Asymmetric reductions using the chiral boronic ester TarB–H: a practical and inexpensive procedure for synthesizing chiral alcohols

Chiral alcohols are prepared in high enantiomeric excesses using an inexpensive and easily synthesized tartaric acid derived boronic ester (TarB–H) with sodium borohydride. The phenylboronic acid could be recovered quantitatively using a simple extraction with sodium hydroxide and diethyl ether. The optimized TarB–H system was used to reduce aromatic and aliphatic ketones in an open flask to chiral alcohols with enantiomeric excesses up to 99%.