Scheme 2.
Reagents and conditions: (a) 1.0 equiv. ZnCl2 (0.5 M in THF), THF, 2-24 h, 40 °C, 93% for 14, 0% for 18, 75% for 4 and 75% for 21; (b) DMF, 120 °C, 2 h, 100%.
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