Optically active epoxide preparation

Is either of the epoxides formed in the preceding reactions chiral Is either epoxide optically active when prepared from the alkene by this method ... 

Allylic alcohols can be converted to epoxy-alcohols with tert-butylhydroperoxide on molecular sieves, or with peroxy acids. Epoxidation of allylic alcohols can also be done with high enantioselectivity. In the Sharpless asymmetric epoxidation,allylic alcohols are converted to optically active epoxides in better than 90% ee, by treatment with r-BuOOH, titanium tetraisopropoxide and optically active diethyl tartrate. The Ti(OCHMe2)4 and diethyl tartrate can be present in catalytic amounts (15-lOmol %) if molecular sieves are present. Polymer-supported catalysts have also been reported. Since both (-t-) and ( —) diethyl tartrate are readily available, and the reaction is stereospecific, either enantiomer of the product can be prepared. The method has been successful for a wide range of primary allylic alcohols, where the double bond is mono-, di-, tri-, and tetrasubstituted. This procedure, in which an optically active catalyst is used to induce asymmetry, has proved to be one of the most important methods of asymmetric synthesis, and has been used to prepare a large number of optically active natural products and other compounds. The mechanism of the Sharpless epoxidation is believed to involve attack on the substrate by a compound formed from the titanium alkoxide and the diethyl tartrate to produce a complex that also contains the substrate and the r-BuOOH. ... 

Table 10 Result of one-pot preparation method of optically active epoxides (67a-d) by a combination of epoxidation of cyclohexenone and enantiomer resolution in a water suspension medium... 

Polyamino acids are easy to prepare by nucleophUe-initiated polymerisation of amino acid JV-carboxyanhydrides. Polymers such as poly-(L)-leucine act as robust catalysts for the epoxi-dation of a wide range of electron-poor alkenes, such as y-substituted a,Ji-unsaturated ketones. The optically active epoxides so formed may be transformed into heterocyclic compounds, polyhydroxylated materials and biologically active compounds such as dUtiazem and taxol side chain. 

CMral tpoxyiutphthoquinones. Pluim and Wynberg1 have prepared a number of Optically active epoxides of 2-alkyl- and 2,3-dialkyl-l,4-naphthoquinones by Oxidation with 30% H202, aqueous NaOH, and benzylquininium chloride. Enantiomeric excesses of 43% can be realized, and these can be improved by Oiyitallization. The authors also report that the most satisfactory method for preparation of 2-alkyl-1,4-naphthoquinones is that of Jacobsen (5, 17 8, 18). 

A new C3-symmetric chiral phase-transfer catalyst that offers multipoint inteaction with a nucleophile has been described (Scheme 7.6) [23]. Thus, various quaternary ammonium salts were prepared through the ring opening of optically active epoxides, followed by quaternization of the resulting amines. Asymmetric benzylation of Schiff s base 20 in the presence of catalyst 24—26 yielded (S)-21 with moderate enantioselectivity. As expected, the C3-symmetric catalyst R,R,R)-26a provided... 

Nucleophilic addition of sulfur ylides to C=0 double bonds is an important means of synthesis of epoxides [198], Because optically active epoxides are widely applied as versatile intermediates in the preparation of, e.g., pharmaceuticals, the asymmetric design of this sulfur ylide-based reaction has attracted much interest [199, 200, 212, 213], One aspect of this asymmetric organocatalytic process which has been realized by several groups is shown in Scheme 6.87A. In the first step a chiral sulfur ylide of type 204 is formed in a nucleophilic substitution reaction starting from a halogenated alkane, a base, and a chiral sulfide of type 203 as organocata-... 

The one-pot method is also applicable to the preparation of optically active epoxides and sulfoxides.25... 

Reactions 1-3 produce intermediates which can be hydrolyzed to give dendrimers with anionic surfaces. Intermediates derived from reaction 11 yield cationic surfaces after quaternization. Dendrimers possessing chiral surfaces were readily prepared from optically active epoxides, as in reaction 4. Hydrophobic surfaces result from reactions 4-10, 11, and 20-22. The resulting dendrimers are soluble in organic solvents, although their interiors might be quite hydrophilic. Conversely, water-soluble dendrimers are obtained from reactions 1-3 (after hydrolysis), 4 (R = H), 14, and 16-19. 

The first group93) chose an approach by which the introduction of the upper side chain was achieved by reaction of the optically active epoxide 165 with the sodium derivative of diethylmalonate to the mixture of the isomers 166 and 167. The desired isomer 166 was isolated in 20% yield by chromatography on silica gel. The stereocontrolled opening of the epoxide which had been prepared out of 163 via the mesylate 164 was the prerequisite for the correct configuration of the prostanoid side chains in compound 171. 

Table 5 lists several reactions in which the Grignard reagent is added to a system that allows ring-opening or displacement, rather than the typical addition reactions described previously. Umezawa [42] patented an approach to make long-chain alcohols from optically active epoxides (see Table 5, entry 1). In this example, they prepared optically pure 4-decyl alcohol in quantitative yield. The product was formed by the addition of a Grignard reagent to the epoxide a catalyst, copper(I) iodide, was also used. 

Using optically active epoxides, chiral N-alkylated pyrazole and imidazole derivative ligands have been prepared by high pressure reactions with 103 or 104 their catalytic efficiency as chiral ligands in the enantioselective addition of diethylzinc to benzaldehyde has been tested. 

The synthesis of 132, starting from S-benzyloxy propanal (131), involved the ring opening of an optically active epoxide 133 with a xanthate anion (Scheme 37)J22 Stereoselective synthesis of 133 by Sharpless epoxidation allowed preparation of the 2-deoxy-4-thio-D- and L-ezyr/zro-pentoses, " which were transformed into the corresponding pyrimidine nucleosides with silylated uracil and McaSiOTf. and then deprotected with Bu NF. 

In addition to the preparation of optically active epoxide intermediates, the titanium/tartrate-catalyzed asymmetric epoxidation can be used to resolve racemic... 

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