Epoxidations Katsuki-Sharpless

The past thirty years have witnessed great advances in the selective synthesis of epoxides, and numerous regio-, chemo-, enantio-, and diastereoselective methods have been developed. Discovered in 1980, the Katsuki-Sharpless catalytic asymmetric epoxidation of allylic alcohols, in which a catalyst for the first time demonstrated both high selectivity and substrate promiscuity, was the first practical entry into the world of chiral 2,3-epoxy alcohols [10, 11]. Asymmetric catalysis of the epoxidation of unfunctionalized olefins through the use of Jacobsen s chiral [(sale-i i) Mi iln] [12] or Shi s chiral ketones [13] as oxidants is also well established. Catalytic asymmetric epoxidations have been comprehensively reviewed [14, 15]. 

The requirement for the presence of an adjacent alcohol group can be regarded as quite a severe limitation to the substrate range undergoing asymmetric epoxidation using the Katsuki-Sharpless method. To overcome this limitation new chiral metal complexes have been discovered which catalyse the epoxidation of nonfunctionalized alkenes. The work of Katsuki and Jacobsen in this area has been extremely important. Their development of chiral manganese (Ill)-salen complexes for asymmetric epoxidation of unfunctionalized olefins has been reviewed1881. 

An important breakthrough in asymmetric epoxidation has been the Katsuki-Sharpless invention [1], The reaction uses a chiral Ti(IV) catalyst, t-butylhydroperoxide as the oxidant and it works only for allylic alcohols as the substrate. In the first report titanium is applied in a stoichiometric amount. The chirality is introduced in the catalyst by reacting titanium tetra-isopropoxide... 

In this chapter, the ligand design for the catalytic enantioselective oxidations developed after the Katsuki-Sharpless epoxidation and the Sharpless AD will be discussed. 

Much attention has been paid to asymmetric amplification where the enantiomeric excess ( ) of the product is higher than that of the chiral catalyst (equation 35)136. The first experiment on asymmetric amplification was reported by Kagan and coworkers in the Katsuki-Sharpless asymmetric epoxidation of allyl alcohols137. Asymmetric amplification has also been studied in the asymmetric addition of dialkylzincs to carbonyl compounds. 

Katsuki-Sharpless asymmetric epoxidation, in situ O-silylation... 

T. Katsuki, V S. Martin, Asymmetric Epoxidation of Allylic Alcohols The Katsuki-Sharpless Epoxidation Reaction, Org. React. 1996, 48, 1-299. 

Before leaving the area of oxene chemistry, there is one further system worthy of mention the manganese Schiff-base complexes. The Schiff-base complexes were prepared in response to the Katsuki-Sharpless system for stereospecific epoxidation (Figure 2.19).57 The Katsuki-Sharpless system consists of titanium(IV) isopropoxide and ( + )- or (—)-diethyl tartrate with... 

Some stereospecific epoxidations such as the Katsuki-Sharpless system only function with alkyl hydroperoxides. 

Asymmetric Epoxidation of Allylic Alcohols The Katsuki-Sharpless Epoxidation Reaction... 

Both chemical and enzymatic synthetic methods for the asymmetric oxidation of the carbon-carbon double bond have been developed [46], but the area of carbon-carbon double bond oxidations has been shaped by the breakthrough discovery of asymmetric epoxidation of allylic alcohols with the Katsuki-Sharpless method [47]. Catalytic asymmetric synthesis of epoxides from alkenes by Jacobsen... 

As far as the epoxidation of enantiomerically pure acyclic allylic alcohols is concerned, the Katsuki-Sharpless enantioselective epoxidation process can be applied (see Section 4.5.2.4.1. and Houben-Weyl, Vol. E13/2, p 1219, and Table 148, pp 1226-1230). If matched substrate/ catalyst combinations are employed, many, otherwise unselective, epoxidations may be rendered highly diastereoselective. 

Katsuki T, Martin VS. Asymmetric epoxidation of allylic alcohols the Katsuki-Sharpless epoxidation reaction. Org. Reactions 1996 48 1. 

After reduction of the enal with diisobutylaluminium hydride, the Wittig olefination of D-glycer-aldehyde acetonide (7 )-24 with Ph3P=CHCHO gives the ( )-allylic alcohol 129. The Katsuki-Sharpless enantioselective epoxidation [89] applied to 129 allows the preparation of D-arabinitol (= D-lyxitol) and ribitol, a meso alditol (Scheme 13.47). Similarly, Wittig olefination of R)-2A with Ph3P=CHCH(OEt)2, followed by acidic hydrolysis of the diethyl acetal and subsequent reduction of the enal with diisobutylaluminium hydride, provides the (Z)-allylic alcohol 130. Diastereoselective epoxidation and hydrolysis leads to D-arabinitol or xylitol, another meso alditol [90a]. 

SCHEME 13.47 Wittig olefination and Katsuki-Sharpless asymmetric epoxidation applied in the conversion of D-glyceraldehyde into pentitols. 

AZT (3 -azido-3 -deoxythymidine) and other modified nucleosides were obtained by Jung and coworkers [216a] starting from crotonaldehyde (Scheme 13.111). The chirality is introduced via Katsuki-Sharpless epoxidation. Enolization of crotonaldehyde with TMSCl and EtaN gives a... 

By applying a Katsuki-Sharpless asymmetric epoxidation, Schreiber and coworkers [218] obtained (+)-KDO from the diallyl alcohol 492 (Scheme 13.113). The method generates... 

The Katsuki-Sharpless asymmetric epoxidation of racemic diol ( )-496 (obtained by allylation of ( )-crotonaldehyde) gives, after chromatographic separation, the erythro-epoxidc (+)-497 (33% yield, >95% ee). Its urethane undergoes assisted epoxide ring opening under acidic conditions, providing a 10 1 mixture of araZ mg-carbonate (+)-498 and the nZ 6>-stereomer. Carbonate hydrolysis and subsequent ozonolysis generates D-olivose (Scheme 13.114). Asymmetric epoxidation of the kinetically resolved dienol (—)-496 (72%) leads to (—)-497... 

A simple, divergent, asymmetric synthesis of the four stereoisomers of the 3-amino-2,3,6-trideoxy-L-hexose family was proposed by Dai and coworkers [222], which is based on the Katsuki-Sharpless asymmetric epoxidation of allylic alcohols (Scheme 13.115). Recently, A-trifluoroacetyl-L-daunosamine, A-trifluoroacetyl-L-acosamine, A-benzoyl-D-acosamine and A-benzoyl-D-nistosamine were derived from methyl sorbate via the methyl 4,5-epoxy-( -hex-2-enoates obtained via a chemoenzymatic method [223]. 

Application of the Katsuki-Sharpless enantioselective epoxidation to racemic mono-0-benzylated divinylglycol allowed the preparation of enantiomerically pure L-lyxo and D-lyxo-pentoses and analogs [224,225]. 

The Katsuki-Sharpless asymmetric epoxidation of ( )-allylic alcohols is the key-step in the total synthesis of all tetroses and hexoses developed by Sharpless and Masamune [259,260] and that are summarized in O Scheme 57 for the L-series. The epoxide obtained by oxidation of... 

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