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Chiral non-racemic

With this epoxidation procedure it is possible to convert the achiral starting material—i.e. the allylic alcohol—with the aim of a chiral reagent, into a chiral, non-racemic product in many cases an enantiomerically highly-enriched product is obtained. The desired enantiomer of the product epoxy alcohol can be obtained by using either the (-1-)- or (-)- enantiomer of diethyl tartrate as chiral auxiliary ... [Pg.254]

Sharpless and Klunder22 are developing a new procedure for conversion of sulfonyl chlorides directly into menthyl sulflnate esters using trimethyl phosphite as a reducing agent (equation 2). This method, starting with sulfonyl chlorides rather than with the much less available sulfinyl chlorides, should allow access to an even wider range of sulfinate esters and, ultimately, to various chiral, non-racemic sulfoxides. [Pg.825]

Modena and colleagues47 have developed use of some chiral, non-racemic terpene alcohols as directing groups for highly diastereoselective m-chloroperbenzoic oxidation of sulfides into sulfoxides. Specifically the isobornyl vinylic sulfides 8 undergo hydroxyl-directed oxidation to give a 9 1 ratio of diastereomeric sulfoxides (equation 11). [Pg.828]

Diastereoselective and enantioselective [3C+2S] carbocyclisations have been recently developed by Barluenga et al. by the reaction of tungsten alkenylcarbene complexes and enamines derived from chiral amines. Interestingly, the regio-chemistry of the final products is different for enamines derived from aldehydes and those derived from ketones. The use of chiral non-racemic enamines allows the asymmetric synthesis of substituted cyclopentenone derivatives [77] (Scheme 30). [Pg.82]

A chiral version of this [4+3] heterocyclisation was achieved using chiral, non-racemic carbene complexes derived from menthol and oximes as depicted... [Pg.103]

Chiral phosphoryl and sulfinyl groups are known as efficient auxiliaries in asymmetric synthesis. As reported below, their asymmetric induction in the a-posi-tion has been used to prepare chiral non-racemic organophosphorus compounds a-substituted by a sulfur function. Such compounds can also be obtained from their a-hydroxy analogues by OH-4 SR stereoselective transformation. [Pg.182]

Interestingly, both invertomers of the obtained M-chloroaziridines 16 were clearly observable in the H-NMR spectrum and they could even be separated by chromatography. The dehydrochlorination was investigated with a variety of bases however, the resulting yields were disappointingly low. Only for R = Ph, a yield of 39% of azirine 17 was obtained using DBU as the base, in all other cases the yields were lower [22]. Davis et al. [23] successfully applied the -elimination of the sulfinyl group in chiral non-racemic N-sulfinylaziridines (Scheme 9), whereby the eliminated sulfenate was trapped by an excess of methyl iodide, which facilitated the isolation of the desired product (18). [Pg.100]

Enzyme-mediated syntheses of chiral non-racemic hetero-organic compounds, with a stereogenic centre located either on a heteroatom or on a carbon atom in a side chain, are comprehensively presented. Particular attention is paid to the use of common hydrolytic and reducing enzymes. On the basis of the results presented, some conclusions are drawn and proposals presented, which concern possible future direchons in the applications of enzymes to the synthesis and transformations of chiral hetero-organic derivatives. [Pg.159]

Another approach to the synthesis of chiral non-racemic hydroxyalkyl sulfones used enzyme-catalysed kinetic resolution of racemic substrates. In the first attempt. Porcine pancreas lipase was applied to acylate racemic (3, y and 8-hydroxyalkyl sulfones using trichloroethyl butyrate. Although both enantiomers of the products could be obtained, their enantiomeric excesses were only low to moderate. Recently, we have found that a stereoselective acetylation of racemic p-hydroxyalkyl sulfones can be successfully carried out using several lipases, among which CAL-B and lipase PS (AMANO) proved most efficient. Moreover, application of a dynamic kinetic resolution procedure, in which lipase-promoted kinetic resolution was combined with a concomitant ruthenium-catalysed racem-ization of the substrates, gave the corresponding p-acetoxyalkyl sulfones 8 in yields... [Pg.163]

However, the most common and important method of synthesis of chiral non-racemic hydroxy phosphoryl compounds has been the resolution of racemic substrates via a hydrolytic enzyme-promoted acylation of the hydroxy group or hydrolysis of the 0-acyl derivatives, both carried out under kinetic resolution conditions. The first attempts date from the early 1990s and have since been followed by a number of papers describing the use of a variety of enzymes and various types of organophosphorus substrates, differing both by the substituents at phosphorus and by the kind of hydroxy (acetoxy)-containing side chain. [Pg.173]

