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Reagents and conditions: (i) trimethylsilylacetylene, Pd2(PPh3)2, CuI, Et3N, THF 65 °C; (ii) KOH, MeOH/THF, rt, 61% for two steps; (iii) 6a-c, CuSO4.5H2O, sodium ascorbate, AcOH, DMF/H2O, rt, 20a (64%), 20b (56%), 20c (47%); (iv) 10a,b, CuSO4.5H2O, sodium ascorbate, AcOH, DMF/H2O, rt, 21a (31%), 21b (85%).
