Racemization studies

If some fields may be empty in the sublevels, all the fields in the main level are required for each entry. A new chiral separation record can be added in CHIRBASE solely if the authors correctly identify both sample and CSP. Since the beginning of the project, our policy has been to contact the authors of all publications containing incomplete, ambiguous or inconsistent data and to ask for additional information. Providing the separations with unique case numbers helps us considerably in this essential task, and also facilitates avoiding redundancies in the database. When chiral separations are reported for the second time in a new publication with exactly the same chromatographic conditions, this is stated in a footnote added in the field comments . In this field, miscellaneous information that cannot appear elsewhere are listed (detection limit, description of a reported chromatogram, racemization study, mobile phase limitations, etc.). 

Stereochemical constraints in cyclic sulfones and sulfoxides impart increased weight to strain and conformational factors in the generation of carbanions and their stability, causing distinct differences between the behavior of cyclic and open-chain systems233, due primarily to the prevention of extensive rotation about the C —S bond, which is the major way that achiral carbanions racemize. Study of the a-H/D exchange rate fce and the racemization rate ka may provide information concerning the acidity-stereochemical relationships in optically active cyclic sulfone and sulfoxide systems. 

Both thermal- and acid-induced equilibrations of 3,3-disubstituted thietane oxides were very slow (K 10-5 s-1)194. The results suggest that thietane oxides are similar to the various acyclic sulfoxides with respect to the rates of thermally induced pyramidal inversion at sulfur238, and that this inversion process, therefore, does not interfere significantly in the above exchange/racemization studies. 

Racemization studies in the synthesis of the tripeptide Z-Gly-Phe-Gly from Z-Gly-Phe and Gly-OC2H5 revealed that in THF at room temperature such racemization occurred to the extent of about 5%, in DMF at -10 °C, however, less than 0.5%. 53 103 In the synthesis of Boc-Val-Tyr-OC2H5 (50%) from Boc-Val and Tyr-OC2H5 with CDI, a small amount of 0-acylation of tyrosine (4%) also occurred in the dipeptide. 11] A V -Carbonyldibenzimidazole was found inferior to CDI in the synthesis of peptides because of poorer yields and more rigorous reaction conditions needed. 53... 

NL Benoiton, K Kuroda, ST Cheung, FMF Chen. Lysyl dipeptide and tripeptide model systems for racemization studies in amino acid and peptide chemistry. Can J Biochem 57, 776, 1979. 

LA Carpino, A El-Faham, F Albericio. Racemization studies during solid-phase peptide synthesis using azabenzotriazole-based coupling reagents. Tetrahedron Lett... 

T Kaiser, GJ Nicholson, H Kohlbau, W Voelter. Racemization studies of Fmoc-Cys(Trt)-OH during stepwise Fmoc-solid phase peptide synthesis. Tetrahedron Lett 37, 1187, 1996. 

Kinetic data on the oxepin-benzene oxide equilibration have been obtained from the temperature-dependent NMR studies. Low values were observed for the enthalpy of isomerization of oxepin (7.1 kJ mol-1) and 2-methyloxepin (1.7 kJ mol-1) to the corresponding benzene oxides (67AG(E)385). The relatively small increase in entropy associated with oxepin formation (5-11 J K 1 mol-1) is as anticipated for a boat conformation in a rapid state of ring inversion. Thermal racemization studies of chrysene 1,2- and 3,4-oxides have allowed accurate thermodynamic parameters for the oxepin-arene oxide equilibration process in the PAH series to be obtained (81CC838). The results obtained from racemization of the 1,2- (Ea 103.7 kJ mol-1, AS 3.7 JK-1 mol-1 and 3,4- (Ea 105.3 kJmoF1, AS 0.7 J K"1 mol ) arene oxides of chrysene are as anticipated for the intermediacy of the oxepins (31) and (32) respectively. 

