Scheme 37.

Synthesis of precursors 273 (A) and 275 (B), as reported by Rodríguez and Spur [64] and by Balas and co-workers [110]. Reagents and conditions: (A). i. KHMDS, THF,0 °C to rt, TMSCl, 0 °C, 276, −78°C to rt, then 1 N HCl, 0 °C to rt, 45%; ii. Ph₃P, imidazole, I₂, 99%; iii. Ph₃P; MeCN; (B). i. 2-methoxypropene, PPTS, EtOAc, 0 °C to rt, 68%; ii. nBuLi, THF, −78 °C, 80%; iii. Dess–Martin oxidation, CH₂Cl₂, rt.