Antipode distribution

In principle several intermediate possibilities exist between the two limiting cases mixture of homopolymers or random copolymer (Scheme 3) The content of the two monomer antipodes in the macromolecules affects the polymer optical rotation however the relationship between enantiomeric purity and rotatory power can depend also on antipode distribution in the polymer. 

In the synthesis of the tetracyclic intermediates for the synthesis of isoarborinol and its CDE-antipode femenol, the stereochemistry of the Diels-Alder reaction can be varied using various Lewis-acid catalysts in aqueous media (Eq. 12.36).97 Their results show that the hydrophobic effects play an important role in enhancing reaction rates and can control product distribution. Novel 2,4-dialkyl-1-alkylideneamino-3-(methoxycarbonylmethyl)azetidines were obtained from aldazines and... 

Since the early times of stereochemistry, the phenomena related to chirality ( dis-symetrie moleculaire, as originally stated by Pasteur) have been treated or referred to as enantiomericaUy pure compounds. For a long time the measurement of specific rotations has been the only tool to evaluate the enantiomer distribution of an enantioimpure sample hence the expressions optical purity and optical antipodes. The usefulness of chiral assistance (natural products, circularly polarized light, etc.) for the preparation of optically active compounds, by either resolution or asymmetric synthesis, has been recognized by Pasteur, Le Bel, and van t Hoff. The first chiral auxiliaries selected for asymmetric synthesis were alkaloids such as quinine or some terpenes. Natural products with several asymmetric centers are usually enantiopure or close to 100% ee. With the necessity to devise new routes to enantiopure compounds, many simple or complex auxiliaries have been prepared from natural products or from resolved materials. Often the authors tried to get the highest enantiomeric excess values possible for the chiral auxiliaries before using them for asymmetric reactions. When a chiral reagent or catalyst could not be prepared enantiomericaUy pure, the enantiomeric excess (ee) of the product was assumed to be a minimum value or was corrected by the ee of the chiral auxiliary. The experimental data measured by polarimetry or spectroscopic methods are conveniently expressed by enantiomeric excess and enantiomeric... 

The distribution of the small quantity of (R) antipode present in the monomer among the fractions of different stereoregularity does not appear to be investigated. 

The template la can be split off by water or methanol to an extent of up to 95% (see Scheme 4.II). The accuracy of the steric arrangement of the binding sites in the resulting imprinted cavity can be tested by the ability of the polymer to resolve the racemate of the template, namely phenyl-a-D,L-mannopyranoside. The polymer was equilibrated in a batch procedure with a solution of the racemate under conditions that allowed a thermodynamically controlled partition of the enantiomers between polymer and solution. The enrichment of the antipodes in the polymer and in solution was determined and the separation factor a, i.e. the ratio of the distribution coefficients of the d- and L-enantiomer between polymer and solution, was calculated. After extensive optimisation of the procedure, a values between 3.5 and 6.0 were obtained [4]. This is an extremely high selectivity for racemic resolution that cannot be reached by most other methods. 

It is necessary to know the velocity distribution of ejected fragments in order to estimate tlie re-accumulation condition for icy bodies. However, this velocity distribution is not known for ice. For this reason, we used the antipodal particle velocity (va), for each collision as the lowest fragment ejection velocity. We also used the criterion, for the re-accumulation condition. In tliis... 

H. Biich, J. Knabe, W. Buzeilo, and W. Rummel, StereospeciRcity of anaesthetic activity, distribution, inactivation and protein binding of the optical antipodes of two W-methylated barbiturates, /. Pharmacol. Exp. Then, 175 709 (1970). 

Morphine and related alkaloids are specific to the genus Papaver (Berberidaceae), although the antipodal series of alkaloids is distributed in the Menisperma-ceae. Early in the biosynthesis of morphine, an inversion at C-1 of (5)-reticuline occurs, followed by ortho-para benzylic coupling to afford salutaridine. Stereospecific reduction and cyclization-elimination affords the 4,5-Ether bridge and thebaine. The dominant pathway from this point involves neopinone, codeinone, codeine, and morphine. Again, most of the enzymes in this sequence were isolated and characterized by Zenk s group (Fig. 30). 

In CSPs, owing to the nature of the polymeric structure, the simultaneous partecipation of several chiral sites or several polymeric chains is conceivable. In CCPs, the chiral sites are distributed at the surface or in the network of the achiral matrix relatively far away from each other and only a bimolecular interaction is generally possible with the optical antipodes. 

The dextrorotatory antipode of the widely distributed (-)-centrolobol (2) was first discovered by Sasaya in A. hirsuta (21) in the company of an interesting tetrahydronaphthopyrone derivative (17), being in evident biogenetic relationship to open chain diarylheptanoids. 

Whereas the optically pure compound 6c resembles the racemic modification in the solid state, the opposite was found to be the case for racemic 6d which proved to be isostructural with optically active 6a (19a-b,20) this material crystallizes in the chiral space group PI with the two enantiomers randomly distributed throughout the lattice. Crystals of unequal R/S composition could be prepared by recrystallizing various mixtures of the optical antipodes. Irradiation of these crystals gave a diastereomeric mixture of optically active dimers, trimers and oligomers, the optical purities of which varied from 0 to 100% depending on the R/S composition of the sample photolyzed. This... 