Additional Methods of Analysis. Racemization studies (11,12,13) hold considerable promise for the dating of ancient bone samples. If the diflBculties of shorter time span compared with those already studied, limited sample universe, and nonuniform burial conditions can be overcome, the application of this procedure to Ancient Near Eastern ivory specimens would be useful. Also of interest is the possibility (19,20,21,... 

Oxalate chelation has long been used as a diagnostic tool in coordination chemistry, and consequently the complex oxalates have been extensively investigated. Since Werner s resolution of K3[Cr(0204)3] with strychnine in 1912, more resolution and racemization studies have been made of oxalato complexes than of any other class of coordination compounds. 

A. Huhtikangas, T. Lehtola, R. Virtanen, and P. Peura, Application of radioimmunoassay to racemization studies, Finn. Chem. Lett., 63-66 (1982). 

An analogous sequence appears in trifluoroseleninic acid as compared to -selenonic acid CF3SeO3H resonates at 3 = 981 whereas the Se chemical shift of CF3SeO2H is only 5=1231 The 3-value of benzeneseleninic acid Ph-SeO2H in a chloroform-methanol mixture is 1188 (3=1152, 1182) and its methyl ester 3= 1236 protonation leads to a ca. 20 ppm shielding. Recently, some further arylseleninic acids have been reported with Se chemical shifts between 3=1200 for Ar = 2,4,6-triisopropylphenyl and 3= 1276 for Ar = 2,4,6-triethylphenyl. These were optically resolved, partially to 100% e.g., on a chiral column and their racemization studied. ... 

Some p-hydroxyamides (246) in different diastereomic mixtures were cyclized to tetrahydrofurane derivatives (247) in the presence of Ph-SeCl, and subsequently, their diastereomeric ratios could be determined by Se NMR analysis.Some arylseleninic acids (Ar-SeO2H S = 1200-1276) were optically resolved by chiral columns and their racemization studied. 

Resolution of racemates by classical separation of diastereomeric salts remains a valid alternative to enantioselective synthesis. A simple but ingenious idea has recently been proposed and shown to work successfully the use of families of resolving agents that would work as a library on the racemate to be resolved, so that the least soluble salt would crystallize out. As a matter of fact, Broxterman [21] has shown that a mixture of acylated tartaric acids like 6a-c of Fig. 8, gave immediate crystallization in 97 out of 100 amine racemates studied, allowing a more rapid choice of resolving agents to be made in a more suitable manner than was previously possible. 

Racemization studies have recently been performed on two model peptides with serine and aspartic acid residues [51]. In this study, the microwave coupling reaction was performed using six cycles of 40 W for 25 s. Between cycles the reaction vessel was externally cooled to maintain the maximum temperature below 30 and 50 °C in different experiments, respectively. Racemization analysis was performed by separation of the l and d enantiomers by HPLC. Analysis showed that after use of microwave energy in the coupling reaction, racemization was less than or comparable with that after conventional coupling (Table 20.4). 

Racemization studies were performed during microwave synthesis of the peptide Ala-Cys-Asp-Glu-Phe-Gly-His-Ile-Lys-Leu-Met-Asn-Pro-Gln-Arg-Ser-Thr-Val-Trp-Tyr-NH2 under a variety of coupling conditions using an Odys-... 

As above H, CD isomerization, racemization studies asymfac, sym/ac isomers... 

Table 2 Unbound Fractions of Enantiomers of Chiral Cardiovascular Drugs in Human Specimens. Plasma and Racemate Studied Unless Otherwise Indicated... 

The covalent character present in many carbanion carbon-metal bonds means that we must use caution in discussing the properties of carbanions based on reactions of organometaUics. One way to study the structures of carbanions is to determine whether chiral carbanions undergo racemization. Studies of noncycHc carbanions indicate that the retention of configuration at a chiral carbanionic center depends on solvent and temperature, with solvents such as diethyl ether decreasing the covalent character of the carbon-metal interaction, and thus facilitating epimerization at the chiral center. 

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