Stereoselective aldol reactions in water

One of the factors hindering the use of boron enolate chemistry on large scale for the synthesis of aldols is the stoichiometric amount of dialkylboron triflates required. The group of Kobayashi in Tokyo have now reported that a stereoselective aldol reaction can be carried out in water with a catalytic amount of boron reagent (Mori Y et al, Angew Chem Int Ed 2001, 40, 2816). The following scheme shows the conditions used. The yields are in the range 61-92% for a number of aldehydes and silylenol ethers. Yields are better than the Lewis acid surfactant combined catalyst (LASCs) reported recently (Manabe K et al, J Am Chem Soc 2000, 122, 7202).