At the beginning of investigations on chiral dendrimers in our own group was the question of how to synthesize chiral, non-racemic derivatives of tris(hydroxymethyl)-methane [82], which we wanted to use as dendrimer center pieces. We have developed efficient diastereoselective syntheses of such triols [83-85] from ( R)-3-hydroxybutanoic acid, readily available from the biopolymer PHB [59,60] (cf. Sect. 2.4). To this end, the acid is converted to the dioxanone 52 [86, 87], from which various alkylation products and different aldol adducts of type 53 were obtained selectively, via the enolate (Fig. 20). These compounds have been reduced to give a variety of enantiopure chiral building blocks for dendrimers, such as the core unit 54, triply branching units 55a and 55b or doubly branching unit 56 [1,88]. [Pg.157]

The design and application of chiral, non-racemic Lewis acids for the asymmetric Diels-Alder reaction has recently been a subject of considerable interest.9 Several methods have been developed in many laboratories1 2 3 4 5 6 7 10 but catalysts are still needed that are more efficient in governing the stereochemical course of the cycloaddition reaction. [Pg.19]

Abstract Phase transfer catalysts including onium salts or crown ethers transfer between heterogeneous different phases and catalytically mediate desired reactions. Chiral non-racemic phase transfer catalysts are useful for reactions producing new stereogenic centers, giving chiral non-racemic products. Recent developments in this rapid expanding area will be presented. [Pg.123]

In contrast the progress of asymmetric synthesis by use of chiral non-racemic phase transfer catalysts had been slow compared to the ordinary phase transfer catalysis. However, recent achievements in this particular area are noteworthy and efficient asymmetric phase transfer catalysis has been increasingly explored.17 101... [Pg.124]

Cinchona alkaloids now occupy the central position in designing the chiral non-racemic phase transfer catalysts because they have various functional groups easily derivatized and are commercially available with cheap price. The quaternary ammonium salts derived from cinchona alkaloids as well as some other phase transfer catalysts are... [Pg.125]

Figure 1. Representative chiral non-racemic phase transfer catalysts. Figure 1. Representative chiral non-racemic phase transfer catalysts.
The phosphonium salt 21 having a multiple hydrogen-bonding site which would interact with the substrate anion was applied to the phase transfer catalyzed asymmetric benzylation of the p-keto ester 20,[18 191 giving the benzylated P-keto ester 22 in 44% yield with 50% ee, shown in Scheme 7 Although the chemical yield and enantiomeric excess remain to be improved, the method will suggest a new approach to the design of chiral non-racemic phase transfer catalysts. [Pg.126]

The first promising asymmetric aldol reactions through phase transfer mode will be the coupling of silyl enol ethers with aldehydes utilizing chiral non-racemic quaternary ammonium fluorides,1371 a chiral version of tetra-n-butylammonium fluoride (TBAF). Various ammonium and phosphonium catalysts were tried138391 in the reaction of the silyl enol ether 41 of 2-methyl-l-tetralone with benzaldehyde, and the best result was obtained by use of the ammonium fluoride 7 (R=H, X=F) derived from cinchonine,1371 as shown in Scheme 14. [Pg.132]

Numbers of asymmetric phase transfer catalysis can now be accomplished efficiently to give a variety of chiral non-racemic products with high enantiomeric excesses. Thus, asymmetric phase transfer catalysis has grown up into practical level in numbers of reactions and some optically pure compounds can be effectively produced on large scale by use of chiral phase transfer catalysts. [Pg.140]

Numbers of reactions have been developed by utilizing the strong affinity of the fluoride anion to the silicon atom. In this context, the use of chiral non-racemic ammonium fluorides137 41 43 751 for asymmetric silyl mediated reactions will be further investigated in future. [Pg.140]

This article provides a brief overview of several recent total syntheses of natural and unnatural products that have benefited from the use of catalytic asymmetric processes. The article is divided by the type of bond formation that the catalytic enan-tioselective reaction accomplishes (e.g C-C or C-0 bond formation). Emphasis is made on instances where a catalytic asymmetric reaction is utilized at a critical step (or steps) within a total synthesis however, cases where catalytic enantioselective transformations are used to prepare the requisite chiral non-racemic starting materials are also discussed. At the close of the article, two recent total syntheses are examined, where asymmetric catalytic reactions along with a number of other catalyzed processes are the significant driving force behind the successful completion of these efforts (Catalysis-Based Total Syntheses). [Pg.146]

