Optimization of the reaction conditions for the synthesis of 6a.

Entry	Copper salt	Solvent/medium	Conventional method		US method
			Timea	Yieldb (%)	Timea	Yieldb (%)
1	—	t-BuOH : H2O	10 h	0	6 h	0
2	CuSO4·5H2O	t-BuOH : H2O	10 h	28	6 h	32
3	CuSO4·5H2O + Na ascorbate	t-BuOH : H2O	10 h	70	6 h	78
4	Cu(OAc)2·H2O	t-BuOH : H2O	10 h	72	6 h	82
5	Cu(OAc)2·H2O + Na ascorbate	t-BuOH : H2O	10 h	72	6 h	83
6	CuI	THF	10 h	70	6 h	80
7	CuCl	THF	10 h	60	6 h	65
8	—	[Et3NH][OAc]	10 h	0	6 h	0
9	Cu(OAc)2·H2O	[Et3NH][OAc]	7 h	90	35 min	95
10	Cu(OAc)2·H2O + Na ascorbate	[Et3NH][OAc]	7 h	90	35 min	95
11	CuSO4·5H2O + Na ascorbate	[Et3NH][OAc]	7 h	81	35 min	84
12	Cu(OAc)2·H2O	[Et3NH][Cl]	7 h	Trace	35 min	10
13	Cu(OAc)2·H2O	[Et3NH][HSO4]	7 h	Trace	35 min	12
14	Cu(OAc)2·H2O	[HDBU][OAc]	7 h	84	45 min	88
15	CuSO4·5H2O + Na ascorbate	[HDBU][OAc]	7 h	81	45 min	84
16	Cu(OAc)2·H2O	[HDBU][HSO4]	7 h	10	45 min	14

^aReaction conditions: 2a (1 mmol), 5a (1 mmol), and 10 mol% [Cu], 1 mL solvent, 30 °C; reaction progress monitored by TLC.

^bIsolated yield, US: ultrasonication.