Table 1. Optimization of the reaction conditions ^a .

Entry	Variation from standard conditions	Yield b (%)	ee c (%)
1	0.5 mol% DPZ in CH2Cl2 at 25 °C and without C1 and any additives, 12 h	62	N.A.
2	None	78	93
3	C2 instead of C1	74	76
4	C3 instead of C1	73	67
5	Ru(bpy)3Cl2·6H2O instead of DPZ	58	89
6	Rose Bengal instead of DPZ	69	91
7	No TBPB	76	89
8	No Na2S2O4	75	90
9	No 5 Å MS	76	90
10	No DPZ	35	88
11	No light	0 d	N.A.
12	Under air	0 e	N.A.
13	No C1	53	N.A.

^aReaction conditions: 1a (0.075 mmol), 2a (0.05 mmol), degassed CPME (1.0 mL), 10 °C, irradiation with blue LED (3 W, 450 nm), 36 h.

^bYield of the isolated product.

^cDetermined by HPLC analysis on a chiral stationary phase.

^dNo reaction was detected.

^e 1a was consumed, but no 3a was obtained. TBPB = tetra-n-butylphosphonium bromide. CPME = cyclopentyl methyl ether. N.A. = not available.