Table 1 Optimization of the reaction conditions^a

^aUnless noted otherwise, the reactions were performed with 1a (0.05 mmol, 1.0 equiv.), cinnamaldehyde 2a (0.09 mmol, 1.8 equiv.), preNHC (20 mol%), DQ (125 mol%), AcOH (30 mol%), additive (50 mg) and base (150 mol%) in solvent (1 mL) under N₂ atmosphere at 30 °C for 48 h.
^bThe dr (the ratio of 3a to 3a’) was determined by ¹HNMR analysis with 1,1,2,2-tetrachloroethane as the internal standard.
^cThe yield of major diastereomer 3a was determined by ¹H NMR using 1,1,2,2-tetrachloroethane as the internal standard.
^dDetermined by chiral HPLC analysis.
^eIsolated yield of 3a with respect to 1a. Mes = 2,4,6-trimethylphenyl.

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