Actually, the fixation of diad configuration in the polymer chain proceeds only when the next monomer molecule is attached. Until this moment, the end unit can still undergo isomerization becoming either planar or its mirror antipode. In the absence of isomerization, the formation of a stereoregular syndiotactic polymer should be expected. In the case when an end unit has the time (before the attack of the following monomer molecule) to adopt a planar configuration, the distribution of diads in the polymer is described by Bernoulli s statistics (binomial distribution) [42]. If the state to two end units of... 

The selective reaction of optical antipodes of NCA should reflect the stereoregularity or the sequential distribution of the antipodal residues in the polymer formed. Infrared spectral studies of the polymer have been known to provide useful information on the stereoregularity and the local conformation. As seen in Fig. 16 and Table 10 for the polymerization of y-benzyl 87%-S-glutamate NCA in dimethylformamide (48), the strength of the absorption at 795 cm"1, which has been reported to depend on the antipodal content of the polymer (50), changes with conversion through a maximum, corresponding to the observed selection of the antipode (Table 10). 

R enantiomer. The plasma protein binding of S-phenprocoumon is greater than that of antipode in human serum (Table 3) [69]. The protein binding of (R,S)-phenprocoumon is between that of the individual enantiomers. The difference in plasma protein binding of phenprocoumon enantiomers contributes to the lower apparent volume of distribution and clearance for S-phenprocoumon than those of its antipode [69]. 

Studies investigating stereoselective binding and distribution of NSAIDs to extrasynovial tissues are scarce. In rats, etodolac displays less stereoselectivity in tissue concentrations than in plasma and was thought to be due to a greater unbound fraction of S enantiomer in plasma relative to antipode [275]. For 2-aryl propionates, which undergo in vivo unidirectional inversion from R to S enantiomer (e.g., ibuprofen), the presence of these NSAIDs in the adipose tissue as hybrid triglycerides has been reported [276]. 

As p TV, at the antipodal point in the projective space, the redshift approaches totality as 2 —> oo. In the stationary Minkowski frame the antipode is infinitely distant at an infinite time coordinate. The conditions leading to the derivation of the redshift formula are not met for radiation with a propagation interval close to a half-circuit in space, and such photons will appear entirely delocalized and severely redshifted, constituting the isotropic microwave background. Segal ascribes the Planddan distribution to the conservation of energy, which is tantamount to the fact that any closed space must eventually impose a Planckian spectrum on stray radiation. 

I. are widely distributed in vegetables and fruits, especially in berries, tea, and tobacco I. are the main components (22%) of violet flower oil. Both optical antipodes are foundThe dominating form in essential... 

Sesquiterpenes (sesquiterpenoids). A structurally highly diverse class of terpenoids with 15 carbon atoms skeleton derived biosynthetically from famesyl pyrophosphate (FPP) ( famesol, isoprene rule, ter-penes). More than 70 different ring systems are formed by enzyme-catalyzed cyclization of the linear parent structure these cyclic structures can be further modified by 1,2- and 1,3-hydride shifts, renewed cycliza-tions, hydroxylations, and other subsequent reactions. S. are widely distributed in plants, fiingi, and animals but are less common in bacteria. Specific biosynthetic routes are often characteristic for certain organisms. Thus, basidiomycetes preferentially use humulene as the basis for the syntheses of protoilludanes, illu-danes, lactaranes, hirsutanes, and related S. skeletons. Individual S. systems are also known for liverworts and marine organisms. In addition, liverworts often contain the optical antipodes of S. known from plants. 

Gil-Av et al. argued that the separation of the antipodes on a suitable asymmetric stationary phase would involve reversible association between the enantiomers and the asymmetric stationary phase molecules. The two antipodes would form diastereoisomeric interaction with somewhat different interatomic distances. Hence, there would be different polar, dispersive and/or steoric interactions between substituents situated round the asymmetric centers of the solute and stationary phase molecules, respectively. Such differences would effect the standard free energy of distribution and the magnitude of the distribution coefficients of the enantiomers. Gil-Av et al. used A-TFA-D-isoleucine lauryl ester and A-TFA-L-isoleucine lauryl ester as the stationary phase which were coated on the walls of a capillary column 100 m long, 250 pm I.D. and was shown to have an efficiency of about 98,000 theoretical plates. The samples of the derivatized amino acids were injected with a split ratio of 1 100. The separation was carried out isothermally at 90°C and the analysis time was just over 4 hours. The results obtained for the... 

The separation of enantiomeric compounds or optical antipodes cannot be performed in a normal distribution system, but requires interaction with another enantiomeric agent, a chiral selector, which gives diastereomers with different distribution properties with the antipodes of the substrate. The binding can be covalent, but it is often preferable to use coordinative bonding and to separate the enantiomers as diastereomeric complexes. The complexing selector may be incorporated in the stationary phase or added to the mobile phase of the chromatographic system. 

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