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Hetero-chiral

The three water ligands located at meridional positions of the J ,J -DBFOX/Ph aqua complexes may be replaced by another molecule of DBFOX/Ph ligand if steric hindrance is negligible. Based on molecular model inspection, the hetero-chiral enantiomer S,S-DBFOX/Ph looks like a candidate to replace the water ligands to form the heterochiral meso-2 l complex J ,J -DBFOX/Ph-S,S-DBFOX/... [Pg.260]

Table 3. l-(oc-Hetero-chiral-substituted)-3-butenols from Allylboronates and a-Hetero Chiral Aldehydes... [Pg.283]

If we assume that the data of Figs. 22 and 23 can be treated by equilibrium thermodynamics, the discontinuities in the ESP versus temperature phase diagram should indicate the presence of a three-way equilibrium between bulk surfactant and two different film types in both homo- and hetero-chiral systems. The surface heats of transition (U) between the two film types in either system may be obtained by relation (15), where IT is the equilibrium... [Pg.92]

Mixtures of N-(a-methylbenzyl) stearamides with both stearoyltyrosine and stearoyltryptophan methyl esters show no discrimination in their pressure-area relationships at 35°C, regardless of the surface pressure to which the films are compressed (Fig. 34). The Yl/A curves for homo- and hetero-chiral pairs are exactly coincident. [Pg.106]

The simplest structural platforms to be exploited with the coenzyme amino acid chimeras were a family of -hairpin peptides. In these peptides, the amino acid side chains are oriented perpendicular to the plane created by the hairpin, with sequential residue side chains positioned on alternating sides of the hairpin. By placing specific residues across the sheet from the coenzyme residue, these peptides offer a potentially simple way of delivering side-chain functionality to the coenzyme derivative. A -hairpin peptide structure is promoted by the inclusion of a hetero-chiral turn sequence to promote type II or 11 -turn formation [26]. [Pg.13]

Stereochemically, even more interesting are the bis-l,3-oxathiane derivatives 98 and 99 <2005STC369>. All compounds are chiral and the chiral elements are carbons C-2/C-2 and the 1,3-oxathine moieties themselves (Equation 9). Due to the bis-structure, compounds 98 and 99 exhibit both homochiral iZR,2 R ZS,Z S) and hetero-chiral (2R,Z S) isomers (Scheme 4) and they reveal rapid equilibration in solution via open-chain intermediates, thereby preventing separation and individual analysis of the isomers in solution. In the solid state, the compounds crystallized either as unique heterochiral isomers or as a mixture of the two as a solid solution. [Pg.759]

The ethylzinc alcoholate (149) prepared from racemic a-terr-butyl-6-phenyl-2-pyridyl-methanol and Et2Zn exists in the solid state as a hetero chiral dimer (Figure 78). Also in this case the alcoholate oxygen atoms bridge between two zinc atoms, affording a central Zn—O—Zn—O plane. The pyridyl nitrogen atoms are coordinated in such a way to the zinc atoms [Zn—N 2.185(2) A] that they are in awfi-position with respect to this plane. [Pg.100]

A. King, B. Howard, A microwave study of the hetero-chiral dimer of butan-2-ol. Chem. Phys. Lett. 348, 343-349 (2001)... [Pg.84]

The concept of supramolecular synthon defines specific substructural units that contain the logical code for self-assembly by noncovalent bonds (consisting mainly of H-bonding interactions) [65]. A chiral counterpart of this concept is the supramolecular chiron [60,66]. It is defined as the minimal homo- or hetero-chiral molecular unit or ensemble capable of generating ordered superstructures by self-assembly through H-bonding or other noncovalent forces, and leading to... [Pg.129]

In this process, enantiomers (R and S) of the hetero-chiral substance of interest interact with the homo-chiral selector (R) to yield two products having a diastereoisomeric relationship. The formation of diastereoisomeric complexes is the basis for enantiomeric separations. The ability to use smaller differences in the rates of formation, stability, and properties of these complexes has been responsible for the major advances in stereochemical separations. [Pg.2156]

Several examples of the 1,3-dipolar cycloadditions between nitrones and 3-(2-alkenoyl)-l,3-oxazolidin-2-ones using the novel hetero-chiral Yb(III) catalyst are shown in Table 32. In most cases, the desired isoxazolidine derivatives were obtained in excellent yields with excellent diastereo- and enantioselectivities. It is noted that high levels of selectivities were attained at room temperature. Nitrones derived from aromatic and heterocyclic aldehydes gave satisfactory re-... [Pg.295]

As for the structure of the hetero-chiral Yb(III) catalyst, the following structure was supported (Scheme 15). Actually, the existence of hydrogen bonds between the phenolic hydrogens of (S)-BINOL and the nitrogens of (R)-MNEA was confirmed by the IR spectra of the catalyst [79,102]. [Pg.296]

