Alcohols enantiomeric purity/absolute

Thus, the enantiomeric contents in a pair of sulphoxides can be determined by the H NMR chemical shifts in the methine or methylene protons in the two diastereomeric complexes which are stabilized by the hydrogen bond between the hydroxyl and the sulphinyl groups (Scheme 13). Similarly, the enantiomeric purity and absolute configurations of chiral sulphinate ester can be determined by measuring the NMR shifts in the presence of the optically active alcohols. ... 

The enantiomeric purity was checked by the H-NMR method of Mislow and Raban (60). The absolute configurations of the major alcohol isomers were all R. 

When an optically active epoxide is treated with bis(iodozincio)methane 3, the homoal-lylic alcohol is obtained with a slight loss of enantiomeric purity (equation 51). The absolute configuration of the product showed that the migrating hydroxymethyl group comes from the front side of the — group. The migration occurs with retention of configuration74-76. 

Distal C-C bond formation can also be used to construct cyclic ethers, but this demands that methods be developed for the enantioselective assembly of complex acyclic ethers. P. Andrew Evans of Indiana University has demonstrated (Angew. Chem. Jnt. Ed. 2004, 43,4788) that Rh-mediated coupling of secondary allylic carbonates such as 7 with secondary alcohols such as 8, both enantiomerically pure, proceeds with clean retention (double inversion) of absolute configuration. Alkene metathesis then delivers the cyclic ether 9 in high diastereomeric and enantiomeric purity. 

The addition of Eu(dpm)3 to camphanate esters of a-deuteriated primary alcohols such as ethanol, benzyl alcohol and geraniol caused shifts in the spectrum that could be used to determine enantiomeric purity and that correlated with the absolute configuration. ... 

Resolution of Acids. The number of acids resolved with brucine is too large to attempt to list even a small portion of them in this synopsis. An excellent tabulation of all published resolutions with brucine up to 1972 is available. Only a few representative examples will be described here (eqs 1-4). In all these cases, the resolved acids were obtained in high yield and with almost absolute enantiomeric purity. The solvents most frequently used for brucine resolutions are acetone and alcohol solvents. However, water, hexane, and others have also been used as cosolvents. 

H]serine formed was degraded into ethanol in such a way that C-3 of serine becomes C-1 of ethanol, and the alcohol dehydrogenase reaction was then used to establish the enantiomeric purity and the absolute configuration of the labelled centre. The sample was in fact 72% (S)-[l- H]ethanol and therefore the tritiated serine contained 72% of the label in the (35) and the remainder in the (3R) isomer (see Fig. 17). 

Martin VS, oodard SS, Katsuki T et al (1981) Kinetic resolution of racemic allylic alcohols by enantioselective epoxidation - a route to substances of absolute enantiomeric purity. J Am Chem Soc 103 6237-6240... 

Note that the aldehyde approaches the alkene from the direction anti to the silicon atom. Therefore, when a chiral allylsilane or allylstannane with a substituent in the a-position is used, chirality transfer takes place, to generate the homoallylic alcohol with essentially no loss in enantiomeric purity. For example, reaction of the aldehyde 157 with the chiral allylsilane 158, using boron trifluoride etherate as the catalyst, gave predominantly the syn product 159 (1.151). The absolute stereochemistry can be determined by using a model in which the hydrogen atom on the a-carbon of the allylsilane eclipses the alkene (the so-called inside hydrogen effect ) in order to minimize steric interactions (1.152). 

Reactions.—The absolute configuration and enantiomeric purity of chiral secondary carbinols/ and also of primary alcohols chiral at can be determined... 

Titanium-catalyzed asymmetric epoxidation of allylic alcohols employing titanium alkoxide, an optically active tartrate ester and an alkyl hydroperoxide. A high degree of enantiomeric purity is attainable having predictable absolute stereochemistry ... 

Enantiomerically pure lactonic pheromones 607-611, of the carpenter bee, blacktailed deer, Japanese beetle, rove beetle, and Oriental hornet, respectively, have been synthesized from racemic cyano alcohols of type 612. The key to the success of the overall approach is the facile separation of diastereomeric carbamates derived from cyano alcohols of type 612 by automated multigram LC. The chosen approach also facilitates the assignment of absolute configurations to the lactone enantiomers and their precursors. In the case of 607, direct determination of enantiomeric purity and absolute configuration is also possible using the chiral solvating agent 2,2,2-trifluoro-l-(9-anthryl)ethanol [439]. 

Me02CCH0/H+ -7.90 eV, vide infra), by the BINOL-Ti catalyst (1) (Table 1). The reaction was carried out by simply adding an olefin and tfien freshly dehydrated and distilled fluoral (2a) at 0 C to the solution of the chiral titanium complex (1) prepared from (/ )- or (5)-BINOL and diisopropoxytitanium dihalide in the presence of molecular sieves MS 4A as described for die glyoxylate-ene reaction (5). The reaction was completed widiin 30 min. The ene-type product, namely homoallylic alcohol (3) was obtained along with the allylic alcohol (4) (entries 1-4). The enantiomeric purities of both products were determined to be more than 95% ee by 1h NMR analysis after transformation to the (5)- and (/ )-MTPA ester derivatives. Thus, the absolute configuration of the products was determined by the Mosher method (8). The sense of asymmetric induction is, therefore, exactly the same as observed for the glyoxylate-ene reaction the (/ )-catalyst provides the (/ )-alcohol products (5). 

Overall, Mosher s reagent, although still useful for the determination of enantiomeric purity by NMR spectroscopy, is not recommended for the determination of the absolute configuration of secondary alcohols by NMR. Such assignments should be made using other, more-reliable reagents. 

NMR spectroscopy was found to be a valuable technique for differentiation between the enantiomers of optically active compounds. The principles of the methods used to distinguish between enantiomers by means of NMR have been developed and reviewed by Mis-low and Raban (217). The best results from the point of view of the determination of optical purity and absolute configuration of chiral sulfur compounds, especially of sulfinyl compounds, have been obtained with the help of chiral solvents (218). Pirkle (86) was the first to demonstrate that enantiomeric sulfoxides have nonidentical NMR spectra when dissolved in chiral alcohols having the following general formula ... 

This was accomplished by anaerobic reduction with Sporotrichum exile in which the 4a(S),8a(R)-enantiomer 51 is reduced six times more rapidly than the other enantiomer. This process in combination with chromic acid oxidation of the derived alcohol gave approximately 70% optically pure enantiomers. Resolution was completed by recrystallization from benzene in which the racemic form is significantly more soluble. The absolute configurations and optical purity of these enantiomeric ketones were confirmed by obtaining the 4a(S),8a(R)-enantiomer 51 from naturally occurring cinchonine (45) via meroquinene... 

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