By Resolution

Ferrocene behaves like an aromatic compound activated for electrophilic substitution reactions. Thus, only minor modifications of experimental procedures developed for aromatics are necessary to obtain ferrocene derivatives (a useful review on general methods is given by Schldgl and Falk [42]). For central chiral ferrocenes, resolution of the racemate is a frequently applied technique. Traditionally, resolutions are best achieved by salt formation between a chiral acid or base and the 

As shown in Fig. 4-8, many amines can be resolved with the help of cheap acids such as (K,il)-tartaric acid [54] or (—)-diacetone-2-keto-l-gulonic acid [(—)-DAG 2,3-4,6-di-0-(l-methylethylidene)-a-Z,-xylo-2-hexulofuranosonicacid] [40]. The resolution of the acidic compounds, e.g., the derivatives of thioglycolic acid (2-mercaptoacetic add), is possible with chiral bases such as ephedrine [45] or phenylethylamine [55], 

If the chiral auxiliary used for the formation of diastereoisomers is covalently attached to the ferrocene derivative, other techniques such as chromatography may be applied for diastereoisomer separation. 1-Phenylethylamine is a comparatively 

With chiral stationary phases, chromatographic separation of enantiomeric ferrocene derivatives is possible. An apparatus for the resolution of ferrocenyl alcohols and other compounds on triacetylcellulose has been described [60]. Analytical enantiomer separation of ferrocenyl alcohols, ethers, sulfides, and amines for the determination of enantiomeric excesses is best achieved on cyclodextrin bonded phases [61]. 

Kinetic resolution of ferrocene derivatives, mainly alcohols, had an important place during the early stage of stereochemical investigations of ferrocene derivatives. The reaction of (partially) resolved ferrocenylalkyl alcohols and amines with racemic 2-phenylbutyric acid anhydride (Korean s method) was the basis for the configurational assignment before the establishment of structures by X-ray crystallography [41]. There has been some debate on the reliability of the method [62, 63], and additional chirality information seems necessary for certainty. Recently, the kinetic resolution of 1-ferrocenylethanol by transesterification with vinyl acetate, catalyzed by a lipase from PseudomonasJluorescens, led to an enantiomeric excess of 90—96% of both enantiomers [64], opening new preparative aspects. 

As shown in Fig. 4-8, many amines can be resolved with the help of cheap acids such as (i ,J ) tartaric acid [54] or (—)-diacetone-2-keto-l-gulonic acid [(—)-DAG  

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A single peak from an ordinary gas chromatogram (a) is revealed as two closely separated peaks by resolution enhancement (b). 

Cromakalim (137) is a potassium channel activator commonly used as an antihypertensive agent (107). The rationale for the design of cromakalim is based on P-blockers such as propranolol (115) and atenolol (123). Conformational restriction of the propanolamine side chain as observed in the cromakalim chroman nucleus provides compounds with desired antihypertensive activity free of the side effects commonly associated with P-blockers. Enantiomerically pure cromakalim is produced by resolution of the diastereomeric (T)-a-meth5lben2ylcarbamate derivatives. X-ray crystallographic analysis of this diastereomer provides the absolute stereochemistry of cromakalim. Biological activity resides primarily in the (—)-(33, 4R)-enantiomer [94535-50-9] (137) (108). In spontaneously hypertensive rats, the (—)-(33, 4R)-enantiomer, at dosages of 0.3 mg/kg, lowers the systoHc pressure 47%, whereas the (+)-(3R,43)-enantiomer only decreases the systoHc pressure by 14% at a dose of 3.0 mg/kg. 

R)-Pantolactone (9) is prepared ia either of two ways by resolution of the (R,5)-pantolactone mixture (18), or by stereoselective reduction of ketopantolactone (19) by chemical or microbial methods (19). 

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). 

RL Dunbrack Jr. Culling the PDB by resolution and sequence identity. 1999. http // www.fccc.edu/research/labs/dunbrack/culledpdb.html... 

The approach used for the initial synthesis of PC s via cyclopentylamine intermediates was shortened and broadened to include the synthesis of 11- i-PG s and also chiral PG s by resolution. 

The chiral keto acid A could be obtained by resolution with (5)-(-)-a-methylben2ylamine. Another route from A to PC s is shown below ... 

Optically pure 5-HETE can be made in quantity by resolution of racemic 5-HETE (Ref. 2). 

The prototype, amphetamine (52), is obtained by reductive amination of phenylacetone by means of ammonia and hydrogen. Isolation of the (+) isomer by resolution gives dextroamphetamine, a somewhat more potent stimulant than the racemate. 

Saturation of the aromatic ring of pentopril analogues is also consistent with ACE inhibition as demonstrated by the oral activity of indolapril (23). The necessary heterocyclic component (21) can in principle be prepared by catalytic perhydrogenation (Rh/C, HOAc) of the corresponding indole. A single isomer predominates. The product is condensed by amide bond formation with the appropriate alanylhomophenylalanyl dipeptide ester 20 to give 22. Selective saponification to 23 could be accomplished by treatment with HCl gas. Use of the appropriate stereoisomers (prepared by resolution processes) produces chiral indolapril [8]. 

