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Carbohydrates stereoselectivity

W. R. Roush and R. J. Brown, Total synthesis of carbohydrates Stereoselective syntheses of 2, 6-dideoxy-D-arabino-hexose and 2,6-dideoxy-D-ribo-hexose, J. Org. Chem., 47 (1982) 1371-1373. [Pg.209]

Triflates of titanium and tin are effective catalysts for various condensations of carbonyl compounds [I2I, 122, 123, 124, 125] Claisen and Dieckmann type condensations between ester functions proceed under mild conditions in the presence of dichlorobis(trifluoromethanesulfonyloxy)titaiiiuin(rV) and a tertiary amine (equations 59 and 60) These highly regio- and stereoselective condensations were used successfully m the synthesis of carbohydrates [122]... [Pg.964]

The nib o-aldol reaction using l,l-diethoxy-2-nib oethane is useful for lengthening of the carbon chain of carbohydrates. The reaction of Eq. 3.86 proceeds in a stereoselective way (ds 75%) to give the syn-niho alcohol in 58% isolated yield.The product is converted into 2-amino-2-deoxyaldoses by reaction with Ha/Raney Ni. [Pg.65]

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]

A low stereoselection is also reported for an Ugi reaction which is the central step in the total synthesis of the antibiotic ( + )-furanomycin80. The carbohydrate-derived intermediate, formed from the following acetal and (+ )-a-methyIbenzylamine, reacts with benzoic acid and tert-butyl isocyanide to deliver the precursor 2 of the antibiotic and its diastcreomer 3 in equal amounts80. [Pg.800]

Kunz H., Hofmeister A., Glaser B. Stereoselective Syntheses Using Carbohydrates as Carriers of Chiral Information in Polysaccharides 1998 539, Ed. Severian D., Pb. Dekker N.Y. [Pg.308]

Lopez J. C., Lukacs G. Pyranose-Derived Dienes and Conjugated Enals. Preparation and Diels-Alder Cycloaddition Reactions ACS Symp. Ser. 1992 494 33-49 Keywords carbohydrate, befera-Diels-Alder reactions, stereoselectivity... [Pg.321]

Kunz H., Mueller B., Pfrengle, W., Rueck K., Staehle W. Carbohydrates As Chiral Templates in Stereoselective [4 + 2 Cycloaddition Reactions ACS Symp. Ser. 1992 494 131-146... [Pg.321]

Kaneko C., Katagiri N., Nomura M., Sato H. A New Method for the Stereoselective Synthesis of Nucleosides by Means of Sodium Borohydride Mediated Reductive C-C or C-N Bond Cleavage Reaction Isr. J. Chem. 1991 31 247-259 Keywords carbohydrates... [Pg.322]

Unfortunately, the phosphorylated form of the starting aldehyde is expensive, and dephosphorylation by a phosphatase requires an additional step. Therefore, the challenge was to obtain a mutant aldolase that not only accepts nonphos-phorylated substrates but also turns over the enantiomeric aldehyde (29) stereoselectively with formation of (30), which is a precursor of carbohydrate (31) (see Scheme 2.8) [74] ... [Pg.46]

Scheme 10.17 illustrates allylation by reaction of radical intermediates with allyl stannanes. The first entry uses a carbohydrate-derived xanthate as the radical source. The addition in this case is highly stereoselective because the shape of the bicyclic ring system provides a steric bias. In Entry 2, a primary phenylthiocar-bonate ester is used as the radical source. In Entry 3, the allyl group is introduced at a rather congested carbon. The reaction is completely stereoselective, presumably because of steric features of the tricyclic system. In Entry 4, a primary selenide serves as the radical source. Entry 5 involves a tandem alkylation-allylation with triethylboron generating the ethyl radical that initiates the reaction. This reaction was done in the presence of a Lewis acid, but lanthanide salts also give good results. [Pg.965]

The synthesis in Scheme 13.44 is also based on a carbohydrate-derived starting material. It controlled the stereochemistry at C(2) by means of the stereoselectivity of the Ireland-Claisen rearrangement in Step A (see Section 6.4.2.3). The ester enolate was formed under conditions in which the T -enolate is expected to predominate. Heating the resulting silyl enol ether gave a 9 1 preference for the expected stereoisomer. The... [Pg.1203]

Nitroalkenes with Chiral Auxiliaries The use of carbohydrates as chiral auxiliary in Diels-Alder reactions for the stereoselective preparation of carbocyclic and heterocyclic chiral rings is well documented.48 For example, D-manno-nitroalkene reacts with 2,3-dimethyl-1,3-butadiene to give a 65 35 mixture of adducts, as shown in Eq. 8.29. The configurations at C-4 and C-5 have been determined to be (4R,5R) and (45,55), respectively. Hydrolysis of the product followed by degradative oxidation of the sugar side chains leads to enantiomerically... [Pg.245]

U. Aich and D. Loganathan, Stereoselective single-step synthesis and X-ray crystallographic investigation of acetylated aryl 1, 2-trans glycopyranosides and aryl 1,2-cis C2-hydroxy-glycopyranosides, Carbohydr. Res., 341 (2006) 19-28. [Pg.88]

Chiral 10 to 12-membered nitrogen and oxygen heterocycles, fused to isoxazo-line rings have been prepared with high regio- and stereoselectivity by INOCs of tethered N- and O-allyl carbohydrate derivatives. The use of a -Y-Ar-CH2 tether, containing a 1,2-disubstituted aromatic ring between the heteroatom attached to... [Pg.87]

Covalently bonded chiral auxiliaries readily induce high stereoselectivity for propionate enolates, while the case of acetate enolates has proved to be difficult. Alkylation of carbonyl compound with a novel cyclopentadienyl titanium carbohydrate complex has been found to give high stereoselectivity,44 and a variety of ft-hydroxyl carboxylic acids are accessible with 90-95% optical yields. This compound was also tested in enantioselective aldol reactions. Transmetalation of the relatively stable lithium enolate of t-butyl acetate with chloro(cyclopentadienyl)-bis(l,2 5,6-di-<9-isopropylidene-a-D-glucofuranose-3-0-yl)titanate provided the titanium enolate 66. Reaction of 66 with aldehydes gave -hydroxy esters in high ee (Scheme 3-23). [Pg.155]

For acrylates, or type I reagents, applied in asymmetric Diels-Alder reactions, several chiral auxiliaries such as menthol derivatives, camphor derivatives,16,3 and oxazolidinones4 are available. Carbohydrate compounds have also been reported as chiral auxiliaries in a recent publication, although the stereoselectivity was not good.5 Here are examples in which asymmetric Diels-... [Pg.269]


See other pages where Carbohydrates stereoselectivity is mentioned: [Pg.321]    [Pg.736]    [Pg.724]    [Pg.321]    [Pg.736]    [Pg.724]    [Pg.239]    [Pg.35]    [Pg.49]    [Pg.423]    [Pg.520]    [Pg.299]    [Pg.783]    [Pg.46]    [Pg.281]    [Pg.286]    [Pg.197]    [Pg.253]    [Pg.573]    [Pg.963]    [Pg.80]    [Pg.49]    [Pg.172]    [Pg.297]    [Pg.176]    [Pg.206]    [Pg.306]    [Pg.1171]    [Pg.49]    [Pg.30]    [Pg.16]    [Pg.29]    [Pg.42]    [Pg.136]   
See also in sourсe #XX -- [ Pg.55 ]

See also in sourсe #XX -- [ Pg.55 ]

See also in sourсe #XX -- [ Pg.55 ]

See also in sourсe #XX -- [ Pg.55 ]

See also in sourсe #XX -- [ Pg.55 ]




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