Direct Aldol Reactions by Asymmetric Organocatalyst in Water

trans-4-(tert-Butyldiphenylsilyloxy)-L-proline (1) is an organic catalyst useful for direct chiral aldol reactions in water. The reaction of aldehyde and ketone in the presence of 10 mol% of substance 1 in water proceeds without the use of organic solvent so as to give the target aldol adduct 2 with high-diastereo and high-enantio-selectivity. It has been confirmed that there is no problem with the yield or the selectivity if the amount of catalyst decreases to 1 mol%. Consequently, it can be also applied to the mass-synthesis.

Typical Procedure: for the 2a^1b)
Catalyst 1 (14.8 mg, 0.04 mmol) is added to a mixture of benzaldehyde (0.041 mL, 0.4 mmol) and cyclohexanone (0.207 mL, 2.0 mmol) in water (0.13 mL) at room temperature. The reaction mixture is stirred for 18 h at this temperature, then the reaction is quenched by the addition of phosphate buffer (pH 7.0). The organic materials are extracted with three portions of ethyl acetate, and the combined organic phases are dried over anhydrous Na₂SO₄, filtered, and concentrated in vacuo. Purificaton by column chromatography on silica gel gives 2a (63.7 mg, Y. 78％, >99％e.e.).