Enantiomers of - complexes

The 5-enantiomer of complex 11 exhibits a similar behavior, finally providing the (15,2f )-cy-clopropanecarboxamide as the major isomer73. It was demonstrated in the racemic series that the diastereoselectivity of the cyclopropanation step proceeds with higher selectivity when the methyl group on the olefin is replaced by larger groups (e.g., isopropyl yield 93% d.r. 96 4)73. 

The chiral recognition properties of crown ethers(4) for racemic Ct-phenylglycine methyl ester perchlorate(5) and 1-phenylethylamine perchlorate(6) were tested by standard extraction experiments.No appreciable amount of crown ethers was detected in aqueous layers. The relative amount of complexed salts to crown ethers in CDCl layers was determined by NMR integrations. When (5) is complexed with (4), the signal of ester methyl protons is moved and separated into two signals for each enantiomei When (6) is complexed with (4), the signal of methyl protons(doublet) is only shifted but not separated. The ratio of enantiomers of complexed salts was determined by HPLC using a chiral column for (5) or by GLC for (6) as diastereomeric isomers of (-)-(L)-N-trifluoroacetylalanine. The results are summarized in Table I and II. 

Inversion of the si,M enantiomer of complex I displays close analogy to one period of the anchor escapement motions. The partial rotation of the ester group and its impact on the coupled elements are identical to those of the escapement wheel in a clock. The Co(CO)3 rotor acts as an anchor and the two carbonyl ligands of the OC-Co-CO fork function as pellets [24]. 

Treatment of the dichloro complex with aqueous cyanide tmder mild reaction conditions led to the isolation of the optically pure (S)-(—)-ProPhos in 95% yield in 2 h. This protocol also allows the opportunity to confirm whether the isolated free phosphine s optical purity has been compromised during the liberation process. This is a standard protocol which is used in all subsequent reactions discussed in this chapter. As seen in Scheme 2, 7a and 7b are the enantiomers of complexes 5a and 5b and, therefore, in the absence of any chiral shift reagent, they exhibit exactly the same chemical shifts. It is important to note that enantiomer (/ )-(+)-ProPhos can also be prepared in a similar efficient manner by using the equally accessible complex (/ )- as the chiral auxiliary. 

Organic chemists often use enantiomencally homogeneous starting materials for the synthe SIS of complex molecules (see Chiral Drugs p 296) A novel preparation of the S enantiomer of compound B has been descnbed using a bacterial cyclohexanone monooxygenase enzyme system... 

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). 

For the cycloaddition reaction in Scheme 4.6 it was found that 3-bromocam-phor, for example, can bind selectively to one enantiomer of the complex [12] and that if the reaction was performed in the presence of the racemic catalyst 8 and 3-bromocamphor, cis-3 was isolated with up to 80% ee compared to 95% ee for the reaction catalyzed by (J )-8b. 

In general, a liquid membrane for chiral separation contains an enantiospecific carrier which selectively forms a complex with one of the enantiomers of a racemic mixture at the feed side, and transports it across the membrane, where it is released into the receptor phase (Fig. 5-1). 

Discrimination between the enantiomers of a racemic mixture is a complex task in analytical sciences. Because enantiomers differ only in their structural orientation, and not in their physico-chemical properties, separation can only be achieved within an environment which is unichiral. Unichiral means that a counterpart of the race-mate to be separated consists of a pure enantiomeric form, or shows at least enrichment in one isomeric form. Discrimination or separation can be performed by a wide variety of adsorption techniques, e.g. chromatography in different modes and electrophoresis. As explained above, the enantioseparation of a racemate requires a non-racemic counterpart, and this can be presented in three different ways ... 

The enantiomerically pure (S)-enantiomer of propanoyl complex 1 has been converted to the aluminum enolate and reacted with 4-pentenal (7) to provide the product (Fe5,2 5,3 / )-8 with good diastereoselectivity 51. 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

For example, the racemic thioester 57 was placed in contact with a certain optically active amide. After 28 days the solution contained 89% of one enantiomer and 11% of the other. To effect the deracemization two conditions are necessary (1) the enantiomers must complex differently with... 

Allylic alcohols can be converted to epoxy-alcohols with tert-butylhydroperoxide on molecular sieves, or with peroxy acids. Epoxidation of allylic alcohols can also be done with high enantioselectivity. In the Sharpless asymmetric epoxidation,allylic alcohols are converted to optically active epoxides in better than 90% ee, by treatment with r-BuOOH, titanium tetraisopropoxide and optically active diethyl tartrate. The Ti(OCHMe2)4 and diethyl tartrate can be present in catalytic amounts (15-lOmol %) if molecular sieves are present. Polymer-supported catalysts have also been reported. Since both (-t-) and ( —) diethyl tartrate are readily available, and the reaction is stereospecific, either enantiomer of the product can be prepared. The method has been successful for a wide range of primary allylic alcohols, where the double bond is mono-, di-, tri-, and tetrasubstituted. This procedure, in which an optically active catalyst is used to induce asymmetry, has proved to be one of the most important methods of asymmetric synthesis, and has been used to prepare a large number of optically active natural products and other compounds. The mechanism of the Sharpless epoxidation is believed to involve attack on the substrate by a compound formed from the titanium alkoxide and the diethyl tartrate to produce a complex that also contains the substrate and the r-BuOOH. ... 

The rhodium complex of the (R,R)-counter-enantiomer of (S,S)-BisP achieved a high level of ee (97%) in the asymmetric hydrogenation of 3-methoxy-substituted substrate (S)-122 (Scheme 25), which constitutes a precursor to the acetylcholinesterase inhibitor SDZ-ENA-713 (123). 

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