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Oxazolidines, chiral

Ester Enolates. Enantiomerically pure amino acids may ultimately be prepared via stereospecific ester enolate generation using an oxazolidine chiral auxiliary (eq 2). Moderate diastere-oselectivity is observed using potassium hexamethyldisilazide. [Pg.356]

Berlan J, Besace Y, Pourcelot G, Cresson P. Addition d organocuprates aux oxazolidines chirales a-p ethyleni-ques I - resultats - effets de sel et de solvent. Tetrahedron 1986 42 4757- 765. [Pg.65]

Besace Y, Berlan J, Pourcelot G, Huche M. Effet de solvant remarquable dans 1 addition diastereoselective d organocuprates sur une oxazolidine chirale a,p insaturee. J. Organomet. Chem. 1983 247 C11-C13. [Pg.65]

Likewise, a cis-2,6-disubstituted piperidine natural product, (-)-lobeline (98, Scheme 8.4.30) was synthesized from the chiral Af-alkyl pyridinium salt ent-80 via a sequence that included addition of a Reformatsky reagent to an intermediate oxazolidine. °... [Pg.368]

A variety of 1,3-oxazolidines have been used as chiral formyl anion equivalents for addition to aldehydes. Thus, for example, reaction of N-protected norephedrine with Bu3Sn-CH(OEt)2 gives 48, and transmetallation with BuLi followed by addition of benzaldehyde affords the expected adduct 49. The selectivity at the newly formed alcohol center is poor, but the situation can be salvaged by oxidation and re-reduction, which affords the product 50 with >95% d.e. It is then a simple matter to hydrolyze off the oxazolidine, although the resulting hydroxyaldehydes... [Pg.95]

Chiral 4,5-disubstituted oxazolidin-2-ones in stereoselective synthesis of (3-hydroxy-a-amino acids 97G475. [Pg.253]

Disubstituted 2-oxazolidines as chiral auxiliaries synthesis from 2-oxazolones 97YZ339. [Pg.254]

Chiral 3-alkenoyl-l,3-oxazolidin-2-ones have been developed and used in highly diastereoselective Diels-Alder reactions by Evans et al. [26] (Scheme 1.34). In this reaction these dienophiles are highly reactive compared with the corresponding... [Pg.24]

Fig. 1.5 Coordination of E12AICI with chiral crotonoyl-1,3-oxazolidin-2-one derivative... Fig. 1.5 Coordination of E12AICI with chiral crotonoyl-1,3-oxazolidin-2-one derivative...
A chiral titanium complex with 3-cinnamoyl-l,3-oxazolidin-2-one was isolated by Jagensen et al. from a mixture of TiCl 2(0-i-Pr)2 with (2R,31 )-2,3-0-isopropyli-dene-l,l,4,4-tetraphenyl-l,2,3,4-butanetetrol, which is an isopropylidene acetal analog of Narasaka s TADDOL [48]. The structure of this complex was determined by X-ray structure analysis. It has the isopropylidene diol and the cinnamoyloxazolidi-none in the equatorial plane, with the two chloride ligands in apical (trans) position as depicted in the structure A, It seems from this structure that a pseudo-axial phenyl group of the chiral ligand seems to block one face of the coordinated cinnamoyloxazolidinone. On the other hand, after an NMR study of the complex in solution, Di Mare et al, and Seebach et al, reported that the above trans di-chloro complex A is a major component in the solution but went on to propose another minor complex B, with the two chlorides cis to each other, as the most reactive intermediate in this chiral titanium-catalyzed reaction [41b, 49], It has not yet been clearly confirmed whether or not the trans and/or the cis complex are real reactive intermediates (Scheme 1.60). [Pg.39]

Among the many chiral Lewis acid catalysts described so far, not many practical catalysts meet these criteria. For a,/ -unsaturated aldehydes, Corey s tryptophan-derived borane catalyst 4, and Yamamoto s CBA and BLA catalysts 3, 7, and 8 are excellent. Narasaka s chiral titanium catalyst 31 and Evans s chiral copper catalyst 24 are outstanding chiral Lewis acid catalysts of the reaction of 3-alkenoyl-l,2-oxazolidin-2-one as dienophile. These chiral Lewis acid catalysts have wide scope and generality compared with the others, as shown in their application to natural product syntheses. They are, however, still not perfect catalysts. We need to continue the endeavor to seek better catalysts which are more reactive, more selective, and have wider applicability. [Pg.48]

