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Reagents and conditions: (i) EtMgBr, Ti(Oi-Pr)4, BF3.Et2O, Et2O, −78 °C, 1 h, 56%; (ii) MeLi, CeCl3, THF, −25 °C, 1 h, 51%; (iii) 2, SOCl2, 80 °C, 16 h then 18 or 20, toluene, 110 °C, 2 h; (iv) cyclopentylamine, Et3N, CH2Cl2, rt, 2 h, 34% for 19 and 25% for 21 over two steps.
