Scheme 3. Thiol construction and cyclization. Conditions: (a) CH₂I₂, MeLi, toluene, –78 °C, 1 h, 67%; (b) potassium thioacetate, DMF, rt, 6 h, 74%; (c) Et₂O, LiAlH₄, 0 °C to rt, 1 h, >90%; (d) 16a or 16b, morpholine, MeOH, 0 °C to rt, 12 h, 67% (17a), 79% (17b).