To achieve catalytic enantioselective aza Diels-Alder reactions, choice of metal is very important. It has been shown that lanthanide triflates are excellent catalysts for achiral aza Diels-Alder reactions [5]. Although stoichiometric amounts of Lewis acids are often required, a small amount of the triflate effectively catalyzes the reactions. On the basis of these findings chiral lanthanides were used in catalytic asymmetric aza Diels-Alder reactions. The chiral lanthanide Lewis acids were first developed to realize highly enantioselective Diels-Alder reactions of 2-oxazolidin-l-one with dienes [6]. [Pg.188]

A frequently used catalytic system used for the catalytic enantioselective carbo-Diels-Alder reaction of N-alkenoyl-l,3-oxazolidin-2-one 4 is the chiral TADDOL-Ti(IV) 6 [14] complexes (Scheme 8.2 see Ghapter 1 in this book, by Hayashi) [15]. [Pg.309]

The related serine derived (4S)-4-methoxycarbonyl-3-(l-oxopropyl)-2-thiono-l,3-oxazolidine 11, and the cysteine derived (4A)-4-methoxycarbonyl-3-(l-oxobntyl)-2-thiono-1,3-thiazolidine 13, also serve as efficient chiral auxiliaries in boron- and tin(II)-mediated aldol condensations98. Thus, conversion of 11 into the boron or tin enolate, followed by reaction with 2-methylpropanal affords predominantly one adduct. Subsequent methanolysis and chromatographic purification delivers the syu-methyl ester in 98% ee. [Pg.499]

Chiral oxazolidines 6, or mixtures with their corresponding imines 7, are obtained in quantitative yield from acid-catalyzed condensation of methyl ketones and ( + )- or ( )-2-amino-l-phcnylpropanol (norephedrine, 5) with azeotropic removal of water. Metalation of these chiral oxazolidines (or their imine mixtures) using lithium diisopropylamide generates lithioazaeno-lates which, upon treatment with tin(II) chloride, are converted to cyclic tin(II) azaenolates. After enantioselective reaction with a variety of aldehydes at 0°C and hydrolysis, ft-hydroxy ketones 8 are obtained in 58-86% op4. [Pg.600]

Reaction of the chiral (45,5R)-oxazolidine 9. obtained from 3-pentanone and (-)-2-amino-l-phenylpropanol, with aldehydes gives predominantly a H -aldol adducts of high enantiomeric purity. The corresponding spn-adducts, formed in low enantiomeric excess, are isolated from the diaslereomeric mixture by chromatography 5. [Pg.600]

The conjugate addition of lithium cuprates to cinnamates 1 bearing a chiral oxazolidine or imidazolidine ring at the ortho position produced 2 in good to excellent yield upon hydrolysis14. [Pg.897]

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... [Pg.182]

PRACTICAL SYNTHESIS OF NOVEL CHIRAL ALLENAMIDES (R)-4-PHENYL-3-(l,2-PRO PADIENYL)OXAZOLIDIN-2-ONE... [Pg.77]

Chiral titanium- and scandium-based catalysts (61 and 62, Figure 3.11) were used to accelerate the cycloadditions of acyl-l,3-oxazolidin-2-ones 60 (Scheme 3.14) with butadiene, isoprene and cyclopentadiene. The cycloadditions... [Pg.118]

Chiral tricyclic fused pyrrolidines 29a-c and piperidines 29d-g have been synthesized starting from L-serine, L-threonine, and L-cysteine taking advantage of the INOC strategy (Scheme 4) [19]. L-Serine (23 a) and L-threonine (23 b) were protected as stable oxazolidin-2-ones 24a and 24b, respectively. Analogously, L-cysteine 23 c was converted to thiazolidin-2-one 24 c. Subsequent N-allylation or homoallylation, DIBALH reduction, and oximation afforded the ene-oximes, 27a-g. Conversion of ene-oximes 27a-g to the desired key intermediates, nitrile oxides 28 a-g, provided the isoxazolines 29 a-g. While fused pyrrolidines 29a-c were formed in poor yield (due to dimerization of nitrile oxides) and with moderate stereoselectivity (as a mixture of cis (major) and trans (minor) isomers), corresponding piperidines 29d-g were formed in good yield and excellent stereoselectivity (as exclusively trans isomers, see Table 3). [Pg.6]

Determined via Mosher analysis of a derivative. Chiral oxazolidine. [Pg.186]