S(-)-x-(l-Naphthyl)ethylamine has been prepared by resolution of the racemic amine with camphoric acid in unspecified yield.2,3... 

Enantiopure (R)- and (S)-nipecotic acid (Nip) derivatives 64 were obtained following classical resolution of ethyl nipecotate with either enantiomer of tartaric acid and successive recrystallization of the corresponding salts [153, 154, 156] or by resolution of racemic nipecotic acid with enantiomerically pure camphorsul-fonic acid [154]. N-Boc protected pyrrolidine-3-carboxylic acid (PCA) 65 for the synthesis of homo-ohgomers [155] was prepared by GeUman from trans-4-hydroxy-L-prohne according to a known procedure [157]. 

R=Me or COOEt) failed.2-Amino-4-phosphonobutanoic acid, the phosphonic acid analogue of glutamic acid, has been obtained in resolved forms by the action of papain, followed by aniline, on 2-benzoylamino-4-(diethoxyphosphinyl)butanoic acid when anilide occurs preferentially for the L- compound. Enantiomers of 4-amino-4-phosphonobutanoic acid were obtained by resolution... 

A complex naturally occurring amino acid 5-hydroxypiperazic acid (5HyPip) 100 was prepared by a multistep procedure that included Diels-Alder addition of 2,4-pentadienoic acid to phthalazinedione 83a as a first step (Scheme 24). Adduct 97 was esterified and oxidized with mercuric acetate to 98, which on hydrogenation over rhodium on alumina and subsequent hydrolysis provided a mixture of enantiomers from which the required enantiomer 99 was obtained by resolution with quinine. Its hydrazinolysis provided 100 [71JCS(C)514 77H119],... 

Enantionmerically enriched pairs of /i-stannylated benzoic acids (+)/(—)-35 and (+)/(—)-36 were obtained by resolution with brucine or strychnine (Scheme 13)20. Again, neither the configuration nor the enantiomeric enrichment could be determined. 

Response to therapy should be measured by resolution of radiologic, serologic, and microbiologic parameters and improvement in signs and symptoms of infection. 

As the representative examples in Scheme 6.11 illustrate, similar stragies may be applied to the corresponding alkenyl ethers (vs. styrenyl ethers) [26], The Zr-catalyzed kinetic resolution/Ru-catalyzed metathesis protocol thus delivers optically pure 2-substituted di-hydrofurans that cannot be accessed by resolution of the five-membered ring heterocycles (see Scheme 6.8). It should be noted, however, that the efficiency of the Zr-catalyzed resolution is strongly dependent, and not in a predictable manner, not only on the presence but the substitution of the acyclic alkene site of the diene substrate. The examples shown in Scheme 6.11 clearly illustrate this issue. 

For a nonracemic mixture of enantiomers prepared by resolution or asymmetric synthesis, the composition of the mixture was given earlier as percent optical purity (equation 1), an operational term, which is determined by dividing the observed specific rotation (Mobs) of a particular sample of enantiomer with that of the pure enantiomer ( max), both of which were measured under identical conditions. Since at the present, the amount of enantiomers in a mixture is often measured by nonpolarimetric methods, use of the term percent optical purity is obsolete, and in general has been replaced by the term percent enantiomeric excess (ee) (equation 2) introduced in 197163, usually equal to the percent optical purity, [/ ] and [5] representing the relative amounts of the respective enantiomers in the sample. 

Cyclopropanation of 2,5-dimethyl-2,4-hexadiene provides chiysanthemic acid, a natural product of the group of pyrethroic acids, used as an insecticide, see Figure 17.3 [3]. The appropriate esters of the 1R stereomers are the active compounds, which are obtained industrially by resolution of the racemates. 

The primary analytical applications of RPLC in the development of biopharmaceuticals are the determination of protein purity and protein identity. Purity is established by analysis of the intact protein, and RPLC is useful in detecting the presence of protein variants, degradation products, and contaminants. Protein identity is most often established by cleavage of the protein with a site-specific protease followed by resolution of the cleavage products by RPLC. This technique, termed peptide mapping, should yield a unique pattern of product peptides for a protein that is homogeneous with respect to primary sequence. 

More than 60 years later Darwish and co-workers (151,152) prepared a series of chiral trialkylsulfonium, dialkylarylsulfonium, and diarylalkylsulfonium perchlorates by resolution of the corresponding (-)-dibenzoyl hydrogen tartrate salts followed by replacement of the optically active acid anion by perchlorate. At the same time, 1-ada-mantylethylmethylsulfonium perchlorate 105 (153) and 1-adaman-tylethylpropenylsulfonium tetrafluoroborate 106 (154) were resolved... 

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