It is believed that bidentate coordination (ex. Yb(III)-3-(2-alkenoyl)-l,3-oxazolidin-2-one) is necessary to obtain high selectivities in many chiral lanthanide-catalyzed reactions [101]. These results are very interesting and promising because it has been shown that even monodentate coordination can achieve good selectivities by using the hetero-chiral Yb(III) catalyst... [Pg.303]

A bond pair (953 and 987 cm-1), which indicated hydrogen bonds (the OH N and O " H+N equilibrium), was observed in the area from 930 to 1000 cm-1 in the IR spectra of the hetero-chiral Yb(III) catalyst [103]. Direct coordination of the amine to Yb(III) is doubtful in light of the fact that the 1,3-dipolar cycloaddition proceeded very slowly when Yb(OTf)3 and (R)-MNEA were first combined and then (S)-BINOL was added... [Pg.303]

SCHEME 24. Enantiomer recognition of zinc alkoxides homochiral versus hetero-chiral dimerization. [Pg.275]

Several examples of the 1,3-dipolar cycloadditions between nitrones and 3-(2-alkenoyl)-l,3-oxazolidin-2-ones using the novel hetero-chiral Yb(III) catalyst are shown in table 36. In most cases, the desired isoxazolidine derivatives were obtained in excellent yields with excellent diastereo- and enantioselectivities. It is noted that high levels of selectivities were attained at room temperature. Nitrones derived from aromatic and heterocyclic aldehydes gave satisfactory results, and even in the reaction using the nitrone derived from an aliphatic aldehyde, the cycloaddition proceeded smoothly to give the endo adduct in an excellent enantiomeric excess, albeit low endolexo selectivity was observed. Moreover, it was found that alkenes which could be employed in the present 1,3-dipolar cycloaddition were not limited to 3-(2-alkenoyl)-l,3-oxazolidin-2-one derivatives. When jV-phenylmaleimide was used as a dipolarophile, the desired isoxazolidine derivative was obtained in a 70% yield with endolexo = > 99/1, and the enantiomeric excess of the endo adduct was 70% ee under the standard reaction conditions. It is believed that bidentate coordination [e.g. Yb(III)-3-(2-alkenoyl)-l,3-oxazolidin-2-one] is necessary to obtain... [Pg.366]

Scheme 16. Novel chiral hetero-chiral Yb(III) catalyst. Scheme 16. Novel chiral hetero-chiral Yb(III) catalyst.
De Feyter. S. Gesquiere, A. Wurst. K. Amabilino. D.B. Veciana. J. De Schryver. F.C. Homo- and hetero-chiral supramolecular tapes from achiral, enantiopure and... [Pg.1400]

Uny et also reported the chemical synthesis of protein polymers based on the (Val-Pro- Ala-Val-Gly) repeat sequence in which glycine is replaced by the D-alanine residue. The hetero-chiral Pro- Ala diad would be erqrected on the basis of stereochemical considerations to adopt a type-II p-tum conformation. Stmctural analyses of small-molecule "Pro- Ala turn models support the formation of the type-II p-mm conformation in solution and the solid state. Polymers based on the (Val-Pro- Ala-Val-Gly) repeat sequence display a thermo-reversible phase transition similar to the corresponding polypeptides derived from the parent (Val-Pro-Gly-Val-Gly) sequence, albeit with a shift of the Tt to approximately 5-10 ° G below the latter due to a slight inaease in hydrophobic character due to the presence of the alanine residue. NMR spectroscopic analyses of the (Val-Pro- Ala-Val-Gly) polymer suggest that the repeat unit retains the p-tum stmcture on the basis of comparison to the corresponding behavior of the (Val-Pro-Gly-Val-Gly) polymer. Stress-strain measurements on cross-linked matrices of the (Val-Pro- Ala-Val-Gly) polymer indicate an elastomeric mechanical response in which the elastic modulus does value in comparison to the (Val-Pro-Gly-Val-Gly) polymer. These smdies of glycine suhstitution support the hypothesis that type-II p-tum formation can he associated with the development of elastomeric behavior with native elastins and elastin-derived polypeptide sequences. Several investigators have proposed that the (Val-Pro-Gly-Val-Gly) pentapeptide represents the minimal viscoelastic unit... [Pg.84]

Attack of the (formal) hydroxide ion on the less hindered (unsubstituted) oxirane carbon atom causes retention of configuration and leads to a hetero-chiral product mixture of enantiomeric diol and nonreacted epoxide. [Pg.123]


See other pages where Hetero-chiral is mentioned: [Pg.48]    [Pg.19]    [Pg.100]    [Pg.52]    [Pg.105]    [Pg.131]    [Pg.128]    [Pg.130]    [Pg.130]    [Pg.157]    [Pg.295]    [Pg.366]    [Pg.367]    [Pg.369]    [Pg.100]    [Pg.123]    [Pg.101]    [Pg.189]    [Pg.189]   
See also in sourсe #XX -- [ Pg.123 ]




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Chiral metal complexes hetero-Diels-Alder reaction

Hetero Chiral lanthanide Lewis acid

Hetero Diels-Alder reaction chiral Lewis acids

Hetero-Diels—Alder reactions chiral auxiliaries

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