Baker's yeast: production of d- and l-3-hydroxy esters

Baker's yeast grown under oxygen limited conditions and used in the reduction of 3-oxo esters results in a shift of the stereoselectivity of the yeast towards -hydroxy esters as compared with ordinary baker's yeast. The highest degree of stereoselectivity was obtained with growing yeast or yeast harvested while growing. In contrast, the stereoselectivity was shifted towards -hydroxy esters when the oxo esters were added slowly to ordinary baker's yeast supplied with gluconolactone as co-substrate. The reduction rate with gluconolactone was increased by active aeration. Ethyl -(S)-3-hydroxybutanoate was afforded in>99% ee. Both enantiomers of ethyl 3-hydroxypentanoate, -(R) in 96% ee and -(S) in 93% ee, and of ethyl 4-chloro-3-hydroxybutanoate, -(S) in 98% ee and -(R) in 94% ee, were obtained. The results demonstrate that the stereoselectivity of baker's yeast can be controlled to a large extent without the use of inhibitors, heat treatment, etc.