Table 1.

Photocycloaddition reaction of phenanthrene‐9‐carboxaldehyde (1 a) and 2,3‐dimethyl‐2‐butene in the presence of chiral Lewis acids 3.

Entry[a]	λ [a] [nm]	Lewis acid[b]	Loading [mol %]	t [a] [h]	Yield[c] [%]	ee [d] [%]
1	424	3 a	50	14	53	56
2	435	3 a	50	9	43	70
3	457	3 a	50	11	49	79
4	457	3 b	50	11	54	84
5	457	3 c	50	11	62	97
6	457	3 c	25	19	79	94
7	457	3 c	10	26	81	94

[a] The reaction was performed at −78 °C with a substrate concentration of c=20 mm in CH₂Cl₂ at the indicated wavelength (λ) and for the indicated period of time (t). The olefin was used in excess (30 equiv). [b] 3 a: Ar=2,4,6‐trifluorophenyl; 3 b: Ar=2‐(trifluoromethyl)phenyl; 3 c: Ar=2,6‐dimethylphenyl. [c] Yield of isolated product. [d] The enantiomeric excess (ee) was determined by chiral‐phase HPLC analysis.