2 Homometallic Alkoxides

676 Alkoxo and Aryloxo Derivatives of Metals
tetraisopropoxide and (C)-diethyl tartrate was a highly efficient enantioselective reagent
for the epoxidation of allylic alcohols using tert-butyl hydroperoxide as the oxygenating
reagent. A wide range of allylic alcohols was converted to the corresponding epoxyalcohol
with enantiomeric excess values (ee) of 90–95%. Subsequent work by Sharpless
and co-workers led to versatile routes to optically pure natural products and drugs. 77
Using racemic secondary allylic alcohols it was shown that one enantiomer reacted
much faster (100ð) than the other giving rise to kinetic resolution by enantioselective
epoxidation. 78 Further research showed that a molar ratio of dialkyltartrate:Ti⊲OPr i ⊳4
of 1.2–1.5 gave optimum results and with the addition of molecular sieves the process
became catalytic with regard to titanium complex (