Table 1. Optimization of the reaction conditions. 1a (0.1 mmol), DBAD (0.15 mmol), CeCl₃·7H₂O (10 mol%), CH₃CN (0.1 M) at 25 °C, 455 nm LED for 24 h.

Entry	Deviation from standard conditions	3a a (%)
1	None	90 (80) b
2	CeCl3 instead of CeCl3·7H2O	80
3	Ce(OTf)3 instead of CeCl3·7H2O	66
4	Ce(OTf)4 instead of CeCl3·7H2O	63
5	K2CO3 instead of Cs2CO3	66
6	Na2CO3 instead of Cs2CO3	55
7	Li2CO3 instead of Cs2CO3	25
8	NaHCO3 instead of Cs2CO3	23
9	Without base	20
10	DCM instead of CH3CN	85
11	DCE instead of CH3CN	86
12	CHCl3 instead of CH3CN	66
13	DMSO instead of CH3CN	61
14	Without light	0
15	Without CeCl3·7H2O	0

^aNMR yields using benzoyl benzoate as internal standard.

^bIsolated yield.