Optimization of Lewis acids for the sequential reaction^a.

Entry	Metal salt	Yieldbb (%)	3aa : 4aac	ee of 4aac (%)
1	Ni(OTf)2	76	82 : 18	92
2	Zn(OTf)2	54	6 : 94	91
3	Mg(OTf)2	52	34 : 66	91
4	Cu(OTf)2	49	1 : 99	93
5dd	Cu(OTf)2	70	1 : 99	93
6ee	Cu(OTf)2	70	1 : 99	93

^aUnless otherwise noted, all reactions were initially carried out with 1a (0.05 mmol), 2a (1.2 equiv.), L5-RaPr2 (10 mol%) and Ni(OTf)₂ (10 mol%) in CH₂Cl₂ (0.2 M) at 35 °C for 16 h. Then, the metal salt (5 mol%) in CH₂Cl₂ (0.1 M) was added and stirred at 30 °C for 16 h under air.

^bIsolated yield of 4aa based on 1a.

^cDetermined by HPLC on a chiral stationary phase.

^d4 Å MS (10 mg) in CHCl₃ (0.05 M) was used for the second transformation for 3 h.

^eIsolated 3aa was treated with Cu(OTf)₂ (5 mol%) and 4 Å MS (10 mg) in CHCl₃ (0.05 M) at 30 °C for 4 h under N₂.