Enantiomer preference

The results of both calculations were in good agreement with the chromatographic resolution data for 40 and 19.211 The lowest or averaged interaction energy between 23a and (S, S)-4() obtained in the calculation is lower than that between 23a and (R, / )-40, whereas an opposite enantiomer preference was observed for 23x and 40. 

In order to obtain a commercially viable process it is necessary to racemize the unwanted amine enantiomer, preferably in situ in a so-called DKR. The paUadium-on-charcoal-catalyzed racemization of amines was first reported by Murahashi et al. [23] and was later combined with Upase-catalyzed acylation, to afford a DKR, by Reetz [24] and others [25]. We were able to achieve a DKR of a-methyl benzyl-amine by performing the hpase-catalyzed acylation in the presence of a palladium nanoparticle catalyst (Scheme 6.10). 

Hoover S. E. R., Lindgren B. S., Keeling C. I. and Slessor K. N. (2000) Enantiomer preference of Trypodendron lineatum and the effect of pheromone dose and trap length on response to lineatin-baited traps in interior British Columbia../. Chem. Ecol. 26, 667-677. 

Anaerobic reductive dechlorination of chiral PCBs confirmed that microbial reductive dechlorination in situ was possible in Lake Hartwell [163]. Microcosms with sediments from the same cores [156] spiked with racemic PCB 132 reductively meto-dechlorinated this congener nonenantioselectively to PCB 91, which in mrn was stereoselectively meta-dechlorinated to achiral PCB 51 (Figure4.12). Similarly, PCB 149wasnonstereoselectively /jflra-dechlorinated to PCB 95, in turn enantioselectively meta-dechlorinated to achiral PCB 53 [163]. The enantiomer preferences for PCB 149 dechlorination were consistent between the laboratory microcosms [163] and field observations, suggesting possible similarities in the microbial consortia in both cases. However, PCB 132 was nonracemic in the cores [156], suggesting that either the microbial consortia and/or environmental conditions affecting microbial activity were different between the laboratory and in situ. Much remains unknown about the microbial strains and enzymes involved in PCB anaerobic reductive dechlorination or the factors controlling stereospecificity. 

The methyl esters of (/ )- or (5)-pyrrolidone-5-carboxylic acid (acronym mepy) respectively form [Rh2(5/ -mepy)4] or [Rh2(55-mepy)4] complexes (see Structure 8). The catalyst is thus available in both configurations, 5R or 5S. Such catalysts deliver often more than 90% ee and conformational energy minima (extended Hiickel) of the carbene-styrene complex predict the observed enantiomer preference [24 a]. 

In lipase-catalyzed racemate separation of alcohols the same enantiomer preference is usually observed in acylation and hydrolysis (Scheme 11.1-15)[SG. ... 

The examples illustrated in Schemes 6.4 and 6.37 may be related to the Brooks experiment [53] as reactions where two different enzymatic pathways operate at the same time, but with opposite enantiomer preferences. It is not clear that the enantiomeric purity of the products from these microbiological experiments was substantially influenced by the existence of pathways that consume each enantiomeric substrate. This issue was not addressed by any of the authors, nor is there much practical consequence if the inherent enantioselectivities are very high as they often are for enzymatic reactions. However, if the enantioselectivities, s, for the competing pathways from each enantiomer are below about 100, then the existence of parallel pathways that consume both enantiomers is an important factor. This has briefly been discussed in Section 6.1 and is summarized below. Such pathways would help to consume both substrate enantiomers at similar rates, and ideally, to maintain the ratio of enantiomers near 1 1 throughout the experiment. If these conditions are satisfied, then the enantiomeric ratio of products would be close... 

Omeprazole can be made by reaction of a substituted diaminobenzene to make the benzoimidazole [38]. Alkylation of the thiol gives the sulfide which is then oxidized to the sulfoxide, omeprazole. The omeprazole can be separated into the two enantiomers. Preferably, asymmetric oxidation of the sulfide is done to selectively prepare the S enantiomer, esomeprazole. 

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