Stereoisomeric effects

Lundberg and Doty [12] first reported that the initiation of the polymerization of racemic monomer by preformed, optically pure, homo- 

Lundberg and Doty [12] suggested that this specificity is connected with the existence of a-helical conformations. It may be envisaged as the result of adsorption of NCA on the terminal residues of the helix by hydrogen-bonding as depicted in Fig. 6 due to Weingarten [59]. Evidence 

Direct evidence for the adsorption of both enantiomorphs of 7-benzyl-glutamate NCA on to the L-polymer in dioxan was obtained by Williams et al. [62] using vapour pressure osmometry to monitor the free-monomer concentration. The results are shown in Table 3. It is of interest that the D-monomer is adsorbed preferentially by an L-polymer (Kld/Kll 1-4). The authors do not state that the amine end-group of the polymer was deactivated in this work if not, complications which might eirise from in situ polymerization appear to have been disregarded. 

It has been known for some time that NCAs of racemic a-amino acids can be converted to optically active poly a-amino acids with the aid of optically active alcohols or combinations of such alcohols with metal alkyls as initiators. Such processes have been termed asymmetric-selective or stereo-selective . Some of these investigations will be described subsequently. Biihrer and Elias [63] have recently made a comprehensive investigation of the kinetics of polymerization of DL-leucine NCA initiated by a series of optically active primary and 

Adsorption of 7-benzyl-glutamate NCA on to poly-7-benzyl-L-glutamate (M = 4000) at 30°C in dioxan (Concentrations in mole 1 ) 

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VII. Stereoisomeric Effects on Thermodynamics and Kinetics of Electrode Processes... 

VII. STEREOISOMERIC EFFECTS ON THERMODYNAMICS AND KINETICS OF ELECTRODE PROCESSES... 

An interesting stereoisomeric effect was reported in the reduction of a formyl group [548] (Fig. 4). Compound XXVII is reduced more difficultly than XXVIII owing to an interaction between the formyl group and the double bond or to a difference in the reaction rate of the ax- and e -substituents the semicarbazones of XXVII and XXVIII, however, do not show any difference because the bulky azometine groups of the both compounds are always equatorial. 

Figure 4. Stereoisomeric effect in the reduction of a formyl group.

Sokolsky-Papkov, M. and Domb, A.J. (2008) Stereoisomeric effect on in vitro drug release from injectable poly(lactic acid co castor oil) polyesters. Polymers for Advanced Technologies, 19(6), 671-679. 

Chloromethylated polysulfone indeed exhibits different kinetic behavior in the quaternization with TEA than its corresponding model compound. From experimental results, it is clear that the rate retardation is not due to steric hindrance, the degree of chloro-methylation on the polymer chain, or a salt effect. Stereoisomeric effects are not a potential factor, since chloromethylated polysulfone consists of only one detectable isomer. In spite of these results, we knew that the polymer backbone must play an important role in this reaction. Under the same experimental conditions used to quaternize chloromethylated polysulfone, poly(vinylbenzyl chloride) exhibited normal second-order kinetics with an Ea of 10.4 kcal/mole, as shown in Figure 6. Noda and Kagawa also observed the same phenomenon in the quaternization of chloromethylated polystyrene with TEA in DMF (Ea = 10.5 kcal/mole)The major difference between these two systems is the polymer backbone polysulfone... 

The stereoisomeric effects comprised by the Aroma5 rule is not modeled by the chemical hardness compactness aromaticity by any of its computed scales, neither semi-empirical or ab initio. 

Canadell J., Fischer H., de WithG., vanBenthemR.A.T.M. (2010), Stereoisomeric effects in thermo-remendable polymer networks based on Diels-Alder crosslink reactions , J. Polym. ScL, Part A Polym. Chem., 48, 3456-3467. 

Meyerson S and Weitkamp AW (1968) Stereoisomeric effects on mass spectra. Organic Mass Spectrometry 1 659-668. 

Joo, M.K., Sohn, Y.S., Jeong, B. Stereoisomeric effect on reverse thermal gelation of poly(ethylene glycol)/poly(lactide) multiblock copolymer. Macromolecules 40, 5111-5115 (2007)... 

