Fig. 2. (a–d) Catalyst optimization. Reactions performed for 20 h in 0.3 mL of Et₂O using 0.2 mmol of 2a, 0.6 mmol of 1a, and 0.04 mmol of catalyst (20 mol%) under irradiation by a single high-power LED (λ_max = 460 nm, irradiance = 100 mW cm⁻²). Yield of 3a determined by ¹H NMR analysis of the crude mixture using trimethyl orthoformate as the internal standard. Enantiomeric ratio (e. r.) of 3a measured by chiral UPC² analysis after derivatization to the corresponding 2,4-dinitrophenyl hydrazone. Note that catalysts A–H gave product 3a with an (R) absolute configuration; the figure shows the (S) major enantiomer of 3a afforded by catalysts I–M. TMS: trimethylsilyl; TBS: tert-butyldimethylsilyl; TIPS: triisopropylsilyl; TDS: thexyldimethylsilyl.