Optimization of the reaction conditions^a.

Entry	Variation of the initial conditionsa	Yield of 3a b (%)	drc
1	None	63	>20 : 1
2	Bi(OTf)3 instead of Sc(OTf)3	40	>20 : 1
3	Cu(OTf)2 instead of Sc(OTf)3	60	2 : 1
4	TMSOTf instead of Sc(OTf)3	47	1 : 1
5	10 mol% Sc(OTf)3 instead of 5 mol%	62	>20 : 1
6	1.4 equiv of 2a instead of 1.2 equiv	57	>20 : 1
7	1.2 equiv of 1a	51	>20 : 1
8	0 °C instead of 25 °C	71	>20 : 1
9	0 °C to rt instead of 25 °C	71 (70)	>20 : 1
10d	DCE instead of CH2Cl2	60	>20 : 1
11d	THF instead of CH2Cl2	69	1 : 1

^aInitial conditions: 1a (0.10 mmol), 2a (0.12 mmol), Sc(OTf)₃ (5 mol%), CH₂Cl₂ (2.0 mL), 25 °C for 12 h.

^bThe ¹H NMR yield of the crude products was determined using CH₂Br₂ as the internal standard and the isolated yield was given in parenthesis.

^cdr value was determined from ¹H NMR of the crude reaction mixture.

^dThe reaction was performed at 0 °C to rt.