2- enantiomeric preference

Applications of the CICC to the prediction of enantiomeric preference in enan-tiosclcctivc reactions arc described in ref s, [37] and [42. ... 

In contrast, hydrogenation of 1,2-diketones that proceeds via 2-hydroxy ketones exhibits marked syn or meso selectivity (Scheme 60), although the enantiomeric preference follows the general sense given in Scheme 47 (92). Thus, (i )-BINAP-Ru-aided hydrogenation of diacetyl gives a 26 74 mixture of enantiomerically pure (R,R)-2,3-butanediol and the meso diol. 

Whether the enantiomeric preference, such as found in the L-configuration of amino acids or the D-configuration of sugars, comes from a deterministic or a random process... 

While these chiral host-guest inclusion compounds have been demonstrated to produce excellent ee s it has proven difficult to predict in an apriori fashion the direction of enantiomeric preference. For example in the cyclization reaction of 21, host 16b produces one enantiomer in 98% ee while 16c produces the opposite enamtioner with 95% ee. It should be noted that 16b and 16c are rather similar and it is therefore difficult without further work to ascribe the exact cause for the different product outcome. ... 

In several tests for cannabimimetic activity (Edery et al. 1971 Jones et al. 1974 Martin 1986), (+)-delta-l-THC (31) was ca 13 to 230 times less active than the (-)-isomer. These results indicate pharmacologic enantiomeric preference rather than absolute stereoselectivity. Indeed, Martin pointed out that, while cannabinoid SAR supports the concept of a specific cannabinoid receptor, a disconcerting element is the apparent lack of greater stereoselectivity in some animal models. However, because the starting material (+) verbenol was not necessarily stereochemically pure, this conclusion is tentative at best. 

On the other hand, the direct alkylation of appropriate carbanions has been extensively investigated with particular regard to the possibility of enantiomeric preferment at the site of alkylation. In common with so many of the reactions described for the synthesis of functionalized phosphonic esters in which two important reactants come together, two approaches are possible in alkylation methodology. 

These transformations serve to illustrate the principles involved in asymmetric synthesis. The requirements for efficient synthetic utilization are (a) an easily available optically active reagent that can carry out the desired transformation, and (b) reaction conditions that lead to a high percentage of enantiomeric preference. In general, it is also desirable to be able to recover the optically active reagent. The Diels-Alder example is a case where this can be accomplished. Hydrolysis or lithium aluminum hydride reduction gives the product and also returns the original alcohol, which can be reused. Similarly, in the synthesis of dialkylacetic acids, the optically active amino alcohol can be recovered by hydrolysis. 

Polylactide is the generaUy accepted term for highly polymeric poly(lactic acid)s. Such polymers are usuaUy produced by polymerization of dilactide the polymerization of lactic acid as such does not produce high molecular weight polymers. The polymers produced from the enantiomeric lactides are highly crystalline, whereas those from the meso lactide are generaUy amorphous. UsuaUy dilactide from L-lactic acid is preferred as a polymerization feedstock because of the avaUabUity of L-lactic acid by fermentation and for the desirable properties of the polymers for various appUcations (1,25). 

Resolution of racemic alcohols by acylation (Table 6) is as popular as that by hydrolysis. Because of the simplicity of reactions ia nonaqueous media, acylation routes are often preferred. As ia hydrolytic reactions, selectivity of esterification may depend on the stmcture of the acylatiag agent. Whereas Candida glindracea Upase-catalyzed acylation of racemic-cx-methylhenzyl alcohol [98-85-1] (59) with butyric acid has an enantiomeric value E of 20, acylation with dodecanoic acid increases the E value to 46 (16). Not only acids but also anhydrides are used as acylatiag agents. Pseudomonasfl. Upase (PFL), for example, catalyzed acylation of a-phenethanol [98-85-1] (59) with acetic anhydride ia 42% yield and 92% selectivity (74). 

Optically active thiiranes have been obtained by resolution of racemic mixtures by chiral tri-o-thymotide. The dextrorotatory thymotide prefers the (5,5)-enantiomer of 2,3-dimethylthiirane which forms a 2 1 host guest complex. A 30% enantiomeric excess of (5,5)-(—)-2,3-dimethylthiirane is obtained (80JA1157). 

Preparation of enantiomerically enriched materials by use of chiral catalysts is also based on differences in transition-state energies. While the reactant is part of a complex or intermediate containing a chiral catalyst, it is in a chiral environment. The intermediates and complexes containing each enantiomeric reactant and a homochiral catalyst are diastereomeric and differ in energy. This energy difference can then control selection between the stereoisomeric products of the reaction. If the reaction creates a new stereogenic center in the reactant molecule, there can be a preference for formation of one enantiomer over the other. 

Whereas the barrier for pyramidal inversion is low for second-row elements, the heavier elements have much higher barriers to inversion. The preferred bonding angle at trivalent phosphorus and sulfur is about 100°, and thus a greater distortion is required to reach a planar transition state. Typical barriers for trisubstituted phosphines are BOSS kcal/mol, whereas for sulfoxides the barriers are about 35-45 kcal/mol. Many phosphines and sulfoxides have been isolated in enantiomerically enriched form, and they undergo racemization by pyramidal inversion only at high temperature. ... 

Each of the following molecules might be resolved into two enantiomers if 1) the molecule s preferred geometry is chiral, and 2) the molecule s enantiomeric forms do not readily interconvert (this interconversion is called configuration inversion ). 

In light of the previous discussions, it would be instructive to compare the behavior of enantiomerically pure allylic alcohol 12 in epoxidation reactions without and with the asymmetric titanium-tartrate catalyst (see Scheme 2). When 12 is exposed to the combined action of titanium tetraisopropoxide and tert-butyl hydroperoxide in the absence of the enantiomerically pure tartrate ligand, a 2.3 1 mixture of a- and /(-epoxy alcohol diastereoisomers is produced in favor of a-13. This ratio reflects the inherent diasteieo-facial preference of 12 (substrate-control) for a-attack. In a different experiment, it was found that SAE of achiral allylic alcohol 15 with the (+)-diethyl tartrate [(+)-DET] ligand produces a 99 1 mixture of /(- and a-epoxy alcohol enantiomers in favor of / -16 (98% ee). 

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