In principle cis 2 butene and trans 2 butene may be mterconverted by rotation about the C 2=C 3 double bond However unlike rotation about the C 2—C 3 single bond in butane which is quite fast mterconversion of the stereoisomeric 2 butenes does not occur under normal circumstances It is sometimes said that rotation about a carbon-carbon double bond is restricted but this is an understatement Conventional lab oratory sources of heat do not provide enough energy for rotation about the double bond m alkenes As shown m Figure 5 2 rotation about a double bond requires the p orbitals of C 2 and C 3 to be twisted from their stable parallel alignment—m effect the tt com ponent of the double bond must be broken at the transition state... 

Stereoisomerism in either the alkamine nucleus or the acyl residue has a considerable effect on the pharmacological action of the tropeines and cocaines. Differences in activity of tropine and i/i-tropine and their benzoyl derivatives have been mentioned already, and there seems to be a consensus of opinion that the i/i-cocaines (alkyl- or aryl- acyl esters of 0-ecgonine) are less toxic and more potent local anfesthetics than the corresponding cocaines, derived from 1-ecgonine. ... 

Finally, since besides the inductive effect of the sulfoxide and the sulfone functional groups, hydrogen bonding, field effects and steric effects to solvation may or may not work in the same direction, the pKx values can be useful in assigning configurations of suitable pairs of stereoisomeric sulfoxide and sulfone carboxylic acids291. 

The effect of HMPA on the reactivity of cyclopentanone enolate has been examined.44 This enolate is primarily a dimer, even in the presence of excess HMPA, but the reactivity increases by a factor of 7500 for a tenfold excess of HMPA at -50° C. The kinetics of the reaction with CH3I are consistent with the dimer being the active nucleophile. It should be kept in mind that the reactivity of regio- and stereoisomeric enolates may be different and the alkylation product ratio may not reflect the enolate composition. This issue was studied with 2-heptanone.45 Although kinetic deprotonation in THF favors the 1-enolate, a nearly equal mixture of C(l) and C(3) alkylation was observed. The inclusion of HMPA improved the C(l) selectivity to 11 1 and also markedly accelerated the rate of the reaction. These results are presumably due to increased reactivity and less competition from enolate isomerization in the presence of HMPA. 

Szeimies recently published an impressive example of a steric effect on a Sr2 reaction at carbon for the addition of thiols to the central bond of bicyclo[ 1.1.0]-systems87. From the radical chain addition of thiophenol to 32 the stereoisomeric cyclobutanes 33a and 33b are obtained exclusively in 56% yield. The thiylradical... 

In contrast to mass spectroscopy, NMR spectroscopy reveals the effect of stereoisomerization. One example is the isomerization of lutein to anhydroltutein induced by cooking (Hentschel et al. 

Since vinyl anions generally retain configuration 39> while isomeric vinyl radicals rapidly interconvert 40) these results constitute evidence that reductions of alkyl iodides do proceed via radical intermediates. Isomerization of stereoisomeric vinyl anions is ruled out by the lack of effect of phenol on the stereochemistry of the products (Scheme III). Since cis and trans-3-hexene are formed in differing proportions from the two halides, it may be concluded that the stereoisomeric vinyl radicals are being intercepted by electron trans-... 

UV radiation hypothetical, but so is the transport of molecules from outer space to Earth. Recent analyses of the Murchison meteorite by two scientists from the University of Arizona, Tucson (Cronin and Pizzarello, 1997 Cronin, 1998) have shown it to contain the four stereoisomeric amino acids DL-a-methylisoleucine and DL-a-methylalloisoleucine. In both cases, the L-enantiomer is present in a clear excess (7.0 and 9.1%). Similar results were obtained for two other a-methyl amino acids, isovaline and a-methylvaline. Contamination by terrestrial proteins can be ruled out, since these amino acids are either not found in nature or are present in only very small amounts. Since the carbonaceous chondrites are thought to have been formed around 4.5 billion years ago (see Sect. 3.3.2), the amino acids referred to above must have been subject to one or more asymmetric effects prior to biogenesis. 

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