Table 1. Screen of Chiral Lewis Acids for Enantioselective [2 + 2] Photocycloaddition.

entry	Lewis acid	ligand	yielda (%)	ee (%)
1	Gd(OTf)3		32
2	Gd(OTf)3	18	0
3	Gd(OTf)3	19	27	0
4	Gd(OTf)3	20	32	–6
5	Gd(OTf)3	21	56	21
6	Gd(OTf)3	22	43	56
7	Eu(OTf)3	22	41	78
8b	Eu(OTf)3	22	74	85
9b,c	Eu(OTf)3	22	71	92

^aYield determined by ¹H NMR using an internal standard.

^bUsing 5 equiv of 16, 5 mol % Ru(bpy)₃Cl₂, 10 mol % Eu(OTf)₃, and 20 mol % ligand 22.

^cReaction conducted at −20 °C.