Molecular enantiomeric

In order to test this very simple idea, the MHOC tail was incorporated into the prototypical bent-core double-Schiff-base structure, providing as target the mesogen (Rac)-MHOBOW (9),53 whose structure and phase sequence is shown in Figure 8.30. The racemate was prepared in order to avoid complications in the interpretation of the results caused by molecular enantiomeric... 

Table 5 LSER solute descriptors of five molecular enantiomeric pairs and the corresponding Eq. (6) enantioselectivity coefficients obtained on the teicoplanin macrocycUc glycopeptide chiral stationary phase... 

What alkene gives a racemic mixture of (2R 3S) and (2S 3R) 3 bromo 2 butanol on treat ment with Br2 in aqueous solution" Hint Make a molecular model of one of the enantiomeric 3 bromo 2 butanols arrange it in a conformation in which the Br and OH groups are anti to one another then disconnect them )... 

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). 

The adjacent iodine and lactone groupings in 16 constitute the structural prerequisite, or retron, for the iodolactonization transform.15 It was anticipated that the action of iodine on unsaturated carboxylic acid 17 would induce iodolactonization16 to give iodo-lactone 16. The cis C20-C21 double bond in 17 provides a convenient opportunity for molecular simplification. In the synthetic direction, a Wittig reaction17 between the nonstabilized phosphorous ylide derived from 19 and aldehyde 18 could result in the formation of cis alkene 17. Enantiomerically pure (/ )-citronellic acid (20) and (+)-/ -hydroxyisobutyric acid (11) are readily available sources of chirality that could be converted in a straightforward manner into optically active building blocks 18 and 19, respectively. 

The emergence of the powerful Sharpless asymmetric epoxida-tion (SAE) reaction in the 1980s has stimulated major advances in both academic and industrial organic synthesis.14 Through the action of an enantiomerically pure titanium/tartrate complex, a myriad of achiral and chiral allylic alcohols can be epoxidized with exceptional stereoselectivities (see Chapter 19 for a more detailed discussion). Interest in the SAE as a tool for industrial organic synthesis grew substantially after Sharpless et al. discovered that the asymmetric epoxidation process can be conducted with catalytic amounts of the enantiomerically pure titanium/tartrate complex simply by adding molecular sieves to the epoxidation reaction mix-... 

A kinetic resolution of racemic sulphoxides was observed in the reduction by chiral polyiminoalanes. The efficiency of this process depends on the molecular structure of the polyiminoalane. With open pseudo-cubic tetra [JV-(l-phenylethyl)]imidoalane, unreacted sulphoxides were isolated in enantiomeric enrichment up to 75%. Optical purity was shown to increase with increasing the reaction temperature, a maximum enrichment being observed between 55 and 70 °C336. 

When enantiomerically pure allyl p-tolyl sulfoxide is deprotonated and then treated with electrophilic 2-cyclopentenone, a conjugate addition occurs forming a new carbon-carbon bond with very high control of absolute stereochemistry (equation 25)65. See also Reference 48. Similarly, using more substituted enantiomerically pure allylic sulfoxides leads to virtually complete diastereocontrol, as exemplified by equations 26 and 27 the double bond geometry in the initial allylic sulfoxide governs the stereochemistry at the newly allylic carbon atom (compare equations 26 vs. 27)66. Haynes and associates67 rationalize this stereochemical result in terms of frontier molecular orbital considerations... 

Alkyldimethylphosphine-boranes 74 underwent enantioselective deprotonation employing (-)-sparteine/s-BuLi, followed by oxidation with molecular oxygen [91, 92] in the presence of triethyl phosphite (Scheme 12) to afford moderate yields of enantiomerically enriched alkyl(hydroxymethyl)methylphosphine-bo-ranes 76, with 91-93% ee in the case of a bulky alkyl group and 75-81% ee in the case of cyclohexyl or phenyl groups [93]. Except for the adamantyl derivative (in which the ee increased to 99%), no major improvement in the ee was observed after recrystallization. 

The complex 65 was synthesized by reaction of the imidazolinium salt with the precursor ruthenium complex 67 (catalytically inactive) in the presence of silver carbonate (Scheme 42). The complex being air-stable and stable on silicagel was isolated in 52% yield after chromatography. The diastereomeric and enantiomeric purity of 65 was determined by HPLC analysis and found to be above 98% (de and ee). The molecular structure was determined by X-ray analysis and showed the unusual twist geometry of this complex. 

Fig. 11 (a) Schematic polymer structure of poly-7 OEt. Phenylene rings are omitted in order to simplify, (b) Molecular model of repeating structure of poly-7 OEt. Four chiral centres on each of two cyclobutane rings in both sides are enantiomeric to each other. 

The molecular modelling approach, taking into account the pyruvate—cinchona alkaloid interaction and the steric constraints imposed by the adsorption on the platinum surface, leads to a reasonable explanation for the enantio-differentiation of this system. Although the prediction of the complex formed between the methyl pyruvate and the cinchona modifiers have been made for an ideal case (solvent effects and a quantum description of the interaction with the platinum surface atoms were not considered), this approach proved to be very helpful in the search of new modifiers. The search strategy, which included a systematic reduction of the cinchona alkaloid structure to the essential functional parts and validation of the steric constraints imposed to the interaction complex between modifier and methyl pyruvate by means of molecular modelling, indicated that simple chiral aminoalcohols should be promising substitutes for cinchona alkaloid modifiers. Using the Sharpless symmetric dihydroxylation as a key step, a series of enantiomerically pure 2-hydroxy-2-aryl-ethylamines... 

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