Metal alkyl peroxides can be used for the epoxidation of electron-deficient alkenes such as enones.12 The use of a combination of diethylzinc, oxygen, and N-methylephedrine gave epoxides in very high yield and generally high enantio-selectivity (Figure 11.8).13
R'OZnOOEt + EtH Reagent w
Figure 11.8 Epoxidation of enones by metal hydroperoxides. 11.4 DIHYDROXYLATION OF ALKENES
Asymmetric dihydroxylation can be achieved using osmium tetroxide in conjunction with a chiral nitrogen ligand.14 The very successful Sharpless procedure uses the natural cinchona alkaloids dihydroquinine (DHQ) and its diastereomer dihy-droquinidine (DHQD), as exemplified in the epoxidation of trans-stilbene
Other catalysts a,»NMe2
Stoichiometric, but can be made catalytic with NMO
Figure 11.9 Dihydroxylation of alkenes.
(Figure 11.9).15'16 Numerous synthetic chiral diamines have also been used, and some examples are shown.17-19
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