Optimization of the reaction conditions^a.

Entry	Variation of the initial conditionsa	Yield of 3ab (%)
1	None	91
2	Sc(OTf)3 instead of Bi(OTf)3	28
3	Yb(OTf)3 instead of Bi(OTf)3	15
4	TfOH instead of Yb(OTf)3	<5
5	DCE instead of CH2Cl2	78
6	THF instead of CH2Cl2	39
7cc	MeCN instead of CH2Cl2	86
8	0 °C instead of 25 °C	84
9	1.5 equiv. of 2a	92
10	5 mol% Bi(OTf)3	83
11	0.06 M CH2Cl2	81

^aInitial conditions: 1a (0.20 mmol), 2a (0.24 mmol), Bi(OTf)₃ (10 mol%), CH₂Cl₂ (1.5 mL), 25 °C for 12 h.

^bGiven are the yield of chromatographically purified 3a.

^c3.0 : 1 diastereomeric ratio was obtained in this case and the dr value was determined from ¹H NMR of the crude reaction mixture.