In a synthesis of furanomycin, this reaction was a stereospecific key step for the construction of the dihydrofuran ring 77 from the chiral non-racemic precursor 76 (Scheme 15.18) [41]. [Pg.887]

The one-electron oxidation of iV-benzylphenothiazine by nitric acid occurs in the presence of /i-cyclodextrin, which stabilizes the radical cation by incorporation into its cavity. The reaction is inhibited by adamantane, which preferentially occupies the cavity. Novel Pummerer-type rearrangements of / -sulfinylphenyl derivatives, yielding /7-quinones and protected dihydroquinones, and highly enantioselective Pummerer-type rearrangements of chiral, non-racemic sulfoxides have been reviewed. A comprehensive study has demonstrated that the redox potential for 7- and 8-substituted flavins is linearly correlated with Hammett a values. DFT calculations in [3.3.n]pro-pellanes highlight low ionization potentials that favour SET oxidative cleavage of the strained central C-C bond rather than direct C-H or C-C bond attack. Oxidations and reductions in water have been reviewed. ... [Pg.245]

A kinetic study of the 1,3-dipolar cycloadditions of alkynyl Fischer carbene complexes with nitrones showed tirst-order kinetics for both nitrones and the alkynyl carbene complexes. The 1,3-dipolar cycloaddition of chiral non-racemic Fischer... [Pg.463]

Two classes of a-hydroxylated lignans have been enantioselectively prepared a) wikstromol (3) [10, 38] and related natural products [39] and b) gomisin A (1) and congeners [40, 41]. In both cases, chiral, non-racemic ita-conic acid derivatives have been synthesized as key compounds for the preparation of -benzyl-y-butyrolactones (either by resolution (g [32]) or by asymmetric hydrogenation (h [25])). [Pg.193]

The total asymmetric syntheses of natural (lS,2S)-norcoronamic 72a and of (lS,2S)-coronamic acid 73 have been obtained from the diastereoselective cyclization of chiral non-racemic 2-(Ar-benzylideneamino)-4-chlorobutyronitriles [98] but one of the shortest syntheses of these attractive amino acids was based on the diastereoselective palladium(0)-catalyzed alkylation and S cyclization of l,4-dichlorobut-2-ene by the anion of 2-aminoacetonitrile derivatives [99]. On the other hand, diastereoselective palladium(0)-catalyzed azidation of chiral non-racemic 1-alkenylcyclopropyl esters provide non-natural (lk,2S)-norcoro-namic acid, enantiomerically pure [100]. [Pg.17]

Microbial reduction of prochiral cyclopentane- and cyclohexane-1,3-diones was extensively studied during the 1960 s in connection with steroid total synthesis. Kieslich, Djerassi, and their coworkers reported the reduction of 2,2-dimethylcyclohexane-l,3-dione with Kloeokera magna ATCC 20109, and obtained (S)-3-hydroxy-2,2-dimethylcyclohexanone. We found that the reduction of the 1,3-diketone can also be effected with conventional baker s yeast, and secured the hydroxy ketone of 98-99% ee as determined by an HPLC analysis of the corresponding (S)-a-methoxy-a-trifluoromethylphenylacetate (MTPA ester).(S)-3-Hydroxy-2,2-dimethy1cyc1ohexanone has been proved to be a versatile chiral non-racemic building block in terpene synthesis as shown in Figure 1. [Pg.31]

Chiral non-racemic 0-(2-ketoalkyl) A-phenylhydroxylamines such as 115 (equation 84) can be prepared through catalytic enantioselective a-aminoxylation of carbonyl compounds catalyzed by proline. This reaction proceeds with a variety of ketones and aldehydes although it has been tried only with a nitrosobenzene component ... [Pg.144]

The synthesis of chiral, non-racemic a-amino acids remains an interesting field of investigation and the synthesis of fully protected a,a-disubstituted a-amino acids 331 via the Beckmann rearrangement of tosylated oximes was achieved (equation 123). As expected, the migrating group was able to retain the original stereochemistry and good yields and excellent enantioselectivities were observed ... [Pg.424]


See other pages where Chiral non-racemic is mentioned: [Pg.186]    [Pg.59]    [Pg.159]    [Pg.160]    [Pg.191]    [Pg.220]    [Pg.192]    [Pg.123]    [Pg.123]    [Pg.125]    [Pg.125]    [Pg.1048]    [Pg.1069]    [Pg.171]    [Pg.525]    [Pg.131]    [Pg.256]    [Pg.767]    [Pg.125]   


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Chiral racemization

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