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Carbohydrate-derived Auxiliaries

Carbohydrate-derived auxiliaries exhibit an efficient stereoselective potential in a number of nucleophilic addition reactions on prochiral imines. a-Amino acids, P amino acids and their derivatives can be synthesized in few synthetic steps, and with high enantiomeric purity. A variety of chiral heterocycles can readily be obtained from glycosyl imines by stereoselective transformations, providing evidence that carbohydrates have now been established as useful auxiliaries in stereoselective syntheses of various interesting classes of chiral compounds. [Pg.127]

Besides monoallylethers of chiral alcohols, allylglycosides bearing a free alcohol functionality have been applied in asymmetric syntheses of enantiomerically pure cyclopropanemethanols with success." The cleavage of the chiral carbohydrate-derived auxiliary was achieved by treatment of the cycloadducts with a. Tf20, pyridine b. DMF/H2O, pyridine, whereas for the compounds derived from 57 a. Tf20, BU4NI and b. nBuLi were applied. [Pg.4]

Carbohydrate-derived Auxiliaries. - Reaction of the fructose-derived acrylate 200 with cyclopentadiene in the presence of Et2AlCl, followed by disconnection of the auxiliary by reduction, gave the cycloadduct 201 in high ee. Use of the same auxiliary in an intramolecular Diels-Alder reaction catalysed by Et2AlCl gave, after reductive disconnection with LiAlH4, the bicycle 2027 It has... [Pg.357]

Charette has reported a highly effective carbohydrate-derived auxiliary for diastereoselective cyclopropanation reactions of olefins with high induction (Scheme 6.13) (see also Chapter 15, Section 15.4) [42]. Cyclopropane 69 produced by treatment of olefin 68 under Furukawa s conditions [43] could be smoothly liberated on treatment of the adduct with triflic anhydride under these conditions the derived 2-0-triflate intermediate undergoes ring contraction upon heating, releasing 70 in 98.4% ee, together with aldehyde 71 [42]. [Pg.196]

Scheme 18.14). Upon treatment of ketones or aldehydes with 62, condensation reactions lead to the formation of nitrones including 63 [71, 72]. Such nitrones were observed to participate in dipolar cycloadditions with olefin 64 and to furnish the corresponding isoxazolidines (cf. 65) in a diastereoselective manner. Facile removal of the chiral carbohydrate-derived auxiliary was effected upon exposure of the adduct to acidic hydrolysis, providing the chiral isoxazolidine 66 (> 90% ee). [Pg.597]

In general, chiral propanoates providing simple diastereoselectivity (in favor of yyn-aldols), combined with a reasonable degree of auxiliary-induced stereoselectivity, are rare. Numerous terpenoid- and carbohydrate-derived propionates do not display satisfactory syn selectivity60. Similarly, the titanium(IV) chloride promoted aldol addition of the following JV-metbylephe-drine derived silylketene acetal leads to the formation of the. mi-adduct in the moderate diastereomeric ratio of 78 22 (syn-adduct sum of the other stereoisomers)61. [Pg.474]

Hultin P. G., Earle M. A. and Sudharshan M. Synthetic Studies with Carbohydrate-Derived Chiral Auxiliaries Tetrahedron 1997 53 14823-14870 Keywords sugar-linked dienes, sugar-linked dienophiles... [Pg.312]

FIGURE 6. Carbohydrate derived chiral auxiliaries for the cyclopropanation of aUylic alcohols... [Pg.267]

Carbohydrates easily complex oxygenophilic electrophiles, e.g., lithium cations and zinc(II) chloride. The coordination between the carbohydrate moiety and the Lewis acids is an important factor which orients the auxiliary-bound substrate. The com-plexation of lithium cations through the carbohydrate moiety influences the reactivity and stereodifferentiation in carbohydrate-derived esters and their enolates [16]. [Pg.105]

A number of commonly used auxiliaries are available in both enantiomeric forms, for example the proline-derived auxiliaries RAMP and SAMP [18] giving rise to the complementary stereoselecting reactions. However, many natural carbohydrates are available in only one configuration. The other enantiomer (usually the L-form) is very expensive in most cases and difficult to prepare. [Pg.105]