Zeijden [112] used chiral M-functionalized cyclopentadiene ligands to prepare a series of transition metal complexes. The zirconium derivative (82 in Scheme 46), as a moderate Lewis acid, catalyzed the Diels-Alder reaction between methacroleine and cyclopentadiene, with 72% de but no measurable enantiomeric excess. Nakagawa [113] reported l,T-(2,2 -bis-acylamino)binaphthalene (83 in Scheme 46) to be effective in the ytterbium-catalyzed asymmetric Diels-Alder reaction between cyclopentadiene and crotonyl-l,3-oxazolidin-2-one. The adduct was obtained with high yield and enantioselectivity (97% yield, endo/exo = 91/9, > 98% ee for the endo adduct). The addition of diisopropylethylamine was necessary to afford high enantioselectivities, since without this additive, the product was essentially... [Pg.129]

Herrmann has prepared several unsymmetrical salts 7 from 1-alkyl-imidazoles (Scheme 6). The chirality was introduced, after N-alkylation of the imidazole by chloro acetonitrile, by addition of enantiomerically pure aminoalcohols onto the nitrile to form an oxazolidine ring [14],... [Pg.198]

In 2001, Braga et al. reported the synthesis of new chiral C2-symmetric oxazolidine disulfide ligands from (R)-cysteine and successfully applied them as catalysts in the asymmetric addition of ZnEt2 to various aldehydes (Scheme 3.23). In the presence of 2mol% of ligand, excellent enantioselectivities of up to >99% ee were obtained even with aliphatic aldehydes such as n-decanal or n-hexanal. These authors proposed that the active catalyst did not maintain its C2-symmetry during the reaction. The disulfide bond was probably cleaved in situ by ZnEt2. [Pg.120]

In addition, other chiral sulfide ligands containing oxazolidines have been tested for the enantioselective addition of ZnEt2 to aldehydes, providing moderate enantioselectivities, as shown in Scheme 3.25. ... [Pg.121]

In 2002, Braga el al. employed a chiral C2-symmetric oxazolidine disulfide as a ligand for the enantioselective synthesis of propargylic alcohols by direct addition of alkynes to aldehydes (Scheme 3.64). Good yields but moderate enantioselectivities (<58% ee) were obtained for the enantioselective alkyny-lation of aldehydes in the presence of ZnEt2. [Pg.144]

In 2005, Carretero et al. reported a second example of chiral catalysts based on S/P-coordination employed in the catalysis of the enantioselective Diels-Alder reaction, namely palladium complexes of chiral planar l-phosphino-2-sulfenylferrocenes (Fesulphos). This new family of chiral ligands afforded, in the presence of PdCl2, high enantioselectivities of up to 95% ee, in the asymmetric Diels-Alder reaction of cyclopentadiene with A-acryloyl-l,3-oxazolidin-2-one (Scheme 5.17). The S/P-bidentate character of the Fesulphos ligands has been proved by X-ray diffraction analysis of several metal complexes. When the reaction was performed in the presence of the corresponding copper-chelates, a lower and opposite enantioselectivity was obtained. This difference of results was explained by the geometry of the palladium (square-planar) and copper (tetrahedral) complexes. [Pg.198]

These examples and those in Scheme 2.6 illustrate the key variables that determine the stereochemical outcome of aldol addition reactions using chiral auxiliaries. The first element that has to be taken into account is the configuration of the ring system that is used to establish steric differentiation. Then the nature of the TS, whether it is acyclic, cyclic, or chelated must be considered. Generally for boron enolates, reaction proceeds through a cyclic but nonchelated TS. With boron enolates, excess Lewis acid can favor an acyclic TS by coordination with the carbonyl electrophile. Titanium enolates appear to be somewhat variable but can be shifted to chelated TSs by use of excess reagent and by auxiliaries such as oxazolidine-2-thiones that enhance the tendency to chelation. Ultimately, all of the factors play a role in determining which TS is favored. [Pg.125]


See other pages where Oxazolidines, chiral is mentioned: [Pg.242]    [Pg.242]    [Pg.307]    [Pg.4]    [Pg.5]    [Pg.23]    [Pg.25]    [Pg.25]    [Pg.26]    [Pg.78]    [Pg.399]    [Pg.853]    [Pg.186]    [Pg.130]    [Pg.215]    [Pg.216]    [Pg.242]    [Pg.37]    [Pg.197]   
See also in sourсe #XX -- [ Pg.234 ]

See also in sourсe #XX -- [ Pg.234 ]

See also in sourсe #XX -- [ Pg.234 ]




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Oxazolidine

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