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Carbohydrate alkenes, nitrile oxide

Optically active aldehydes are available in abundance from amino and hydroxy acids or from carbohydrates, thereby providing a great variety of optically active nitrile oxides via the corresponding oximes. Unfortunately, sufficient 1,4- or 1,3-asymmetric induction in cycloaddition to 1-alkenes or 1,2-disubstituted alkenes has still not been achieved. This represents an interesting problem that will surely be tackled in the years to come. On the other hand, cycloadditions with achiral olefins lead to 1 1 mixtures of diastereoisomers, that on separation furnish pure enantiomers with two or more stereocenters. This process is, of course, related to the separation of racemic mixtures, also leading to both enantiomers with 50% maximum yield for each. There has been a number of applications of this principle in synthesis. Chiral nitrile oxides are stereochemicaUy neutral, and consequently 1,2-induction from achiral alkenes can fully be exploited (see Table 6.10). [Pg.400]

Intramolecular 1,3-dipolar additions of nitrones and nitrile oxides to carbohydrate alkene groups have met with success. Thus, treatment of the unsaturated heptose ether 68 (Scheme 17), which can be made following 1,3-dithianyl anion addition to C-l of 2,3,4-tri-0-benzyl-5,6-dideoxy-D-xy/o-hex-5-enose, with IV-methylhydroxylamine in refluxing methanol, affords the nitrone 69 that cyclizes to give the bicyclic isoxazolidine 70 (60% isolated) together with the epimer at the new asymmetric center carrying the methylene carbon atom (16% isolated) [35]. [Pg.582]

Acetyl- and 3-benzoylisoxazoles 389 (and isoxazolines) have been prepared by one-pot reactions of alkynes (and alkenes) with ammonium cerium(iv) nitrate (CAN(lv)) or ammonium cerium(lll) nitrate tetrahydrate (CAN(m))-formic acid, in acetone or acetophenone. These processes probably involve 1,3-dipolar cycloaddition of nitrile oxides produced via nitration of the carbonyl compound by cerium salts. The existence of nitrile oxides as reaction intermediates was proved by the formation of the dimer furoxan 390 when the above reaction was carried out in absence of any dipolarophile (Scheme 95) <2004T1671>. An analogous improved procedure has been applied to alkynyl glycosides as dipolarophiles for the preparation of carbohydrate isoxazoles <2006SL1739>. [Pg.430]

Further attempts to increase the kinetics and biocompatibility of 1,3-dipolar cycloadditions led organic chemists to explore altemative dipoles that react with multiple-bond reaction partners. Nitrile oxides are highly reactive dipoles that can react with various alkenes and alkynes to provide isoxazolines and isoxazoles, respectively. In the absence of a suitable reacting partner, nitrile oxides tend to dimerize to form fiiroxane derivatives, or can alternatively act as electrophiles. However, when generated in situ from suitable precursors such as hydroximoyl chlorides or by mild oxidation directly from oximes, nitrile oxides were successfully applied to the labeling of nucleic acids [43], peptides [44] and carbohydrates [45] (Fig. 7). [Pg.18]


See other pages where Carbohydrate alkenes, nitrile oxide is mentioned: [Pg.82]    [Pg.106]    [Pg.18]    [Pg.158]   


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4- - 1-alkene nitrile

Alkenes nitrile oxides

Alkenes oxidant

Alkenes, oxidative

Carbohydrate alkenes, nitrile oxide cycloadditions

Carbohydrates nitriles

Carbohydrates oxidation

Nitrile oxides

Nitriles nitrile oxides

Oxidative nitriles

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