Page 67 - Catalysts for Fine Chemical Synthesis Vol 1 - Robert & Poignant

Catalysts for Fine Chemical Synthesis: Hydrolysis, Oxidation and Reduction. Volume 1
Edited by Stan M Roberts and Geraldine Poignant
Copyright  2002 John Wiley & Sons, Ltd.
ISBN: 0-471-98123-0

3 Asymmetric Epoxidation

CONTENTS
3.1 Introduction . . . . . . . . . . . . . . . . . . . . 51
References . . . . . . . . . . . . . . . . . . . . . . . 52

3.1 INTRODUCTION

The stereoselective oxidation of organic compounds is dominated by studies of
epoxidation (Figure 3.1). Epoxides are useful in organic synthesis, they are
versatile intermediates and easily undergo stereoselective ring-opening reac-
[1]
tions to form bifunctional compounds . This explains the development of
many methods for the synthesis of the enantiomerically pure epoxides. We
will describe in Chapters 4±6 different methods of asymmetric epoxidation of
functionalized and unfunctionalized alkenes. This chapter expands the infor-
mation given in Chapter 1 and forms a consolidated introductory section for
the next three chapters
In 1980 a useful level of asymmetric induction in the epoxidation of some
[2]
alkenes was reported by Katsuki and Sharpless . The combination of titanium
(IV) alkoxide, an enantiomerically pure tartrate ester and tert-butyl hydroper-
oxide was used to epoxidize a wide variety of allylic alcohols in good yield and
enantiomeric excess (usually >90 %). This reaction is now one of the most
[3]
widely applied reactions in asymmetric synthesis .
Concerning the oxidation of electron-deficient alkenes such as chalcone
derivatives, in 1980, Julia  et al. reported an example of a highly stereoselective
epoxidation of an electron deficient alkene using a triphasic catalysis system.
This method involves alkaline aqueous hydrogen peroxide, an organic solvent
and an insoluble polyamino acid. [4±7] A refined method now employs a biphasic
system consisting of an oxidant, a non-nucleophilic base, a polyamino acid and
[8]
an organic solvent .

Catalyst O O
H H R R R R
+ [O] or
R R
H H H H
Figure 3.1 Catalytic asymmetric epoxidation of alkenes.