Considerable work was done to induce chirality via chiral auxiliaries. Reac tions with aromatic a-ketoesters like phenylglyoxylates 21 and electron-rich al kenes like dioxoles 22 and furan 23 were particularly efficient (Scheme 6). Yield up to 99% and diastereoselectivities higher than 96% have been observed whet 8-phenylmenthol 21a or 2-r-butylcyclohexanol 21b were used as chiral auxiliarie [14-18]. It should be noted that only the exoisomers 24 and 25 were obtained from the reaction of dioxoles 22. Furthermore, the reaction with furan 23 wa regioselective. 24 were suitable intermediates in the synthesis of rare carbohydrate derivatives like branched chain sugars [16], Other heterocyclic compounds liki oxazole 28 [19] and imidazole 29 [20] derivatives as well as acyclic alkenes 3fl 31, and 32 [14,15,21,22] were used as olefinic partners. Numerous cyclohexane derived alcohols [18,21-24] and carbohydrate derivatives [25] were used as chiri... [Pg.184]

The examples outlined in this chapter show that carbohydrates are efficient stereodifferentiating auxiliaries, which offer possibilities for stereochemical discrimination in a wide variety of chemical reactions. Interesting chiral products are accessible, including chiral carbo- and heterocycles, a- and 3-amino acid derivatives, 3-lactams, branched carbonyl compounds and amines. Owing to the immense material published since the time of the earlier review articles on carbohydrates in asymmetric synthesis [9,10], the examples discussed in this chapter necessarily focused on the use of carbohydrates as auxiliaries covalently linked to and cleavable from the substrate. Given the scope of this chapter, a discussion of other interesting asymmetric reactions has not been permitted — for example, reactions in which carbohydrate-derived Lewis acids, such as cyclopentadienyl titanium carbohydrate complexes, exhibit stereocontrol in aldol reactions [180]. Similarly, processes in which in situ glycosylation induces reactivity and stereodifferentiation — for example, in Mannich reactions of imines [181] — have also been excluded from this discussion. [Pg.494]

Hultin, P G, Earle, M A, Sudharshan, M, Synthetic studies with carbohydrate-derived chiral auxiliaries. Tetrahedron, 53, 14823-14870, 1997. [Pg.495]

Multifunctional carbohydrate derivatives have been used widely as chiral synthons [240], and they have more recently found application as chiral auxiliaries [241], In this chapter, examples of carbohydrates in which an alcohol functional group is the point of attachment to the reagent wall be presented. Alkylations of ester etiolates of the D-allofiiranose derivative 1.47 or of the diacetoneghicose... [Pg.55]

L. L. Huang, M. H. Xu, and G. Q. Lin, A new entry to asymmetric synthesis of optically active a,y-substituted y-butyrolactones, using a carbohydrate derived amide as both a chiral auxiliary and a proton source, J. Org. Chem., 70 (2005) 529-532. [Pg.394]

Reactions of ketenes, e.g. dihaloketenes, with electron-rich alkenes bearing the chiral auxiliary, for example enolethers, carbohydrate-derived anomeric enolethers or ketene acetals, lead to cyclobutanones with moderate stereoselectivities. [Pg.17]

This year has again seen a considerable number of uses of carbohydrate derivatives as either removable auxiliaries or as chiral ligands/catalysts for asymmetric reactions. [Pg.389]

A final example of the use of carbohydrate-derived dienophiles in Diels-Alder reactions involves the role of the carbohydrate as a chiral auxiliary. Shing and coworkers have described the Diels-Alder reaction of arabinose-derived dienophile 31 with butadiene under Lewis acid catalyzed conditions (Scheme 5). A mixture of cycloadducts 32 with R and S configurations at the cyclohexane ring stereocenter was obtained in 68% yield. The ratio was 73 27 in favor of the / -product when a benzyl glycoside was used lower selectivities were observed with the corresponding methyl glycoside (77). [Pg.7]

A different facet of our studies of the Bradsher cycloaddition was the use of carbohydrate-derived chiral auxiliaries in order to obtain face selective cycloadditions with isoquinoline salts.Our results are illustrated in Scheme 5. It is interesting to note that the a-anomer in our series gives very useful face selectivity. However, the rather... [Pg.29]


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Carbohydrate derivatives

Carbohydrates chiral auxiliaries derived from

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