Optimization of reaction conditions for 3aa ^a.

Entry	Variation from the standard conditions	Yieldb (3aa) (%)
1	None	70
2	FeCl3 instead of BF3·Et2O	45
3	Bi(OTf)3 instead of BF3·Et2O	53
4	p-TSA instead of BF3·Et2O	n.d.
5	MeCN instead of toluene	35
6	DCE instead of toluene	46
7	Fluorobenzene instead of toluene	69
8	BF3·Et2O (0.2 equiv.)	86
9	BF3·Et2O (0.1 equiv.)	76
10	1a : 2a = 1 : 1.5	78

^aStandard conditions: 1a (0.5 mmol), 2a (2.0 equiv), BF₃·Et₂O (1 equiv.), toluene (2.0 mL), 40 °C, 1 h.

^bIsolated yields.