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Reagents and conditions: (a) methyl propiolate for 18/ethyl propiolate for 19/diethyl acetylenedicarboxylate for 22, CuSO4, sodium ascorbate, MeOH/H2O 10/1 v/v, 55 °C; (b) 1 M NaOH, EtOH, RT; (c) p-fluorobenzyl bromide, K2CO3, dimethylformamide (DMF), RT for 21.
