eq.), allyl alcohol, CH₂Cl₂, 4 Å MS powder, 25 °C, 1 h, 86%; (b) PdCl₂, CH₂Cl₂–MeOH (1 : 4), 25 °C, 4 h, 90% towards 19; (c) 11, CH₂Cl₂, DMAP, 0–25 °C, 3 h, 78% for 19 and 83% for 2 over two steps; (d) NaOMe, MeOH, 25 °C, 1 h, 94%; (e) 8 mol% chloro[tris(2,4-di-tert-butylphenyl)phosphite] gold, 8 mol% AgOTf, CH₂Cl₂, 4 Å MS powder, 25 °C, 15 min, 93%.